Showing 1–50 of 57 results for author: Richings, A J

The effect of galaxy interactions on star formation rates in the COLIBRE simulations

Authors: Quinten W. E. van Zegveld, Evgenii Chaikin, Joop Schaye, David R. Patton, Sara L. Ellison, Alejandro Benítez-Llambay, Filip Huško, Robert J. McGibbon, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller

Abstract: Observations and theory indicate that galaxy interactions enhance star formation rates (SFRs). However, the degree of enhancement and its dependence on the properties of the interacting galaxies vary across different studies. In this work, we use the COLIBRE simulations of galaxy formation to investigate the effect of interactions on the SFRs of star-forming galaxies at redshift $z\approx0$. The C… ▽ More Observations and theory indicate that galaxy interactions enhance star formation rates (SFRs). However, the degree of enhancement and its dependence on the properties of the interacting galaxies vary across different studies. In this work, we use the COLIBRE simulations of galaxy formation to investigate the effect of interactions on the SFRs of star-forming galaxies at redshift $z\approx0$. The COLIBRE simulations capture the multiphase nature of the interstellar medium and have volumes up to $200^3$ and $400^3$ cMpc$^3$ at m6 (gas and dark-matter particle mass $\sim10^6~\mathrm{M_\odot}$) and m7 ($\sim10^7~\mathrm{M_\odot}$) resolutions, respectively. After constructing samples of interacting galaxies (with mass ratios $>0.1$) and isolated controls, matched in stellar mass, large- and small-scale environment, and redshift, we show that the average specific SFR (sSFR) of interacting galaxies is enhanced by up to a factor of $\approx2$ for separations of $\approx10$ kpc. The enhancement decreases with pair separation but remains significant out to $\approx200$ kpc. The enhancement increases with increasing numerical resolution, is more pronounced in the central regions of galaxies, and decreases with increasing stellar mass at fixed separation. Mergers with higher mass ratios induce stronger sSFR enhancement. We compare our results with observational data from the SDSS, finding good agreement in the dependence of the mean sSFR enhancement on separation, but underpredicting its normalisation by a factor of $\approx2$. Finally, we show that the pre-merger sSFR enhancement of resolved interactions accounts for $\approx2$ per cent of the $z\approx0$ cosmic SFR density. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Comments: 21 pages, 20 figures; submitted to MNRAS

The influence of feedback on the baryonic content of haloes in the COLIBRE simulations

Authors: Jonathan J. Davies, Joop Schaye, Filip Huško, Ruby J. Wright, Evgenii Chaikin, Matthieu Schaller, Robert J. McGibbon, Alejandro Benítez-Llambay, Sylvia Ploeckinger, Alexander J. Richings

Abstract: We present predictions for the relation between the halo gas mass fraction and halo mass, $f_{\rm gas}^{200}-M_{200}$, from the COLIBRE cosmological simulations of galaxy formation, and explore how the gas content of haloes is influenced by feedback from supernovae and active galactic nuclei (AGN) over time. The $f_{\rm gas}^{200}-M_{200}$ relation in COLIBRE is non-monotonic, with a peak at… ▽ More We present predictions for the relation between the halo gas mass fraction and halo mass, $f_{\rm gas}^{200}-M_{200}$, from the COLIBRE cosmological simulations of galaxy formation, and explore how the gas content of haloes is influenced by feedback from supernovae and active galactic nuclei (AGN) over time. The $f_{\rm gas}^{200}-M_{200}$ relation in COLIBRE is non-monotonic, with a peak at $M_{200}\sim 10^{11.5-12}$~M$_\odot$. Below this mass, feedback from supernovae efficiently expels gas from the haloes of dwarf galaxies, and above it, AGN feedback efficiently depletes the haloes of galaxy groups. The fiducial COLIBRE model yields gas fractions for galaxy groups and clusters that agree with constraints from Chandra and XMM-Newton X-ray data, but which are high relative to gas fractions inferred from eROSITA stacks and measurements of the kinetic Sunyaev-Zel-dovich (kSZ) effect. COLIBRE's hybrid AGN feedback model, which combines thermal and jet-driven feedback, produces lower gas fractions in better agreement with eROSITA and kSZ measurements. COLIBRE produces lower gas fractions for groups and clusters than EAGLE and other contemporary simulations, and better agreement with observational constraints. We investigate the origin of this improvement relative to EAGLE, and how the resolution of the simulation affects the impact of feedback. Our results demonstrate that halo gas fractions are a sensitive probe of feedback physics, and that they can differ significantly between simulations that otherwise produce very similar galaxy populations. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Comments: 25 pages, 11 figures. Submitted to MNRAS, comments welcome

The evolution of galaxy dust scaling relations in the COLIBRE simulations

Authors: Aswin P. Vijayan, James W. Trayford, Joop Schaye, Sylvia Ploeckinger, Andrea Gebek, Nick Andreadis, Maarten Baes, Alejandro Benítez-Llambay, Evgenii Chaikin, Carlos S. Frenk, Filip Huško, Robert J. McGibbon, Alexander J. Richings, Matthieu Schaller

Abstract: We present dust scaling relations across cosmic time ($0 \le z \le 15$) for galaxies in the COLIBRE cosmological simulations. COLIBRE self-consistently tracks dust production, growth, destruction, and grain size evolution within a multiphase interstellar medium. Using volumes up to $(400\, {\rm cMpc})^3$ at three mass resolutions ($10^{5}-10^7$ M$_{\odot}$), we predict the dust mass function, cosm… ▽ More We present dust scaling relations across cosmic time ($0 \le z \le 15$) for galaxies in the COLIBRE cosmological simulations. COLIBRE self-consistently tracks dust production, growth, destruction, and grain size evolution within a multiphase interstellar medium. Using volumes up to $(400\, {\rm cMpc})^3$ at three mass resolutions ($10^{5}-10^7$ M$_{\odot}$), we predict the dust mass function, cosmic dust mass density, and key dust scaling relations (dust-to-gas ratio, dust-to-metal ratio, grain species fractions, and grain sizes) as functions of galaxy metallicity, stellar mass, and dust mass. The model broadly reproduces most observed relations across cosmic time, matching closest at the highest resolution. We find that silicates dominate the dust mass ($\gtrsim 70\%$) at all epochs, and while large grains dominate in the early Universe ($z \ge 5$), their mass fraction declines to become comparable to small grains by $z=0$. At $z < 1$, the simulated dust mass functions align well with observations, but the cosmic dust mass density is systematically high by $\lesssim 0.3$ dex, while in agreement with observations at higher redshifts. Additionally, the simulations underpredict the extreme dust masses of bright sub-millimeter galaxies at $z \ge 2$. We demonstrate that scaling relations are sensitive to numerical resolution only in the low-redshift, low-mass regime; while their normalisation is influenced by gas-phase selection. These findings highlight both the predictive power and resolution-dependent limits of cosmological dust models, providing essential insights to refine ISM physics. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Comments: 24 pages and 13 figures in main text. Submitted to MNRAS. COLIBRE simulation project website: https://colibre.strw.leidenuniv.nl

Cosmological simulations of the high-redshift galaxy population adopting a variable stellar initial mass function

Authors: Anna Durrant, Robert A. Crain, Cedric G. Lacey, Joop Schaye, Renske Smit, Andrea Gebek, Matthieu Schaller, Shengdong Lu, Evgenii Chaikin, Nick Andreadis, Maarten Baes, Matthew R. Bate, Alejandro Benítez-Llambay, Carlos S. Frenk, Filip Huško, Robert J. McGibbon, Sylvia Ploeckinger, Alexander J. Richings

Abstract: JWST surveys reveal a greater space density of high-redshift UV-bright galaxies than predicted by conventional galaxy formation models. We present results from a $L=100$ cMpc cosmological simulation evolved to $z=5$ with a variation of the COLIBRE galaxy formation model that adopts a density-dependent stellar initial mass function (IMF), such that stellar populations formed from dense gas are born… ▽ More JWST surveys reveal a greater space density of high-redshift UV-bright galaxies than predicted by conventional galaxy formation models. We present results from a $L=100$ cMpc cosmological simulation evolved to $z=5$ with a variation of the COLIBRE galaxy formation model that adopts a density-dependent stellar initial mass function (IMF), such that stellar populations formed from dense gas are born with a top-heavy IMF. Crucially, heavy element and dust yields, and supernova feedback energetics, are self-consistently adjusted to the changing IMF. We model UV/optical emission (including nebular emission) from galaxies and its attenuation by dust. By allowing a significant fraction of high-redshift star formation to proceed with a top-heavy IMF, the rest-frame far-UV luminosities of early galaxies are elevated by up to a factor of $\simeq4$ with respect to the fiducial COLIBRE L100m6 simulation, which assumes a universal Chabrier IMF. This enables the formation of galaxies with observed brightness up to $M_{\rm UV} \simeq -20$ at $z=15$ (c.f. $M_{\rm UV} \simeq -18.5$ in the fiducial simulation), illustrating the potential of star formation with a top-heavy IMF to alleviate tensions with JWST data. Later, the boost in far-UV emission is partly offset by attenuation due to increased dust surface densities from i) additional dust grain ejection from core-collapse supernovae and ii) efficient grain growth promoted by more metal-rich interstellar gas. The simulation reproduces the $z=5$ galaxy stellar mass function and rest-frame optical luminosity function with comparable accuracy to the fiducial simulation, and both simulations exhibit UV continuum slopes that are consistent with JWST observations. △ Less

Submitted 17 July, 2026; originally announced July 2026.

Comments: Main text 22 pages and 11 figures, submitted to MNRAS

The COLIBRE-SKIRT pipeline: Calibration-free dust radiative transfer postprocessing for cosmological simulations

Authors: Andrea Gebek, Maarten Baes, Nick Andreadis, Joop Schaye, Anand Utsav Kapoor, Connor Bottrell, Shengdong Lu, Cedric G. Lacey, Alejandro Benítez-Llambay, Peter Camps, Evgenii Chaikin, Anna Durrant, Carlos S. Frenk, Filip Huško, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, James W. Trayford, Aswin P. Vijayan

Abstract: Context. Three-dimensional dust radiative transfer provides a powerful framework to connect cosmological galaxy simulations to multiwavelength observations. Until recently, in large-volume simulations, the formation of a cold ISM phase was prevented and dust was not evolved self-consistently. This required calibration of dust-to-metal ratios and extra subgrid dust attenuation in birth clouds, ther… ▽ More Context. Three-dimensional dust radiative transfer provides a powerful framework to connect cosmological galaxy simulations to multiwavelength observations. Until recently, in large-volume simulations, the formation of a cold ISM phase was prevented and dust was not evolved self-consistently. This required calibration of dust-to-metal ratios and extra subgrid dust attenuation in birth clouds, thereby reducing the predictive power. Aims. We present the COLIBRE-SKIRT pipeline, a calibration-free dust radiative transfer framework for the novel COLIBRE suite of large-volume cosmological simulations, which include a live dust model and directly simulate the multiphase ISM. Our primary aim is to establish a reference pipeline for generating multiwavelength mock observables from these simulations. As a first application, we produce far-ultraviolet (FUV) to far-infrared (FIR) spatially integrated spectra and assess them by comparison with the observed low-redshift cosmic spectral energy distribution (CSED). Methods. We apply the SKIRT dust radiative transfer code to the COLIBRE simulations. Dust masses and species fractions are taken directly from the simulation, and no birth cloud model is added in postprocessing. We introduce a "split & scale" approach that maps the simulated two-size, multi-species dust distribution onto continuous grain size distributions without introducing free parameters. Results. We find that, for the first time, a large-volume cosmological simulation directly reproduces the local Universe CSED without calibrating the postprocessing routine a priori. Residual tensions in the mid-infrared (~0.2 dex) point towards insufficient heating of the hottest dust components and uncertainties in the modelling of the PAH-emission carriers. This framework can be readily applied at low and high redshift to create synthetic spectra and images from the FUV to the FIR. △ Less

Submitted 16 July, 2026; originally announced July 2026.

Comments: Main text 23 pages and 8 figures. Submitted to A&A

Columba: isolated dwarf galaxy populations in diverse cosmological environments simulated with a cold interstellar medium

Authors: Jemima M. Briggs, Azadeh Fattahi, Robert A. Crain, Yannick M. Bahe, Sylvia Ploeckinger, Matthieu Schaller, Alejandro Benítez-Llambay, Evgenii Chaikin, Alexander J. Richings, Joop Schaye

Abstract: We introduce a suite of LambdaCDM cosmological, hydrodynamical simulations that track the evolution of a large population of dwarf galaxies. The suite comprises zoom-in simulations of 25 spherical, under-dense regions of r=5cMpc, selected to span $\approx1.5$ dex in mean enclosed density, covering voids to filamentary structures, whilst excluding haloes of Milky Way-mass or larger. The simulations… ▽ More We introduce a suite of LambdaCDM cosmological, hydrodynamical simulations that track the evolution of a large population of dwarf galaxies. The suite comprises zoom-in simulations of 25 spherical, under-dense regions of r=5cMpc, selected to span $\approx1.5$ dex in mean enclosed density, covering voids to filamentary structures, whilst excluding haloes of Milky Way-mass or larger. The simulations achieve a mass resolution of $\sim 10^5$ solar masses with a galaxy formation model including cold, dense interstellar gas and whose subgrid stellar feedback efficiency reproduces the z=0 galaxy stellar mass function. We investigate the impact of the cosmic environment on dwarf galaxy formation and evolution. We find that the 5 cMpc environment influences the normalisation of the halo and galaxy mass functions, but does not significantly affect the stellar mass - halo mass (SMHM) relation and halo occupation fraction for galaxies with $M_{\star}=10^6-10^9$ solar masses. Instead, host halo concentration, estimated from DM-only counterparts, is more important: both the fraction of haloes hosting a resolved galaxy and the scatter about the SMHM relation correlate positively with concentration. Owing to halo assembly bias, concentration also influences galaxy formation times, such that at fixed halo mass more concentrated haloes host galaxies that are both older and more massive. The offset from the mean SMHM relation also anti-correlates with $t_{90}$, the time at which 90 percent of a galaxy's stellar mass has assembled. These correlations between halo properties and galaxy star formation histories present testable predictions for forthcoming observational surveys. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Comments: accepted for publication in MNRAS

The evolution of the galaxy gas-phase mass-metallicity relation from $z=15$ to $z=0$ in the COLIBRE cosmological simulations

Authors: Piyush Sharda, Joop Schaye, Robert J. McGibbon, Alejandro Benítez-Llambay, Evgenii Chaikin, Carlos S. Frenk, Jacqueline Hodge, Filip Huško, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller

Abstract: We present the evolution of the galaxy gas-phase mass-metallicity relation (MZR) from $z=15$ to $z=0$ in the COLIBRE cosmological hydrodynamical simulations. Amongst other novel features, COLIBRE follows the multiphase interstellar medium with gas allowed to cool to $\sim 10\,\mathrm{K}$, and includes a new chemistry model in which hydrogen and helium are tracked in non-equilibrium, metals are all… ▽ More We present the evolution of the galaxy gas-phase mass-metallicity relation (MZR) from $z=15$ to $z=0$ in the COLIBRE cosmological hydrodynamical simulations. Amongst other novel features, COLIBRE follows the multiphase interstellar medium with gas allowed to cool to $\sim 10\,\mathrm{K}$, and includes a new chemistry model in which hydrogen and helium are tracked in non-equilibrium, metals are allowed to mix and diffuse, and the chemical network is coupled to a self-consistent live dust model. Using fiducial COLIBRE runs spanning particle masses from $10^5\,\mathrm{M_{\odot}}$ to $10^7\,\mathrm{M_{\odot}}$ and box sizes $25 - 400\,\mathrm{cMpc}$, we derive the median, mass-weighted MZRs for star-forming galaxies and compare them with a comprehensive compilation of observational data and other simulations. COLIBRE reproduces the observed MZR across cosmic time, notwithstanding the systematic uncertainties in observational measurements of the gas-phase oxygen abundances. The simulations show excellent numerical convergence and uniquely probe the full stellar mass range sampled by current observations across all redshifts. We find that the MZR is already in place at cosmic dawn ($z \approx 10$), and shows no evolution until $z \approx 5$. The slope of the MZR becomes shallower at low redshifts. The turnover at the high-mass end is largely governed by feedback from active galactic nuclei (AGN), whereas the low-mass end of the MZR sensitively depends on the strength of feedback from core collapse supernovae. Variations in the star formation efficiency or depletion of oxygen on dust grains have a more minor impact on the MZR. We identify key physical processes that shape the MZR across cosmic time and highlight where future observations can further constrain galaxy formation models. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Comments: 26 pages, 17 figures, submitted to MNRAS. To access the data underlying all the plots in the paper, visit https://github.com/psharda/colibre_mzr_sharda26

Investigating black hole accretion and feedback self-regulation in Seyfert galaxies using the FIRE-3 cosmological hydrodynamic simulations

Authors: Jonathan Mercedes-Feliz, Daniel Anglés-Alcázar, Jose Cevallos, Boon Kiat Oh, Santiago García-Burillo, Rachel K. Cochrane, Cristina Ramos Almeida, Claude-André Faucher-Giguère, Almudena Alonso-Herrero, Alexander J. Richings, Miguel Pereira-Santaella, Jorge Moreno, Niranjan Chandra Roy, Tanio Díaz-Santos, Philip F. Hopkins

Abstract: Recent observations of local Seyfert galaxies show an intriguing connection between Active Galactic Nuclei (AGN) luminosity and a deficit of molecular gas on ~50pc scales compared to 200pc, the plausible imprint of AGN feedback. Motivated by these findings, we investigate the interplay between supermassive black hole (BH) accretion, AGN feedback, and nuclear gas reservoirs using high-resolution co… ▽ More Recent observations of local Seyfert galaxies show an intriguing connection between Active Galactic Nuclei (AGN) luminosity and a deficit of molecular gas on ~50pc scales compared to 200pc, the plausible imprint of AGN feedback. Motivated by these findings, we investigate the interplay between supermassive black hole (BH) accretion, AGN feedback, and nuclear gas reservoirs using high-resolution cosmological hydrodynamic simulations implementing FIRE-3 multi-phase interstellar medium (ISM) physics and multi-component BH accretion and feedback models. Focusing on the late-time evolution of four Milky Way-mass galaxies, we find recurrent cycles of increased gas inflow toward the accretion disc, enhanced BH accretion, feedback self-regulation, and suppressed gas inflow rate until the next fueling event. AGN winds interact with the ISM and escape preferentially through low-density polar channels after opening central cavities on ~10-500pc scales, regulating BH growth and producing episodic behaviour on ~10-100Myr timescales. The simulations reproduce the observed diversity of nuclear morphologies, gas concentrations, and AGN luminosities in late-type Seyfert galaxies, but do not exhibit a clear anti-correlation between gas concentration and AGN luminosity. Higher-luminosity AGN ($L_X$~$10^{41.5-43}$ erg s$^{-1}$) powered by the accretion disc reservoir can coexist with feedback-driven cavities, consistent with observations, but they are more common in simulated galaxies with centrally-peaked gas distributions. Although differences in sample selection, tracer choice, spatial resolution, and stochasticity in AGN fueling may impact underlying concentration-luminosity trends, the apparent tension between simulations and observations points to the timing between gas inflow, accretion-disc depletion, and feedback-driven clearing on ~50-200pc scales as a key constraint on AGN self-regulation models. △ Less

Submitted 23 June, 2026; originally announced June 2026.

Comments: 21 pages, 11 figures. To be submitted to MNRAS

Little Red Dots on FIRE: Exploring the formation and observational signatures of ultra-compact early galaxies

Authors: Niranjan Chandra Roy, Daniel Anglés-Alcázar, Rachel K. Cochrane, Alexander J. Richings, Jonathan Mercedes-Feliz, Christopher C. Hayward, Claude-André Faucher-Giguère, Erini Lambrides, Robert Feldmann, Boon Kiat Oh, Andrew Marszewski, Guochao Sun, Kelcey Davis, Jed McKinney, Caitlin M. Casey, Tanio Díaz-Santos, Madisyn Brooks, Grace Farrell

Abstract: Little Red Dots (LRDs) are compact sources with broad Balmer lines, Balmer breaks, anomalous UV emission, rising red continuum, and uncertain origin. We use FIRE cosmological simulations, 3D dust radiative transfer, and synthetic emission-line data cubes to test whether ultra-compact early galaxies can reproduce LRD-like observables without invoking AGN. In progenitors of present-day group halos (… ▽ More Little Red Dots (LRDs) are compact sources with broad Balmer lines, Balmer breaks, anomalous UV emission, rising red continuum, and uncertain origin. We use FIRE cosmological simulations, 3D dust radiative transfer, and synthetic emission-line data cubes to test whether ultra-compact early galaxies can reproduce LRD-like observables without invoking AGN. In progenitors of present-day group halos ($M_{\rm halo} > 10^{13.5} M_{\odot}$), we identify transient phases at $z \approx 4-8$ lasting $\sim 150-400$ Myr in which strong dissipative inflows build massive ($M_{\star} \sim 10^{8.5}-10^{10.5} M_{\odot}$), UV-bright ($-23 \lesssim M_{\rm UV} \lesssim -20$), ultra-compact ($R_{\rm eff} < 300$ pc) stellar cores with extreme circular velocity ($V_{\rm circ} > 500$ km s$^{-1}$) and consistent with several LRD properties: strong Balmer breaks ($F_ν(4200{\rm Å})/F_ν(3500{\rm Å}) \sim 2$); blue UV beta slopes ($β_{\rm UV} \approx -1.25$); dust masses; ALMA non-detections; and Balmer-line widths up to $\sim 1500$ km s$^{-1}$ broadened by galaxy-scale dynamics. However, stellar emission and host-galaxy kinematics alone do not reproduce the red rest-optical continuum, more extreme Balmer breaks ($\gtrsim 2.5$) and line widths ($\gtrsim 2000$ km s$^{-1}$), or the broad-Balmer/narrow-forbidden-line signature of broad-line AGN. The same ultra-compact conditions efficiently fuel central BHs, suggesting a hybrid stellar+AGN scenario in which compact stars explain the UV continuum, Balmer break, and intermediate line widths while AGN supply the red optical continuum and more extreme line properties. With halo masses $M_{\rm halo} \sim 10^{11-12.5} M_\odot$ and comoving abundance $\sim 2 \times 10^{-5} {\rm cMpc}^{-3}$ (for $\sim 20\%$ duty-cycle at $z \approx 4-8$), ultra-compact galaxies can contribute to the massive, bright LRD population. △ Less

Submitted 22 June, 2026; originally announced June 2026.

Comments: 24 pages, 11 figures. Comments most welcome

Unveiling the population of massive quenched galaxies at $z\ge2$ in the COLIBRE simulations -- II. The role of AGN feedback and environment on their emergence

Authors: Ángel Chandro-Gómez, Claudia del P. Lagos, Chris Power, Willian M. Baker, Alejandro Benítez-Llambay, Evgenii Chaikin, Harry G. Chittenden, Camila Correa, Carlos S. Frenk, Filip Huško, Kei Ito, Robert J. McGibbon, Themiya Nanayakkara, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, Joop Schaye, James W. Trayford, Francesco Valentino

Abstract: Early ($z \gtrsim 2$) Massive ($M_{\star} \gtrsim 10^{10}\,\mathrm{M_{\odot}}$), Quenched Galaxies (MQGs) challenge current galaxy formation models. In this series, we study these systems using the new COLIBRE cosmological hydrodynamical simulations. Following the broad agreement between its predictions and observations found in the first paper, this second paper explores the processes driving gal… ▽ More Early ($z \gtrsim 2$) Massive ($M_{\star} \gtrsim 10^{10}\,\mathrm{M_{\odot}}$), Quenched Galaxies (MQGs) challenge current galaxy formation models. In this series, we study these systems using the new COLIBRE cosmological hydrodynamical simulations. Following the broad agreement between its predictions and observations found in the first paper, this second paper explores the processes driving galaxies to become massive and quenched in COLIBRE, identifying Active Galactic Nucleus (AGN) feedback as the primary quenching mechanism in both the thermal (L200m6 simulation) and hybrid (thermal+jet, L200m7h simulation) AGN feedback models implemented. However, the two models behave differently: while the thermal model efficiently quenches massive galaxies at $z>3$, the hybrid model is less effective because black holes (BHs) grow more slowly in the early Universe, and the jet component, which dominates the feedback energy, acts on longer timescales to impact galaxies. Both models predict quasar-like MQGs (AGN with $L_{\rm bol}\gtrsim10^{45}\,\mathrm{erg\,s^{-1}}$), with the most luminous systems associated with more recently quenched galaxies. Compared to star-forming galaxies of similar mass, MQGs host more massive BHs and exhibit higher star formation efficiencies. These differences arise primarily from their environments before quenching, particularly at local ($\rm 0.3\,cMpc$) to intermediate scales ($\rm 1.0\,cMpc$), where overdense regions are associated with enhanced gas inflows, higher BH accretion and, hence, feedback power. We find that about $54\%$ ($20\%$) of the $z=3$ MQGs survive as the main progenitors of $z=0$ galaxies, although up to $56\%$ ($60\%$) experience rejuvenation episodes at $z<3$ in L200m6 (L200m7h). Our results highlight the central role of BH growth, AGN feedback and environment in driving rapid quenching in the early Universe. △ Less

Submitted 29 May, 2026; originally announced May 2026.

Comments: Submitted for publication in MNRAS. This work is Part II of a two-paper series (Part I: arXiv:2512.16208). 23 pages (17 of main body and 6 of appendices). Comments welcome

The galaxy ultraviolet luminosity function from $z=7$ to $15$ in the COLIBRE simulations

Authors: Shengdong Lu, Carlos S. Frenk, Cedric G. Lacey, Andrea Gebek, Joop Schaye, Shaun Cole, Sownak Bose, Anna Durrant, Nick Andreadis, Maarten Baes, Alejandro Benítez-Llambay, Evgenii Chaikin, Camila Correa, Robert A. Crain, Filip Huško, Robert J. McGibbon, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, James W. Trayford

Abstract: JWST has enabled the detection of galaxies in the earliest stages of cosmic history. We compare the ultraviolet luminosity functions (UVLFs) at redshifts $z=7-15$ predicted by the new cosmological hydrodynamics simulations, COLIBRE with observations, including those from JWST. The UV luminosities of COLIBRE galaxies are derived using the radiative transfer code SKIRT, which tracks stellar emission… ▽ More JWST has enabled the detection of galaxies in the earliest stages of cosmic history. We compare the ultraviolet luminosity functions (UVLFs) at redshifts $z=7-15$ predicted by the new cosmological hydrodynamics simulations, COLIBRE with observations, including those from JWST. The UV luminosities of COLIBRE galaxies are derived using the radiative transfer code SKIRT, which tracks stellar emission and its processing through the multi-phase interstellar medium and dust distribution predicted by COLIBRE. We find that although COLIBRE is consistent with the observed evolution of the stellar mass function up to $z=12$, its dust-attenuated UVLFs fall systematically below the observations at the bright end: at the number density of $10^{-6}\,\mathrm{Mpc^{-3}\,mag^{-1}}$, the brightest galaxies are underluminous by $\approx 1\,\rm mag$ at $z=7$, increasing to $\approx 2.5\,\rm mag$ at $z=15$. Accounting for observational uncertainties brings the COLIBRE UVLFs closer to the observational data, but does not fully resolve the discrepancy. Ignoring dust attenuation allows COLIBRE to produce sufficiently bright galaxies at $7\lesssim z \lesssim 12$, while at $z=15$, COLIBRE still underpredicts the luminosities of the brightest galaxies, indicating the need for additional physical mechanisms to boost the UV luminosities at the earliest cosmic epochs, such as a ''top-heavy'' stellar initial mass function. We fit the COLIBRE UVLFs with Schechter functions and calculate the evolution of the best-fit parameters. We find that the galaxy number density decreases, the characteristic luminosity becomes fainter and the faint-end slope becomes steeper towards higher redshifts. The UV luminosity density decreases by a factor of $\approx 300$ from $z = 7$ to $z = 15$. △ Less

Submitted 7 May, 2026; originally announced May 2026.

Comments: 28 pages, 16 figures; submitted to MNRAS

Galaxy luminosity functions from far-UV to submillimetre at $z=0$ in the COLIBRE simulations

Authors: Shengdong Lu, Carlos S. Frenk, Cedric G. Lacey, Andrea Gebek, Joop Schaye, Shaun Cole, Sownak Bose, Nick Andreadis, Maarten Baes, Alejandro Benítez-Llambay, Evgenii Chaikin, Robert A. Crain, Anna Durrant, Filip Huško, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, James W. Trayford

Abstract: We present predictions from the recent COLIBRE cosmological hydrodynamical simulations of galaxy formation for the present-day galaxy luminosity functions (LFs) at wavelengths ranging from the far-ultraviolet (FUV) to the submillimetre. The simulations are post-processed with the radiative transfer code SKIRT, accounting for dust attenuation and emission using the distribution and properties of du… ▽ More We present predictions from the recent COLIBRE cosmological hydrodynamical simulations of galaxy formation for the present-day galaxy luminosity functions (LFs) at wavelengths ranging from the far-ultraviolet (FUV) to the submillimetre. The simulations are post-processed with the radiative transfer code SKIRT, accounting for dust attenuation and emission using the distribution and properties of dust grains predicted directly by COLIBRE. Results from simulations varying in mass resolution by a factor of $\sim 10^2$ ($\sim 10^5 - 10^7\,\mathrm{M_{\odot}}$) show very good convergence over most luminosity ranges. The COLIBRE-SKIRT LFs match the data remarkably well from the FUV to the near-infrared ($3.4\,\mathrm{μm}$) and also in the far-infrared and submillimetre wavelength range ($70-850\,\mathrm{μm}$). In the mid-infrared (MIR; $8-24\,\mathrm{μm}$), COLIBRE-SKIRT matches the data well at low luminosities but significantly underpredicts the luminosities of MIR-bright galaxies, with the discrepancy increasing towards longer wavelengths. The total infrared LF, obtained by integrating the spectral energy distributions over $8-1000\,\mathrm{μm}$, also matches observations well at the faint end but underpredicts the number of very bright galaxies. The unprecedented agreement at all other wavelengths indicates that COLIBRE, coupled with this calibration-free SKIRT post-processing framework, successfully predicts the properties of stellar populations at the present day and the amount and distribution of interstellar dust. △ Less

Submitted 3 May, 2026; originally announced May 2026.

Comments: 25 pages, 16 figures. Submitted to MNRAS

The morphologies of present-day galaxies in the COLIBRE simulations

Authors: Victor J. Forouhar Moreno, Joop Schaye, Matthieu Schaller, Aaron Ludlow, Robert J. McGibbon, Alejandro Benítez-Llambay, Evgenii Chaikin, Carlos S. Frenk, Filip Huško, Sylvia Ploeckinger, Alexander J. Richings, James W. Trayford

Abstract: The diversity of galaxy morphologies and their relations with galaxy and halo properties is fundamental to understanding galaxy formation. Cosmological simulations of representative volumes can help disentangle the origin of observed correlations, but most suffer from two main limitations that affect morphologies: an over-pressurised interstellar medium and spurious interactions between stellar an… ▽ More The diversity of galaxy morphologies and their relations with galaxy and halo properties is fundamental to understanding galaxy formation. Cosmological simulations of representative volumes can help disentangle the origin of observed correlations, but most suffer from two main limitations that affect morphologies: an over-pressurised interstellar medium and spurious interactions between stellar and dark matter particles. We present an overview of galaxy morphologies in the COLIBRE simulations, which address these limitations and reproduce many observed galaxy scaling relations. To quantify galaxy morphology, we use four (strongly-correlated) theory-space metrics, three kinematic and one spatial. We explore how different choices and limitations affect these indicators, including luminosity- versus mass-weighting, aperture size and shot noise. Overall, we find good convergence in present-day morphologies across two orders of magnitude in mass resolution. COLIBRE predicts that kinematic morphology correlates strongly with stellar mass and colour, and that galaxies with stellar masses of $\approx(1-2)\times 10^{10}\,\mathrm{M}_{\odot}$ tend to be the most rotationally-dominated. At fixed stellar mass, the morphology of central galaxies correlates weakly with the properties of their host halo. Morphology correlates more strongly with internal galaxy properties, with more disky galaxies being more gas-rich, having higher star formation rates and exhibiting younger and more extended stellar populations. Other properties, like the mass of the most massive black hole, the fraction of stars that are accreted and stellar metallicity, also correlate with morphology, but with correlation strengths sensitive to the stellar mass of the galaxy and whether it is a central or satellite. △ Less

Submitted 26 July, 2026; v1 submitted 3 April, 2026; originally announced April 2026.

Comments: Accepted for publication in MNRAS. 23 main text pages (13 figures) and 5 appendix pages (6 figures)

The tidal evolution of satellite galaxies in cosmological simulations: insights from COLIBRE

Authors: Feihong He, Jiaxin Han, Joop Schaye, Wenting Wang, Zhaozhou Li, Sylvia Ploeckinger, Evgenii Chaikin, Robert J. McGibbon, Filip Huško, Matthieu Schaller, Alejandro Benítez-Llambay, Alexander J. Richings, James W. Trayford, Carlos S. Frenk, Fangzhou Jiang

Abstract: We investigate the co-evolution of the stellar and dark matter mass of satellite galaxies using the COLIBRE cosmological hydrodynamical simulations with subhaloes resolved by the history-based HBT-HERONS subhalo finder. We identify a universal tidal track connecting stellar mass loss to subhalo mass loss characterized by two distinct phases, which can be well described by the two-parameter model.… ▽ More We investigate the co-evolution of the stellar and dark matter mass of satellite galaxies using the COLIBRE cosmological hydrodynamical simulations with subhaloes resolved by the history-based HBT-HERONS subhalo finder. We identify a universal tidal track connecting stellar mass loss to subhalo mass loss characterized by two distinct phases, which can be well described by the two-parameter model. The initial phase consists primarily of dark matter stripping, whereas stellar stripping becomes significant only after the subhalo bound mass fraction drops below a critical value ($\sim 0.057$). We find a bimodal mass loss rate distribution of subhaloes. In satellites with modest mass loss rates, the stellar mass is largely frozen. By contrast, the galaxy quickly becomes unresolved, along with the dark matter component for the extreme-mass-loss population, naturally explaining the lack of ``orphan'' galaxies in previous hydrodynamical simulations. Our model also predicts the formation condition for dark-matter-deficient galaxies (DMDGs), whose abundance peaks at $m_{*}\sim 10^{9.5}\,\rm{M}_{\odot}$. The abundance of DMDGs can be very sensitive to numerical effects, with COLIBRE resolving a much larger DMDG population than previous hydrodynamical simulations. We also estimate the influence of artificial disruption on the satellite stellar mass function, which can amount to 20 (50) per cent at $m_* \sim 10^{9} (10^{8}) \, \rm M_\odot$, given a baryonic mass resolution of $\sim 10^{6}\,\rm{M}_{\odot}$. △ Less

Submitted 20 July, 2026; v1 submitted 3 April, 2026; originally announced April 2026.

Comments: accepted by MNRAS

Journal ref: Mon Not R Astron Soc (2026)

A subgrid model for chemical enrichment in cosmological simulations

Authors: Camila A. Correa, Joop Schaye, Matthieu Schaller, James W. Trayford, Evgenii Chaikin, Alejandro Benitez-Llambay, Carlos S. Frenk, Sylvia Ploeckinger, Alexander J. Richings

Abstract: We present the modules for stellar nucleosynthesis, stellar mass loss, and turbulent diffusion of the new COLIBRE subgrid model for cosmological hydrodynamical simulations of galaxy formation. COLIBRE models the thermal evolution of the multi-phase interstellar medium, dust grains, star formation, and stellar and AGN feedback. This work focuses on the model for chemical enrichment. We track the ev… ▽ More We present the modules for stellar nucleosynthesis, stellar mass loss, and turbulent diffusion of the new COLIBRE subgrid model for cosmological hydrodynamical simulations of galaxy formation. COLIBRE models the thermal evolution of the multi-phase interstellar medium, dust grains, star formation, and stellar and AGN feedback. This work focuses on the model for chemical enrichment. We track the evolution of 12 chemical elements produced by a broad range of nucleosynthetic channels, including core-collapse supernovae and stellar winds, Type Ia supernovae, and asymptotic giant branch (AGB) stars. Enrichment from $s$- and $r$-process elements is modelled via contributions from AGB stars, neutron star mergers, common envelope supernovae, and collapsars. We present an updated compilation of stellar yields taken from the literature, which we release alongside this work. Small-scale element mixing is implemented through a turbulent diffusion process. While diffusion has only a minimal impact on basic integrated galaxy properties, it does reduce the slope of the gas-phase metallicity-mass relation compared with simulations that do not include it. The distribution of element ratios of individual stellar particles is sensitive to diffusion, but only at low metallicities ($Z \lesssim 10^{-1}\,\rm{Z}_\odot$). The model is tested using redshift $z=0$ results from a set of cosmological simulations, mostly of (25 Mpc)$^3$ volumes, demonstrating generally good agreement with Milky Way stellar abundance trends from the APOGEE survey. The model also reproduces the alpha-element enhancement relations observed in galaxies from SDSS, ATLAS-3D, and the Local Group. △ Less

Submitted 1 April, 2026; originally announced April 2026.

Comments: Accepted for publication in MNRAS

The evolution of the sizes and angular momentum content of galaxies in the COLIBRE simulations

Authors: Aaron D. Ludlow, Katy L. Proctor, Joop Schaye, Filip Huško, Victor J. Forouhar Moreno, Danail Obreschkow, Evgenii Chaikin, Matthieu Schaller, Sylvia Ploeckinger, Alejandro Benítez-Llambay, Kyle A. Oman, Robert J. McGibbon, James W. Trayford, Carlos S. Frenk, Alexander J. Richings

Abstract: We analyse the sizes and specific angular momentum content of galaxies in the Colibre cosmological hydrodynamical simulations spanning two orders of magnitude in mass resolution. We compare the predicted size-mass and angular momentum-mass relations to a broad range of observational measurements spanning redshifts $z=0$ to $4$. At $z=0$, Colibre reproduces observed size-mass relations over the sam… ▽ More We analyse the sizes and specific angular momentum content of galaxies in the Colibre cosmological hydrodynamical simulations spanning two orders of magnitude in mass resolution. We compare the predicted size-mass and angular momentum-mass relations to a broad range of observational measurements spanning redshifts $z=0$ to $4$. At $z=0$, Colibre reproduces observed size-mass relations over the sampled mass range $10^8 \lesssim M_\star/{\rm M_\odot}\lesssim 10^{11.5}$, and for multiple size definitions, including two- and three-dimensional stellar half-mass radii, half-light radii across several wavelengths, as well as alternative measures such as baryonic half-mass radii and characteristic radii defined by stellar surface density thresholds. The simulations also recover the observed segregation of galaxies in the size-mass plane by morphological type and star formation rate, and reproduce the distinct, approximately parallel sequences followed by star-forming discs and quenched spheroids in the stellar specific angular momentum-mass plane. The angular momentum content of star-forming Colibre galaxies matches that of observed systems out to $z\approx 1.5$. At higher redshifts, massive galaxies ($ 10^{9.5}\lesssim M_\star/{\rm M_\odot}\lesssim 10^{11}$) in the simulations are somewhat smaller than observed, and the separation between star-forming and passive populations in the size-mass plane is reduced relative to observations, while at lower masses the agreement remains good. This apparent discrepancy may reflect the effects of dust attenuation, which is neglected in our analysis and may preferentially obscure the central regions of observed systems. Overall, our findings highlight the close connection between galaxy size, angular momentum, and morphology over cosmic time. △ Less

Submitted 27 July, 2026; v1 submitted 27 March, 2026; originally announced March 2026.

Comments: 23 pages, 20 figures

Journal ref: Mon Not R Astron Soc (2026)

Unveiling the population of massive quenched galaxies at $z\ge2$ in the COLIBRE simulations - I. Galaxy demographics

Authors: Ángel Chandro-Gómez, Claudia del P. Lagos, Chris Power, William M. Baker, Alejandro Benítez-Llambay, Evgenii Chaikin, Harry G. Chittenden, Camila Correa, Carlos S. Frenk, Filip Huško, Robert J. McGibbon, Themiya Nanayakkara, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, Joop Schaye, James W. Trayford

Abstract: The James Webb Space Telescope has uncovered a substantial population of Massive ($M_{\star} > 10^{10}\,\mathrm{M_{\odot}}$), Quenched Galaxies (MQGs) in the early Universe ($z \ge 2$), whose properties challenge current galaxy formation models. In this series, we examine this population of MQGs within the new COLIBRE cosmological hydrodynamical simulations, which introduce key innovations in thei… ▽ More The James Webb Space Telescope has uncovered a substantial population of Massive ($M_{\star} > 10^{10}\,\mathrm{M_{\odot}}$), Quenched Galaxies (MQGs) in the early Universe ($z \ge 2$), whose properties challenge current galaxy formation models. In this series, we examine this population of MQGs within the new COLIBRE cosmological hydrodynamical simulations, which introduce key innovations in their sub-grid physics. In this first paper, we find a dependence of MQG number densities on both mass resolution and the Active Galactic Nucleus feedback implementation, as well as a significant impact from potential observational uncertainties. Using the fiducial $(200\,\rm cMpc)^3$ volume L200m6 simulation, which provides adequate volume, mass and spatial resolution to study these systems, we report number densities and stellar mass functions in broad agreement with the latest observations. The predicted quenching and formation timescales are qualitatively consistent with observational inferences, indicating extended formation (medians $t_{50}\approx0.5-1.5\,\mathrm{Gyr}$) followed by rapid quenching (medians $t_{\mathrm{q}}\lesssim0.6\,\mathrm{Gyr}$) with strong starburst episodes. Leveraging the state-of-the-art physics in COLIBRE, the model predicts that MQGs have dust and $\rm H_{2}$ fractions more than $1$~dex lower than their massive star-forming counterparts; generally consistent with the (scarce) observational estimates. MQGs and massive star-forming systems show broadly similar stellar sizes and kinematics, suggesting that size or morphological transformations occur after quenching in COLIBRE. Our results provide robust predictions for MQGs and show that tensions with observations are reduced when an effective observational uncertainty is forward-modelled. △ Less

Submitted 15 August, 2026; v1 submitted 18 December, 2025; originally announced December 2025.

Comments: Accepted for publication in MNRAS (after minor changes). This work is Part I of a two-paper series (Part II: arXiv:2605.31052). 25 pages (18 of main body and 7 of appendices)

Kennicutt-Schmidt relation of galaxies over 13 billion years in the COLIBRE hydrodynamical simulations

Authors: Claudia del P. Lagos, Joop Schaye, Matthieu Schaller, Danail Obreschkow, Yannick M. Bahe, Alejandro Benitez-Llambay, Evgenii Chaikin, Camila Correa, Timothy A. Davis, Carlos S. Frenk, Filip Husko, Melanie Kaasinen, Robert J. McGibbon, Kyle Oman, Sylvia Ploeckinger, Alexander J. Richings, James W. Trayford, Jing Wang, Ruby J. Wright

Abstract: We investigate the correlation between star formation rate (SFR) surface density and gas surface density (known as the Kennicutt-Schmidt, KS, relation) at kiloparsec (kpc) scales across cosmic time ($0\le z \le 8$) for galaxies with stellar masses $>10^9\,\rm M_{\odot}$, using the COLIBRE state-of-the-art cosmological hydrodynamical simulations. These simulations feature on-the-fly non-equilibrium… ▽ More We investigate the correlation between star formation rate (SFR) surface density and gas surface density (known as the Kennicutt-Schmidt, KS, relation) at kiloparsec (kpc) scales across cosmic time ($0\le z \le 8$) for galaxies with stellar masses $>10^9\,\rm M_{\odot}$, using the COLIBRE state-of-the-art cosmological hydrodynamical simulations. These simulations feature on-the-fly non-equilibrium chemistry coupled to dust grain evolution and detailed radiative cooling down to $\approx 10$~K, enabling direct predictions for the atomic (HI) and molecular (H$_2$) KS relations. At $z\approx 0$, COLIBRE reproduces the observed (spatially-resolved) KS relations for HI and H$_2$, including the associated scatter, which we predict to be significantly correlated with stellar surface density, local specific SFR (sSFR), and gas metallicity. We show that the HI KS relation steepens for lower-mass galaxies, while the H$_2$ KS relation shifts to higher normalisation in galaxies with higher sSFRs. The H$_2$ depletion time decreases by a factor of $\approx 20$ from $z = 0$ to $z = 8$, primarily due to the decreasing gas-phase metallicity. This results in less H$_2$ and more HI being associated with a given SFR at higher redshift. We also find that galaxies with higher sSFRs have a larger molecular gas content and higher star formation efficiency per unit gas mass on kpc scales. The predicted evolution of the H$_2$ depletion time and its correlation with a galaxy's sSFR agree remarkably well with observations in a wide redshift range, $0\le z\le 5$. △ Less

Submitted 19 May, 2026; v1 submitted 12 December, 2025; originally announced December 2025.

Comments: 27 pages; 19 main body of the paper. Accepted for publication MNRAS after minor revisions

Journal ref: Mon Not R Astron Soc (2026)

Dynamical disequilibrium in dwarf galaxies: rethinking gas dynamics, rotation curves, and dark matter inference

Authors: Diego Dado, Kyle A. Oman, Katherine E. Harborne, Francesca Fragkoudi, Joop Schaye, Matthieu Schaller, Alejandro Benítez-Llambay, Evgenii Chaikin, Carlos S. Frenk, Filip Huško, Sylvia Ploeckinger, Alexander J. Richings

Abstract: We quantify departures from hydrodynamical and centrifugal equilibrium in the gas discs of low-mass ($10^{10.75}<M_\mathrm{200c}/\mathrm{M}_\odot<10^{11}$) galaxies from the COLIBRE cosmological hydrodynamical simulations. We evaluate the full Eulerian acceleration balance in the midplane and show that disequilibrium is widespread: equilibrium-based circular velocity estimates typically have error… ▽ More We quantify departures from hydrodynamical and centrifugal equilibrium in the gas discs of low-mass ($10^{10.75}<M_\mathrm{200c}/\mathrm{M}_\odot<10^{11}$) galaxies from the COLIBRE cosmological hydrodynamical simulations. We evaluate the full Eulerian acceleration balance in the midplane and show that disequilibrium is widespread: equilibrium-based circular velocity estimates typically have errors of $\geq 10$ per cent ($\approx 75$ per cent of midplane gas by mass). Disequilibrium is strongest and the largest associated errors occur in the inner few kiloparsecs that are crucial for constraining the dark matter density profile. Correcting the circular velocity to account for pressure and convective terms does not reliably improve its recovery in strongly perturbed systems where time-dependent forces dominate the residual acceleration budget. Stellar feedback, self-gravitating gas clumps and AGN energy injection account for most strong local perturbations, and large-scale gravitational asymmetries act as a scaffold for disequilibrium. We classify gas discs into coherent, perturbed, and slow/erratic rotators and show that this classification correlates with galaxy properties like mass, morphology and tracers of recent feedback. A majority of galaxies in our sample would be unsuitable for standard rotation curve analysis. Much of the observed diversity in the shapes of dwarf galaxy rotation curves may stem from non-equilibrium gas motions rather than diversity in mass profiles - resolving the discrepancy is then first and foremost a problem in gas dynamics. △ Less

Submitted 13 March, 2026; v1 submitted 11 December, 2025; originally announced December 2025.

Comments: MNRAS accepted version

Journal ref: Mon Not R Astron Soc (2026)

Gravitational potential drives the concentration dependence of the stellar mass-halo mass relation

Authors: Kai Wang, Joop Schaye, Alejandro Benítez-Llambay, Evgenii Chaikin, Carlos S. Frenk, Filip Huško, Robert J. McGibbon, Sylvia Ploeckinger, Alexander J. Richings, Matthieu Schaller, James W. Trayford

Abstract: We investigate the origin of the scatter in the stellar mass-halo mass (SMHM) relation using the \colibre cosmological hydrodynamical simulations. At fixed halo mass, we find a clear positive correlation between stellar mass and halo concentration, particularly in low-mass haloes between $10^{11}$ and $10^{12}\,\rm M_\odot$, where all halo properties are computed from the corresponding dark-matter… ▽ More We investigate the origin of the scatter in the stellar mass-halo mass (SMHM) relation using the \colibre cosmological hydrodynamical simulations. At fixed halo mass, we find a clear positive correlation between stellar mass and halo concentration, particularly in low-mass haloes between $10^{11}$ and $10^{12}\,\rm M_\odot$, where all halo properties are computed from the corresponding dark-matter-only simulation. Two scenarios have been proposed to explain this trend: the earlier formation of higher-concentration haloes allows more time for star formation, or the deeper gravitational potential wells of higher-concentration haloes enhance baryon retention. To distinguish between them, we examine correlations between halo concentration, stellar mass, stellar age, and stellar metallicity. While, at fixed halo mass, halo concentration correlates with stellar age, stellar age itself shows only a weak correlation with stellar mass, indicating that early formation alone cannot account for the concentration-dependence in the scatter of the SMHM relation. In contrast, both stellar metallicity and halo concentration exhibit correlations with stellar mass. The connection between halo concentration and stellar metallicity persists even when simultaneously controlling for both halo mass and stellar mass. These results support the scenario in which the deeper gravitational potentials in higher-concentration haloes suppress feedback-driven outflows, thereby enhancing both baryon and metal retention. △ Less

Submitted 13 August, 2026; v1 submitted 25 November, 2025; originally announced November 2025.

Comments: 9 pages, 3 + 1 figures, published on MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 546, Issue 2, February 2026, stag110

The role of galactic winds fueling central starbursts and quasars in the FIRE cosmological simulations

Authors: Jonathan Mercedes-Feliz, Daniel Anglés-Alcázar, Boon Kiat Oh, Rachel K. Cochrane, Sarah Wellons, Alexander J. Richings, Jorge Moreno, Claude-André Faucher-Giguère, Philip F. Hopkins, Dušan Kereš

Abstract: Central starbursts and Active Galactic Nuclei (AGN) are thought to be fueled by either galaxy interactions or secular processes in gravitationally unstable discs. We employ cosmological hydrodynamic simulations from the Feedback in Realistic Environments (FIRE) project to propose a new nuclear fueling scenario based on the transition that galaxies undergo from bursty to smooth star formation and f… ▽ More Central starbursts and Active Galactic Nuclei (AGN) are thought to be fueled by either galaxy interactions or secular processes in gravitationally unstable discs. We employ cosmological hydrodynamic simulations from the Feedback in Realistic Environments (FIRE) project to propose a new nuclear fueling scenario based on the transition that galaxies undergo from bursty to smooth star formation and from prominent global galactic winds to inefficient stellar feedback as they grow above $M_{\star}\sim 10^{10-10.5}\,{\rm M}_{\odot}$: the last major galactic wind event shuts down star formation, evacuates gas from the galaxy, and slows down gas accretion from the circumgalactic medium (CGM), creating a $\sim$$10^{10}\,{\rm M}_{\odot}$ pileup of gas in the inner CGM which later accretes coherently onto the galaxy, achieving a tenfold increase in inflow rate over pre-outflow conditions. We explicitly track the accumulation of gas along the outflow pathway owing to hydrodynamic interactions and show that $\sim$50% of gas fueling the central $\sim$10-100$\,{\rm pc}$ over the subsequent $\sim$15$\,{\rm Myr}$ can be traced back to pileup gas having experienced $>$50% change in infall velocity owing to the wind interaction. This galactic wind pileup effect may thus represent a significant fueling mode for compact starbursts and luminous AGN. Galactic winds at earlier times or AGN-driven outflows can have qualitatively similar effects, but the pileup of gas driven by the last major galactic wind event refuels the galaxy precisely when the deepening stellar potential prevents further gas evacuation by stellar feedback, providing the ideal conditions for quasar fueling at the time when AGN feedback is most needed to regulate central star formation in massive galaxies at their peak of activity. △ Less

Submitted 22 October, 2025; originally announced October 2025.

Comments: 19 pages, 11 figures. Submitted to MNRAS

Non-explosive pre-supernova feedback in the COLIBRE model of galaxy formation

Authors: Alejandro Benítez-Llambay, Sylvia Ploeckinger, Joop Schaye, Alexander J. Richings, Evgenii Chaikin, Matthieu Schaller, James W. Trayford, Carlos S. Frenk, Filip Huško, Camila Correa

Abstract: We present the implementation and testing of a subgrid non-explosive pre-supernova (NEPS) feedback module for the COLIBRE model of galaxy formation. The NEPS module incorporates three key physical processes sourced by young, massive stars that act immediately following star formation: momentum injection from stellar winds and radiation pressure, and thermal energy from photoheating in HII regions.… ▽ More We present the implementation and testing of a subgrid non-explosive pre-supernova (NEPS) feedback module for the COLIBRE model of galaxy formation. The NEPS module incorporates three key physical processes sourced by young, massive stars that act immediately following star formation: momentum injection from stellar winds and radiation pressure, and thermal energy from photoheating in HII regions. The age- and metallicity-dependent energy and momentum budgets are derived from BPASS stellar population models and are coupled self-consistently to the local gas properties. We test the model using a suite of smoothed particle hydrodynamics simulations of isolated, unstable gaseous disks at various numerical resolutions (gas particle masses in the range $10^4-10^6$ $\rm M_{\odot}$). We find that the NEPS module successfully regulates star formation by providing pressure support that prevents catastrophic gas collapse. This regulation improves the numerical convergence of star formation rates and disk structure. In our model, feedback from HII regions is the dominant regulatory mechanism. Furthermore, we demonstrate a crucial synergy with subsequent supernova feedback; NEPS feedback pre-processes the interstellar medium, creating a more homogeneous environment that moderates the effect of explosive feedback from supernova events. Our NEPS module thus provides a physically motivated and numerically robust framework that mitigates resolution-dependent artefacts and promotes self-regulated galaxy growth. △ Less

Submitted 10 February, 2026; v1 submitted 29 September, 2025; originally announced September 2025.

Comments: Accepted for publication in MNRAS

Journal ref: Mon Not R Astron Soc (2026)

Emission line tracers of galactic outflows driven by stellar feedback in simulations of isolated disk galaxies

Authors: Elliot L. Howatson, Alexander J. Richings, Elke Roediger, Claude-Andre Faucher-Giguere, Tom Theuns, Yuankang Liu, Tsang Keung Chan, Oliver Thompson, Cody Carr, Daniel Angles-Alcazar

Abstract: Hydrodynamic simulations can connect outflow observables to the physical conditions of outflowing gas. Here, we use simulations of isolated disk galaxies ranging from dwarf mass ($M_{200} = 10^{10}\mathrm{M}_{\odot}$) to Milky Way mass ($M_{200} = 10^{12}\mathrm{M}_{\odot}$), based on the FIRE-2 subgrid models to investigate multiphase galactic outflows. We use the CHIMES non-equilibrium chemistry… ▽ More Hydrodynamic simulations can connect outflow observables to the physical conditions of outflowing gas. Here, we use simulations of isolated disk galaxies ranging from dwarf mass ($M_{200} = 10^{10}\mathrm{M}_{\odot}$) to Milky Way mass ($M_{200} = 10^{12}\mathrm{M}_{\odot}$), based on the FIRE-2 subgrid models to investigate multiphase galactic outflows. We use the CHIMES non-equilibrium chemistry module to create synthetic spectra of common outflow tracers ([CII]$_{158\rm{μm}}$, $\mathrm{CO}_{J(1-0)}$, H$α$ and $[\mathrm{OIII}]_{5007\text{A}}$). Using our synthetic spectra we measure the mass outflow rate, kinetic power and momentum flux using observational techniques. In [CII]$_{158\rm{μm}}$ we measure outflow rates of $10^{-4}$ to $1$ $\mathrm{M_{\odot}yr^{-1}}$ across an SFR range of $10^{-3}$ to $1$ $\text{M}_{\odot}\text{yr}^{-1}$, which is in reasonable agreement with observations. The significant discrepancy is in $\mathrm{CO}_{J(1-0)}$, with the simulations lying $\approx1$ dex below the observational sample. We test observational assumptions used to derive outflow properties from synthetic spectra. We find the greatest uncertainty lies in measurements of electron density, as estimates using the SII doublet can overestimate the actual electron density by up to 2 dex, which changes mass outflow rates by up to 4 dex. We also find that molecular outflows are especially sensitive to the conversion factor between CO luminosity and H2 mass, with outflow rates changing by up to 4 dex in our least massive galaxy. Comparing the outflow properties derived from the synthetic spectra to those derived directly from the simulation, we find that [CII]$_{158\rm{μm}}$ probes outflows at greater distances from the disk, whilst we find that molecular gas does not survive at large distances within outflows within our modestly star-forming disk galaxies simulated in this work. △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 19 pages, 14 figures. Accepted for publication in MNRAS

The HyLight model for hydrogen emission lines in simulated nebulae

Authors: Yuankang Liu, Tom Theuns, Tsang Keung Chan, Alexander J. Richings, Anna F. McLeod

Abstract: Hydrogen recombination lines provide key diagnostics of ionized gas in galaxies, yet most hydrodynamical simulations estimate hydrogen level populations using interpolated emissivity tables rather than computing them directly from local physical conditions. We present HyLight, a Python-based atomic model that calculates hydrogen level populations and line emissivities from the gas density, tempera… ▽ More Hydrogen recombination lines provide key diagnostics of ionized gas in galaxies, yet most hydrodynamical simulations estimate hydrogen level populations using interpolated emissivity tables rather than computing them directly from local physical conditions. We present HyLight, a Python-based atomic model that calculates hydrogen level populations and line emissivities from the gas density, temperature, and ionization state, enabling accurate predictions in both equilibrium and non-equilibrium environments. Benchmark comparisons show that HyLight reproduces Cloudy predictions for Balmer, Paschen, and Brackett emissivities to within 1 per cent under typical photoionized nebular conditions, while discrepancies of several tens of per cent arise relative to other published calculations. As an illustrative application, we use HyLight to compute photoionization-to-line intensity ratios in an HII nebula and generate synthetic hydrogen emission maps from a radiation-hydrodynamical simulation that includes non-equilibrium thermochemistry. Combining physical consistency with flexibility, HyLight provides a robust framework for connecting hydrodynamical simulations with observational diagnostics of photoionized regions, and enhances our ability to interpret hydrogen emission in complex, non-equilibrium astrophysical environments. △ Less

Submitted 25 February, 2026; v1 submitted 24 September, 2025; originally announced September 2025.

Comments: 30 pages, 28 figures. Accepted for publication in MNRAS. The HyLight package is available at https://github.com/YuankangLiu/HyLight . It can also be accessed via PyPI at https://pypi.org/project/hylightpy/ . The input scripts for the Cloudy models used in this paper can be accessed at https://doi.org/10.5281/zenodo.16911175

Journal ref: Mon Not R Astron Soc (2026)

The evolution of the galaxy stellar mass function and star formation rates in the COLIBRE simulations from redshift 17 to 0

Authors: Evgenii Chaikin, Joop Schaye, Matthieu Schaller, Sylvia Ploeckinger, Alejandro Benítez-Llambay, Carlos S. Frenk, Filip Huško, Robert McGibbon, Alexander J. Richings, James W. Trayford

Abstract: We investigate the evolution of the galaxy stellar mass function (GSMF) and star formation rates (SFRs) across cosmic time in the COLIBRE simulations of galaxy formation. COLIBRE includes a multiphase interstellar medium, radiative cooling rates coupled to a model for the evolution of dust grains, and employs prescriptions for stellar and AGN feedback calibrated to reproduce the $z=0$ observed GSM… ▽ More We investigate the evolution of the galaxy stellar mass function (GSMF) and star formation rates (SFRs) across cosmic time in the COLIBRE simulations of galaxy formation. COLIBRE includes a multiphase interstellar medium, radiative cooling rates coupled to a model for the evolution of dust grains, and employs prescriptions for stellar and AGN feedback calibrated to reproduce the $z=0$ observed GSMF and stellar mass - size relation. We present the evolution of the GSMF from simulations at three resolutions: $m_{\rm gas}\approx m_{\rm dm}\sim 10^7$, $10^6$, and $10^5~\mathrm{M_\odot}$, in cosmological volumes of up to $400^3$, $200^3$, and $100^3$ cMpc$^3$, respectively. We demonstrate that COLIBRE is consistent with the observed GSMF over the full redshift range for which there are observations to compare with ($0<z<12$), with maximum systematic deviations of $\approx 0.3$ dex reached at $2<z<4$. We also examine the evolution of the star-forming main sequence, cosmic SFR density, stellar mass density, and galaxy quenched fraction, making predictions for both the fiducial COLIBRE model with thermally-driven AGN feedback and its variant with hybrid (thermal + kinetic jet) AGN feedback, and finding good agreement with observations. Notably, we show that COLIBRE matches the number density of massive quiescent galaxies at high redshifts reported by JWST, while predicting a stellar-to-halo mass relation that evolves little with redshift. We conclude that neither a redshift-dependent star formation efficiency, nor a variable stellar initial mass function, nor a deviation from $Λ\mathrm{CDM}$ is necessary to reproduce the high-redshift JWST stellar masses and SFRs. △ Less

Submitted 16 April, 2026; v1 submitted 9 September, 2025; originally announced September 2025.

Comments: 37 pages; accepted for publication in MNRAS; images, videos, and interactive visualizations are available on the COLIBRE website at https://colibre-simulations.org/

Journal ref: Mon Not R Astron Soc (2026)

A hybrid active galactic nucleus feedback model with spinning black holes, winds and jets

Authors: Filip Huško, Cedric G. Lacey, Joop Schaye, Matthieu Schaller, Evgenii Chaikin, Sylvia Ploeckinger, Alejandro Benítez Llambay, Alexander J. Richings, James W. Trayford

Abstract: We present a hybrid active galactic nucleus (AGN) feedback model that features three accretion disc states (the thick, thin, and slim discs at low, moderate, and super-Eddington accretion rates, respectively), and two feedback modes: thermal isotropic and kinetic jets. The model includes black hole (BH) spin evolution due to gas accretion, BH mergers, jet spindown, and Lense-Thirring torques. The… ▽ More We present a hybrid active galactic nucleus (AGN) feedback model that features three accretion disc states (the thick, thin, and slim discs at low, moderate, and super-Eddington accretion rates, respectively), and two feedback modes: thermal isotropic and kinetic jets. The model includes black hole (BH) spin evolution due to gas accretion, BH mergers, jet spindown, and Lense-Thirring torques. The BH spin determines the jet directions and affects the feedback efficiencies. The model is implemented in the SWIFT code and coupled with the COLIBRE galaxy formation model. We present the first results from hybrid AGN feedback simulations run as part of the COLIBRE suite, focusing on the impact of new parameters and calibration efforts. Using the new hybrid AGN feedback model, we find that AGN feedback affects not just massive galaxies, but all galaxies down to $M_*\approx10^8$ $\mathrm{M}_\odot$. BH spins are predicted to be near-maximal for intermediate-mass BHs ($M_\mathrm{BH}\in[10^6,10^8]$ $\mathrm{M}_\odot$), and lower for other BH masses. These trends are in good agreement with observations. The intergalactic medium is hotter and impacted on larger scales in the hybrid AGN feedback simulations compared to those using purely thermal feedback. In the hybrid AGN simulations, we predict that half of the cumulative injected AGN energy is in thermal and the other half in jet form, broadly independent of BH mass and redshift. Jet feedback is important at all redshifts and dominates over thermal feedback at $z<0.5$ and $z>1.5$, but only mildly. △ Less

Submitted 17 February, 2026; v1 submitted 5 September, 2025; originally announced September 2025.

Comments: Accepted for publication in MNRAS. Images, videos, and interactive visualizations are available on the COLIBRE website at https://colibre.strw.leidenuniv.nl/

COLIBRE: calibrating subgrid feedback in cosmological simulations that include a cold gas phase

Authors: Evgenii Chaikin, Joop Schaye, Matthieu Schaller, Sylvia Ploeckinger, Yannick M. Bahé, Alejandro Benítez-Llambay, Camila Correa, Victor J. Forouhar Moreno, Carlos S. Frenk, Filip Huško, Roi Kugel, Robert McGibbon, Alexander J. Richings, James W. Trayford, Josh Borrow, Robert A. Crain, John C. Helly, Cedric G. Lacey, Aaron Ludlow, Folkert S. J. Nobels

Abstract: We present the calibration of stellar and active galactic nucleus (AGN) feedback in the subgrid model for the new COLIBRE hydrodynamical simulations of galaxy formation. COLIBRE directly simulates the multi-phase interstellar medium and the evolution of dust grains, which is coupled to the chemistry. COLIBRE is calibrated at three resolutions: particle masses of… ▽ More We present the calibration of stellar and active galactic nucleus (AGN) feedback in the subgrid model for the new COLIBRE hydrodynamical simulations of galaxy formation. COLIBRE directly simulates the multi-phase interstellar medium and the evolution of dust grains, which is coupled to the chemistry. COLIBRE is calibrated at three resolutions: particle masses of $m_{\rm gas} \approx m_{\rm dm} \sim 10^7$ (m7), $10^6$ (m6), and $10^5~\mathrm{M_\odot}$ (m5). To calibrate the COLIBRE feedback at m7 resolution, we run Latin hypercubes of $\approx 200$ simulations that vary up to four subgrid parameters in cosmological volumes of ($50~\mathrm{cMpc}$)$^{3}$. We train Gaussian process emulators on these simulations to predict the $z=0$ galaxy stellar mass function (GSMF) and size - stellar mass relation (SSMR) as functions of the model parameters, which we then fit to observations. The trained emulators not only provide the best-fitting parameter values but also enable us to investigate how different aspects of the prescriptions for supernova and AGN feedback affect the predictions. In particular, we demonstrate that while the observed $z=0$ GSMF and SSMR can be matched individually with a relatively simple supernova feedback model, simultaneously reproducing both necessitates a more sophisticated prescription. We show that the calibrated m7 COLIBRE model not only reproduces the calibration target observables, but also matches various other galaxy properties to which the model was not calibrated. Finally, we apply the calibrated m7 model to the m6 and m5 resolutions and, after slight manual adjustments of the subgrid parameters, achieve a similar level of agreement with the observed $z=0$ GSMF and SSMR. △ Less

Submitted 25 February, 2026; v1 submitted 4 September, 2025; originally announced September 2025.

Comments: 41 pages, 20 figures (including appendices); accepted for publication in MNRAS; images, videos, and interactive visualizations are available on the COLIBRE website at https://colibre-simulations.org

Journal ref: Mon Not R Astron Soc (2026)

The COLIBRE project: cosmological hydrodynamical simulations of galaxy formation and evolution

Authors: Joop Schaye, Evgenii Chaikin, Matthieu Schaller, Sylvia Ploeckinger, Filip Huško, Rob McGibbon, James W. Trayford, Alejandro Benítez-Llambay, Camila Correa, Carlos S. Frenk, Alexander J. Richings, Victor J. Forouhar Moreno, Yannick M. Bahé, Josh Borrow, Anna Durrant, Andrea Gebek, John C. Helly, Adrian Jenkins, Cedric G. Lacey, Aaron Ludlow, Folkert S. J. Nobels

Abstract: We present the COLIBRE galaxy formation model and the COLIBRE suite of cosmological hydrodynamical simulations. COLIBRE includes new models for radiative cooling, dust grains, star formation, stellar mass loss, turbulent diffusion, pre-supernova stellar feedback, supernova feedback, supermassive black holes and active galactic nucleus (AGN) feedback. The multiphase interstellar medium is explicitl… ▽ More We present the COLIBRE galaxy formation model and the COLIBRE suite of cosmological hydrodynamical simulations. COLIBRE includes new models for radiative cooling, dust grains, star formation, stellar mass loss, turbulent diffusion, pre-supernova stellar feedback, supernova feedback, supermassive black holes and active galactic nucleus (AGN) feedback. The multiphase interstellar medium is explicitly modelled without a pressure floor. Hydrogen and helium are tracked in non-equilibrium, with their contributions to the free electron density included in metal-line cooling calculations. The chemical network is coupled to a dust model that tracks three grain species and two grain sizes. In addition to the fiducial thermally-driven AGN feedback, a subset of simulations uses black hole spin-dependent hybrid jet/thermal AGN feedback. To suppress spurious transfer of energy from dark matter to stars, dark matter is supersampled by a factor 4, yielding similar dark matter and baryonic particle masses. The subgrid feedback model is calibrated to match the observed $z \approx 0$ galaxy stellar mass function, galaxy sizes, and black hole masses in massive galaxies. The COLIBRE suite includes three resolutions, with particle masses of $\sim 10^5$, $10^6$, and $10^7\,\text{M}_\odot$ in cubic volumes of up to 100, 200, and 400 cMpc on a side, respectively. The largest runs use 136 billion ($5 \times 3008^3$) particles. We describe the model, assess its strengths and limitations, and present both visual impressions and quantitative results. Comparisons with various low-redshift galaxy observations generally show very good numerical convergence and excellent agreement with the data. △ Less

Submitted 22 February, 2026; v1 submitted 28 August, 2025; originally announced August 2025.

Comments: Accepted for publication in MNRAS. Differences w.r.t. v1: Added appendix comparing with EAGLE and TNG, various minor changes. For visualisations, see the COLIBRE website at http://colibre-simulations.org/

Journal ref: Mon Not R Astron Soc (2026)

SPARCS -- combining radiation hydrodynamics with non-equilibrium metal chemistry in the SWIFT astrophysical code

Authors: Tsang Keung Chan, Alexander J. Richings, Tom Theuns, Yuankang Liu, Matthieu Schaller, Mladen Ivkovic

Abstract: We present SPARCS, which combines the moment-based radiative transfer method SPH-M1RT with the non-equilibrium metal chemistry solver CHIMES in the modern highly-parallel astrophysical code SWIFT. SPARCS enables on-the-fly radiation hydrodynamics simulations, with multi-frequency ultraviolet radiative transfer coupled with all ionisation states of 11 major elements, in the presence of dust, cosmic… ▽ More We present SPARCS, which combines the moment-based radiative transfer method SPH-M1RT with the non-equilibrium metal chemistry solver CHIMES in the modern highly-parallel astrophysical code SWIFT. SPARCS enables on-the-fly radiation hydrodynamics simulations, with multi-frequency ultraviolet radiative transfer coupled with all ionisation states of 11 major elements, in the presence of dust, cosmic ray ionization and heating, and self-gravity. Direct radiation pressure on gas and dust is also accounted for. We validate SPARCS against analytic solutions and standard photo-ionization codes such as CLOUDY in idealized tests. As an example application, we simulate an ionization front propagating through an inhomogeneous interstellar medium with solar metallicity. We produce mock optical emission line observations with the level population calculation code HyLight and the diagnostic radiative transfer code RADMC3D. We find that non-equilibrium effects and inhomogeneities can boost the low ion fractions by up to an order of magnitude. Possible applications of SPARCS include studying the dynamical impact of radiation on gas in star-forming regions, and in the interstellar and circumgalactic medium, as well as interpreting line diagnostics in such environments, and galactic or AGN outflows. △ Less

Submitted 20 March, 2026; v1 submitted 18 August, 2025; originally announced August 2025.

Comments: 24 pages, 17 figures

Journal ref: Mon Not R Astron Soc (2026) Volume 546, Issue 2, February 2026, stag004

Hybrid-chimes: A model for radiative cooling and the abundances of ions and molecules in simulations of galaxy formation

Authors: Sylvia Ploeckinger, Alexander J. Richings, Joop Schaye, James W. Trayford, Matthieu Schaller, Evgenii Chaikin

Abstract: Radiative processes play a pivotal role in shaping the thermal and chemical states of gas across diverse astrophysical environments, from the interstellar medium (ISM) to the intergalactic medium. We present a hybrid cooling model for cosmological simulations that incorporates a comprehensive treatment of radiative processes, including parameterizations of the interstellar radiation field, cosmic… ▽ More Radiative processes play a pivotal role in shaping the thermal and chemical states of gas across diverse astrophysical environments, from the interstellar medium (ISM) to the intergalactic medium. We present a hybrid cooling model for cosmological simulations that incorporates a comprehensive treatment of radiative processes, including parameterizations of the interstellar radiation field, cosmic ray rates, and dust physics. The model uses the chimes chemical network and combines on-the-fly non-equilibrium calculations with quasi-equilibrium cooling rates. The quasi-equilibrium rates account for the time-dependent free electron fractions of elements tracked in non-equilibrium, balancing computational efficiency with physical accuracy. We evaluate the performance under various conditions, including the thermal evolution of primordial gas at the cosmic mean density, the properties of the warm and cold neutral media in Milky Way-like galaxies, and the atomic-to-molecular hydrogen transition. We demonstrate that thermal equilibrium predictions for the neutral phases of the ISM underestimate the median gas pressures in simulations of isolated galaxies by up to 0.5 dex. Finally we find that the atomic-to-molecular hydrogen transition is shifted to lower densities by up to 1 dex if oxygen is not included in the chemical network. Our work provides a robust framework for studying the multi-phase ISM and its role in galaxy formation and evolution. △ Less

Submitted 27 August, 2025; v1 submitted 18 June, 2025; originally announced June 2025.

Comments: 26 pages, 19 figures, accepted for publication in MNRAS; version matches the accepted manuscript

Modelling the evolution and influence of dust in cosmological simulations that include the cold phase of the interstellar medium

Authors: James W. Trayford, Joop Schaye, Camila Correa, Sylvia Ploeckinger, Alexander J. Richings, Evgenii Chaikin, Matthieu Schaller, Alejandro Benitez-Llambay, Carlos Frenk, Filip Husko

Abstract: While marginal in mass terms, dust grains play an outsized role in both the physics and observation of the interstellar medium (ISM). However, explicit modelling of this ISM constituent remains uncommon in large cosmological simulations. In this work, we present a model for the life-cycle of dust in the ISM that couples to the forthcoming COLIBRE galaxy formation model, which explicitly simulates… ▽ More While marginal in mass terms, dust grains play an outsized role in both the physics and observation of the interstellar medium (ISM). However, explicit modelling of this ISM constituent remains uncommon in large cosmological simulations. In this work, we present a model for the life-cycle of dust in the ISM that couples to the forthcoming COLIBRE galaxy formation model, which explicitly simulates the cold ISM. We follow 6 distinct grain types: 3 chemical species, including carbon and two silicate grains, with 2 size bins each. Our dust model accounts for seeding of grains from stellar ejecta, self-consistent element-by-element metal yields and growth by accretion, grain size transfer (shattering and coagulation) and destruction of dust by thermal sputtering in the ISM. We detail the calibration of this model, particularly the use of a clumping factor, to account for unresolved gas clouds in which dust readily evolves. We present a fiducial run in a 25$^3$~cMpc$^3$ cosmological volume that displays good agreement with observations of the cosmic evolution of dust density, as well as the $z=0$ galaxy dust mass function and dust scaling relations. We highlight known tensions between observational datasets of the dust-to-gas ratio as a function of metallicity depending on which metallicity calibrator is used; our model favours higher-normalisation metallicity calibrators, which agree with the observations within 0.1~dex for stellar masses $>10^9 \; {\rm M_\odot}$. We compare the grain size distribution to observations of local galaxies, and find that our simulation suggests a higher concentration of small grains, associated with more diffuse ISM and the warm-neutral medium (WNM), which both play a key role in boosting H$_2$ content. Putting these results and modelling approaches in context, we set the stage for upcoming insights into the dusty ISM of galaxies using the COLIBRE simulations. △ Less

Submitted 8 January, 2026; v1 submitted 19 May, 2025; originally announced May 2025.

Comments: 29 pages, 16 figures, Published in MNRAS

Journal ref: Mon Not R Astron Soc (2025)

Hydrodynamic simulations of the Disc of Gas Around Supermassive black holes (HDGAS) -II; The transition from neutral atomic to molecular gas phases

Authors: Mojtaba Raouf, Serena Viti, Reihaneh Karimi, Alexander J. Richings

Abstract: We use HDGAS hydrodynamic simulations to study the impact of active galactic nucleus (AGN) feedback on the conversion of atomic-gas to molecular-gas within the circumnuclear-disc (CND) of a typical AGN-dominated galaxy. The comparison of CI, CII, and CO line intensities and their ratios in the HDGAS post-processing radiative-transfer analysis reveals the complex interplay between AGN-activity, col… ▽ More We use HDGAS hydrodynamic simulations to study the impact of active galactic nucleus (AGN) feedback on the conversion of atomic-gas to molecular-gas within the circumnuclear-disc (CND) of a typical AGN-dominated galaxy. The comparison of CI, CII, and CO line intensities and their ratios in the HDGAS post-processing radiative-transfer analysis reveals the complex interplay between AGN-activity, cold molecular gas properties, and the physical processes governing the evolution of star-formation in galaxies. Our results demonstrate that the CI/CO intensity ratio serves as a reliable indicator of the atomic-to-molecular gas transition. We present the probability distribution function (PDF) and abundance trends of various metal species related to molecular H$2$ gas, highlighting differences in clumpiness and intensity maps between AGN feedback and NoAGN models. The profile of the integrated intensity (moment-0) maps shows that the AGN-feedback model exhibits a lower CI/CO intensity ratio in the vicinity of the supermassive black hole (< 50 pc), indicating a smaller atomic-gas abundance and the presence of positive AGN-feedback. Our simulations have successfully predicted the presence of faint-CO emissions extending to larger radii from the galactic center. We also explore the relationships between CII/CO and CI/CII intensity ratios, as well as the ratios versus CO intensity, which provides insights into the "CO-dark" issues. One notable feature in the later time-scale of the AGN model is the presence of a "CO-dark" region, where the intensity of CO emission ($\rm I_{CO}$) is depleted relative to the H$_2$ column density ($N_{\rm H_2}$) compared to the NoAGN model. △ Less

Submitted 2 April, 2025; originally announced April 2025.

Comments: 12 pages and 8 figures; accepted for publication in MNRAS

Predictions for CO emission and the CO-to-H$_2$ conversion factor in galaxy simulations with non-equilibrium chemistry

Authors: Oliver A. Thompson, Alexander J. Richings, Brad K. Gibson, Claude-André Faucher-Giguère, Robert Feldmann, Christopher C. Hayward

Abstract: Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H$_2$ gas mass of the region using the CO-to-H$_2$ conversion factor (Xco). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulat… ▽ More Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H$_2$ gas mass of the region using the CO-to-H$_2$ conversion factor (Xco). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H$_2$ column densities, and the relation between Xco and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H$_2$ mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5\%) for our high-mass galaxies, though still affects the H$_2$ mass and Lco by $\approx$25\%. △ Less

Submitted 12 June, 2024; v1 submitted 21 March, 2024; originally announced March 2024.

Comments: 19 pages, 16 figures (including appendices). Accepted for publication in MNRAS

Dense stellar clump formation driven by strong quasar winds in the FIRE cosmological hydrodynamic simulations

Authors: Jonathan Mercedes-Feliz, Daniel Anglés-Alcázar, Boon Kiat Oh, Christopher C. Hayward, Rachel K. Cochrane, Alexander J. Richings, Claude-André Faucher-Giguère, Sarah Wellons, Bryan A. Terrazas, Jorge Moreno, Kung Yi Su, Philip F. Hopkins

Abstract: We investigate the formation of dense stellar clumps in a suite of high-resolution cosmological zoom-in simulations of a massive, star forming galaxy at $z \sim 2$ under the presence of strong quasar winds. Our simulations include multi-phase ISM physics from the Feedback In Realistic Environments (FIRE) project and a novel implementation of hyper-refined accretion disk winds. We show that powerfu… ▽ More We investigate the formation of dense stellar clumps in a suite of high-resolution cosmological zoom-in simulations of a massive, star forming galaxy at $z \sim 2$ under the presence of strong quasar winds. Our simulations include multi-phase ISM physics from the Feedback In Realistic Environments (FIRE) project and a novel implementation of hyper-refined accretion disk winds. We show that powerful quasar winds can have a global negative impact on galaxy growth while in the strongest cases triggering the formation of an off-center clump with stellar mass ${\rm M}_{\star}\sim 10^{7}\,{\rm M}_{\odot}$, effective radius ${\rm R}_{\rm 1/2\,\rm Clump}\sim 20\,{\rm pc}$, and surface density $Σ_{\star} \sim 10^{4}\,{\rm M}_{\odot}\,{\rm pc}^{-2}$. The clump progenitor gas cloud is originally not star-forming, but strong ram pressure gradients driven by the quasar winds (orders of magnitude stronger than experienced in the absence of winds) lead to rapid compression and subsequent conversion of gas into stars at densities much higher than the average density of star-forming gas. The AGN-triggered star-forming clump reaches ${\rm SFR} \sim 50\,{\rm M}_{\odot}\,{\rm yr}^{-1}$ and $Σ_{\rm SFR} \sim 10^{4}\,{\rm M}_{\odot}\,{\rm yr}^{-1}\,{\rm kpc}^{-2}$, converting most of the progenitor gas cloud into stars in $\sim$2\,Myr, significantly faster than its initial free-fall time and with stellar feedback unable to stop star formation. In contrast, the same gas cloud in the absence of quasar winds forms stars over a much longer period of time ($\sim$35\,Myr), at lower densities, and losing spatial coherency. The presence of young, ultra-dense, gravitationally bound stellar clumps in recently quenched galaxies could thus indicate local positive feedback acting alongside the strong negative impact of powerful quasar winds, providing a plausible formation scenario for globular clusters. △ Less

Submitted 17 April, 2024; v1 submitted 30 October, 2023; originally announced October 2023.

Comments: 14 pages, 12 figures. Accepted for publication in MNRAS; doi:10.1093/mnras/stae1021

Tests of subgrid models for star formation using simulations of isolated disk galaxies

Authors: Folkert S. J. Nobels, Joop Schaye, Matthieu Schaller, Sylvia Ploeckinger, Evgenii Chaikin, Alexander J. Richings

Abstract: We use smoothed-particle hydrodynamics simulations of isolated Milky Way-mass disk galaxies that include cold, interstellar gas to test subgrid prescriptions for star formation (SF). Our fiducial model combines a Schmidt law with a gravitational instability criterion, but we also test density thresholds and temperature ceilings. While SF histories are insensitive to the prescription for SF, the Ke… ▽ More We use smoothed-particle hydrodynamics simulations of isolated Milky Way-mass disk galaxies that include cold, interstellar gas to test subgrid prescriptions for star formation (SF). Our fiducial model combines a Schmidt law with a gravitational instability criterion, but we also test density thresholds and temperature ceilings. While SF histories are insensitive to the prescription for SF, the Kennicutt-Schmidt (KS) relations between SF rate and gas surface density can discriminate between models. We show that our fiducial model, with an SF efficiency per free-fall time of 1 per cent, agrees with spatially-resolved and azimuthally-averaged observed KS relations for neutral, atomic and molecular gas. Density thresholds do not perform as well. While temperature ceilings selecting cold, molecular gas can match the data for galaxies with solar metallicity, they are unsuitable for very low-metallicity gas and hence for cosmological simulations. We argue that SF criteria should be applied at the resolution limit rather than at a fixed physical scale, which means that we should aim for numerical convergence of observables rather than of the properties of gas labelled as star-forming. Our fiducial model yields good convergence when the mass resolution is varied by nearly 4 orders of magnitude, with the exception of the spatially-resolved molecular KS relation at low surface densities. For the gravitational instability criterion, we quantify the impact on the KS relations of gravitational softening, the SF efficiency, and the strength of supernova feedback, as well as of observable parameters such as the inclusion of ionized gas, the averaging scale, and the metallicity. △ Less

Submitted 24 September, 2023; originally announced September 2023.

Comments: Submitted to MNRAS, 23 pages, 20 figures

Hydrodynamic simulations of the Disk of Gas Around Supermassive black holes (HDGAS) -I; Molecular Gas Dynamics

Authors: Mojtaba Raouf, Serena Viti, S. García-Burillo, Alexander J. Richings, Joop schaye, Ashley Bemis, Folkert S. J. Nobels, Matteo Guainazzi, Ko-Yun Huang, Matthieu Schaller, Violette Impellizzeri, Jon Holdship

Abstract: We present hydrodynamic simulations of the interstellar medium (ISM) within the circumnuclear disk (CND) of a typical AGN-dominated galaxy influenced by mechanical feedback from an active galactic nucleus(AGN). The simulations are coupled with the CHIMES non-equilibrium chemistry network to treat the radiative-cooling and AGN-heating. A focus is placed on the central 100 pc scale where AGN outflow… ▽ More We present hydrodynamic simulations of the interstellar medium (ISM) within the circumnuclear disk (CND) of a typical AGN-dominated galaxy influenced by mechanical feedback from an active galactic nucleus(AGN). The simulations are coupled with the CHIMES non-equilibrium chemistry network to treat the radiative-cooling and AGN-heating. A focus is placed on the central 100 pc scale where AGN outflows are coupled to the ISM and constrained by observational Seyfert-2 galaxies. AGN-feedback models are implemented with different wind-velocity and mass-loading factors. We post-process the simulation snapshots with a radiative-transfer code to obtain the molecular emission lines. We find that the inclusion of an AGN promotes the formation of CO in clumpy and dense regions surrounding supermassive-blackholes (SMBH). The CO(1-0) intensity maps ($<$6 Myr) in the CND seem to match well with observations of NGC 1068 with a best match for a model with 5000 $\rm km/s$ wind-velocity and a high mass-loading factor. We attempt to discern between competing explanations for the apparent counter-rotating gas disk in the NGC 1068 through an analysis of kinematic maps of the CO line emission. We suggest that mechanical AGN-feedback could explain the alignment-stability of position-angle across the different CND radii around the SMBH through momentum and energy loading of the wind. It is the wind-velocity that drives the disk out of alignment on a 100 pc scale for a long period of time. The position-velocity diagrams are in broad agreement with the predicted Keplerian rotation-curve in the model without-AGN, but the AGN models exhibit a larger degree of scatter, in better agreement with NGC 1068 observations. △ Less

Submitted 26 June, 2023; originally announced June 2023.

Comments: 16 pages, 13 figures. Accepted for publication in MNRAS

$\rm [C_{II}]$ 158 $\rm μm$ emission as an indicator of galaxy star formation rate

Authors: Lichen Liang, Robert Feldmann, Norman Murray, Desika Narayanan, Christopher C. Hayward, Daniel Anglés-Alcázar, Luigi Bassini, Alexander J. Richings, Claude-André Faucher-Giguère, Dongwoo T. Chung, Jennifer Y. H. Chan, Doǧa Tolgay, Onur Çatmabacak, Dušan Kereš, Philip F. Hopkins

Abstract: Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($L_{\rm [C_{II}]}$) and star formation rate (SFR), suggesting that $L_{\rm [C_{II}]}$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $L_{\rm [C_{II}]}{}/{}\rm SFR$ than the local SFGs, including the infrared-lumin… ▽ More Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($L_{\rm [C_{II}]}$) and star formation rate (SFR), suggesting that $L_{\rm [C_{II}]}$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $L_{\rm [C_{II}]}{}/{}\rm SFR$ than the local SFGs, including the infrared-luminous, starburst galaxies at low and high redshifts, as well as some moderately star-forming galaxies at the epoch of re-ionization (EoR). The origin of this `$\rm [C_{II}]$ deficit' is unclear. In this work, we study the $L_{\rm [C_{II}]}$-SFR relation of galaxies using a sample of $z=0-8$ galaxies with $M_*\approx10^7-5\times10^{11}\,M_\odot$ extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (FIRE) project. We find a simple analytic expression for $L_{\rm [C_{II}]}$/SFR of galaxies in terms of the following parameters: mass fraction of $\rm [C_{II}]$-emitting gas ($f_{\rm [C_{II}]}$), gas metallicity ($Z_{\rm gas}$), gas density ($n_{\rm gas}$) and gas depletion time ($t_{\rm dep}{}={}M_{\rm gas}{}/{}\rm SFR$). We find two distinct physical regimes, where $t_{\rm dep}$ ($Z_{\rm gas}$) is the main driver of the $\rm [C_{II}]$ deficit in $\rm H_2$-rich ($\rm H_2$-poor) galaxies. The observed $\rm [C_{II}]$ deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that $\rm [C_{II}]$ deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant $L_{\rm [C_{II}]}$-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming $\rm [C_{II}]$ line intensity mapping experiments. △ Less

Submitted 6 December, 2023; v1 submitted 10 January, 2023; originally announced January 2023.

Comments: 44 page, 24 figures, 8 tables and 8 appendices, accepted for publication in MNRAS. Key figures: fig. 16 & 17. Comments are welcome

Local positive feedback in the overall negative: the impact of quasar winds on star formation in the FIRE cosmological simulations

Authors: Jonathan Mercedes-Feliz, Daniel Anglés-Alcázar, Christopher C. Hayward, Rachel K. Cochrane, Bryan A. Terrazas, Sarah Wellons, Alexander J. Richings, Claude-André Faucher-Giguère, Jorge Moreno, Kung Yi Su, Philip F. Hopkins, Eliot Quataert, Dušan Kereš

Abstract: Negative feedback from accreting supermassive black holes is regarded as a key ingredient in suppressing star formation and quenching massive galaxies. However, several models and observations suggest that black hole feedback may have a positive effect, triggering star formation by compressing interstellar medium gas to higher densities. We investigate the dual role of black hole feedback using co… ▽ More Negative feedback from accreting supermassive black holes is regarded as a key ingredient in suppressing star formation and quenching massive galaxies. However, several models and observations suggest that black hole feedback may have a positive effect, triggering star formation by compressing interstellar medium gas to higher densities. We investigate the dual role of black hole feedback using cosmological hydrodynamic simulations from the Feedback In Realistic Environments (FIRE) project, including a novel implementation of hyper-refined accretion-disc winds. Focusing on a massive, star-forming galaxy at $z \sim 2$ ($M_{\rm halo} \sim 10^{12.5} \, {\rm M}_{\odot}$), we show that strong quasar winds with kinetic power $\sim$10$^{46}$ erg/s acting for $>$20$\,$Myr drive the formation of a central gas cavity and can dramatically reduce the star formation rate surface density across the galaxy disc. The suppression of star formation is primarily driven by reducing the amount of gas that can become star-forming, compared to directly evacuating the pre-existing star-forming gas reservoir (preventive feedback dominates over ejective feedback). Despite the global negative impact of quasar winds, we identify several plausible signatures of local positive feedback, including: (1) spatial anti-correlation of wind-dominated regions and star-forming clumps, (2) higher local star formation efficiency in compressed gas near the edge of the cavity, and (3) increased local contribution of outflowing material to star formation. Stars forming under the presence of quasar winds tend to do so at larger radial distances. Our results suggest that positive and negative AGN feedback can coexist in galaxies, but local positive triggering of star formation plays a minor role in global galaxy growth. △ Less

Submitted 1 August, 2023; v1 submitted 4 January, 2023; originally announced January 2023.

Comments: 17 pages, 12 figures. Accepted for publication in MNRAS

The effects of local stellar radiation and dust depletion on non-equilibrium interstellar chemistry

Authors: Alexander J. Richings, Claude-Andre Faucher-Giguere, Alexander B. Gurvich, Joop Schaye, Christopher C. Hayward

Abstract: Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionises and heats the gas, and (2) the depletion of meta… ▽ More Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionises and heats the gas, and (2) the depletion of metals onto dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M$_\odot$ per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Way-mass, using the FIRE galaxy formation models together with the CHIMES non-equilibrium chemistry and cooling module. In our fiducial model, we couple the chemistry to the stellar fluxes calculated from star particles using an approximate radiative transfer scheme, and we implement an empirical density-dependent prescription for metal depletion. For comparison, we also run simulations with a spatially uniform radiation field, and without metal depletion. Our fiducial model broadly reproduces observed trends in HI and H2 mass with stellar mass, and in line luminosity versus star formation rate for [CII] 158$μ$m, [OI] 63$μ$m, [OIII] 88$μ$m, [NII] 122$μ$m and H$α$ 6563A. Our simulations with a uniform radiation field predict fainter luminosities, by up to an order of magnitude for [OIII] 88$μ$m and H$α$ 6563A, while ignoring metal depletion increases the luminosity of carbon and oxygen lines by a factor $\approx$2. However, the overall evolution of the galaxy is not strongly affected by local stellar fluxes or metal depletion, except in dwarf galaxies where the inclusion of local fluxes leads to weaker outflows and hence higher gas fractions. △ Less

Submitted 30 August, 2022; v1 submitted 3 August, 2022; originally announced August 2022.

Comments: 29 pages, 24 figures (including appendices). Accepted for publication in MNRAS. Minor changes relative to v1

Rapid disc settling and the transition from bursty to steady star formation in Milky Way-mass galaxies

Authors: Alexander B. Gurvich, Jonathan Stern, Claude-André Faucher-Giguère, Philip F. Hopkins, Andrew Wetzel, Jorge Moreno, Christopher C. Hayward, Alexander J. Richings, Zachary Hafen

Abstract: Recent observations and simulations indicate substantial evolution in the properties of galaxies with time, wherein rotationally-supported and steady thin discs (like those frequently observed in the local universe) emerge from galaxies that are clumpy, irregular, and have bursty star formation rates (SFRs). To better understand the progenitors of local disc galaxies we carry out an analysis of th… ▽ More Recent observations and simulations indicate substantial evolution in the properties of galaxies with time, wherein rotationally-supported and steady thin discs (like those frequently observed in the local universe) emerge from galaxies that are clumpy, irregular, and have bursty star formation rates (SFRs). To better understand the progenitors of local disc galaxies we carry out an analysis of three FIRE-2 simulated galaxies with a mass similar to the Milky Way at redshift z=0. We show that all three galaxies transition from bursty to steady SFRs at a redshift between z=0.5 and z=0.8, and that this transition coincides with a rapid (< ~1 Gyr) emergence of a rotationally-supported interstellar medium (ISM).In the late phase with steady SFR, the rotational energy comprises > ~90% of the total kinetic + thermal energy in the ISM, and is roughly half the gravitational energy. By contrast, during the early phase with bursty star formation, the ISM has a quasi-spheroidal morphology and its energy budget is dominated by quasi-isotropic flows including turbulence and coherent inflows/outflows. This result, that rotational support is subdominant at early times, challenges the common application of equilibrium disc models to the high-redshift progenitors of Milky Way-like galaxies. We further find that the formation of a rotation-supported ISM coincides with the formation of a thermal energy-supported inner circumgalactic medium (CGM). Before this transition, the inner CGM is also supported by turbulence and coherent flows, indicating that at early times there is no clear boundary between the ISM and inner CGM. △ Less

Submitted 8 March, 2022; originally announced March 2022.

Comments: 16 pages, 14 figures, submitted to MNRAS

Unravelling the physics of multiphase AGN winds through emission line tracers

Authors: Alexander J. Richings, Claude-Andre Faucher-Giguere, Jonathan Stern

Abstract: Observations of emission lines in Active Galactic Nuclei (AGN) often find fast (~1000 km s^-1) outflows extending to kiloparsec scales, seen in ionised, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines tr… ▽ More Observations of emission lines in Active Galactic Nuclei (AGN) often find fast (~1000 km s^-1) outflows extending to kiloparsec scales, seen in ionised, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient ISM or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [OIV] 25 $μ$m / [NeII] 12 $μ$m , [NeV] 14 $μ$m / [NeII] 12 $μ$m and [NIII] 57 $μ$m / [NII] 122 $μ$m constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurised compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that >50 per cent of the mass outflow rate, momentum flux and kinetic energy flux of the outflow are traced by lines such as [NII] 122 $μ$m and [NeIII] 15 $μ$m (produced in the 10^4 K phase) and [CII] 158 $μ$m (produced in the transition from 10^4 K to 100 K). △ Less

Submitted 23 February, 2021; v1 submitted 11 December, 2020; originally announced December 2020.

Comments: 19 pages, 16 figures (including appendix). Accepted for publication in MNRAS; minor changes relative to previous version

Pressure balance in the multiphase ISM of cosmologically simulated disk galaxies

Authors: Alexander B. Gurvich, Claude-André Faucher-Giguère, Alexander J. Richings, Philip F. Hopkins, Michael Y. Grudić, Zachary Hafen, Sarah Wellons, Jonathan Stern, Eliot Quataert, T. K. Chan, Matthew E. Orr, Dušan Kereš, Andrew Wetzel, Christopher C. Hayward, Sarah R. Loebman, Norman Murray

Abstract: Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. W… ▽ More Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyze how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the midplane. Bulk flows (e.g., inflows and fountains) are important at a few disk scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total midplane pressure is well-predicted by the weight of the disk gas, and we show that it also scales linearly with the star formation rate surface density (Sigma_SFR). These results support the notion that the Kennicutt-Schmidt relation arises because Sigma_SFR and the gas surface density (Sigma_g) are connected via the ISM midplane pressure. △ Less

Submitted 16 November, 2020; v1 submitted 26 May, 2020; originally announced May 2020.

Comments: 22 pages, 16 figures, accepted in MNRAS

The changing circumgalactic medium over the last 10 Gyr I: physical and dynamical properties

Authors: Ezra Huscher, Benjamin D. Oppenheimer, Alice Lonardi, Robert A. Crain, Alexander J. Richings, Joop Schaye

Abstract: We present an analysis of the physical and dynamical states of two sets of EAGLE zoom simulations of galaxy haloes, one at high redshift ($z=2-3$) and the other at low redshift ($z=0$), with masses of $\approx 10^{12} M_{\odot}$. Our focus is how the circumgalactic medium (CGM) of these $L^*$ star-forming galaxies change over the last 10 Gyr. We find that the high-$z$ CGM is almost equally divided… ▽ More We present an analysis of the physical and dynamical states of two sets of EAGLE zoom simulations of galaxy haloes, one at high redshift ($z=2-3$) and the other at low redshift ($z=0$), with masses of $\approx 10^{12} M_{\odot}$. Our focus is how the circumgalactic medium (CGM) of these $L^*$ star-forming galaxies change over the last 10 Gyr. We find that the high-$z$ CGM is almost equally divided between the "cool" ($T<10^5$ K) and "hot" ($T\geq 10^5$ K) phases, while the low-$z$ hot CGM phase contains $5\times$ more mass. The high-$z$ hot CGM contains 60% more metals than the cool CGM, while the low-$z$ cool CGM contains 35% more metals than the hot CGM content. The metals are evenly distributed radially between the hot and cool phases throughout the high-$z$ CGM. At high $z$, the CGM volume is dominated by hot outflows, cool gas is mainly inflowing, but cool metals are flowing outward. At low $z$, the cool metals dominate the interior and the hot metals are more prevalent at larger radii. The low-$z$ cool CGM has tangential motions consistent with rotational support out to $0.2 R_{200}$, often exhibiting $r \approx 40$ kpc disc-like structures. The low-$z$ hot CGM has several times greater angular momentum than the cool CGM, and a more flattened radial density profile than the high-$z$ hot CGM. This study verifies that, just as galaxies demonstrate significant evolutionary stages over cosmic time, the gaseous haloes surrounding them also undergo considerable changes of their own both in physical characteristics of density, temperature and metallicity, and dynamic properties of velocity and angular momentum. △ Less

Submitted 22 May, 2020; v1 submitted 13 May, 2020; originally announced May 2020.

Comments: 16 pages, 15 figures. Submitted to MNRAS. Comments welcome. (revision submitted on May 22, 2020 fixes Fig. 3. right panel that mistakenly displayed metal mass and now correctly displays total gas mass)

Reproducing the CO-to-H$_2$ conversion factor in cosmological simulations of Milky Way-mass galaxies

Authors: Laura C. Keating, Alexander J. Richings, Norman Murray, Claude-Andre Faucher-Giguere, Philip F. Hopkins, Andrew Wetzel, Dusan Keres, Samantha Benincasa, Robert Feldmann, Sarah Loebman, Matthew E. Orr

Abstract: We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results… ▽ More We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which in our models sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H$_2$ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T < 300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H$_{2}$ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model which controls the amount of radiation that penetrates giant molecular clouds. △ Less

Submitted 28 September, 2020; v1 submitted 22 January, 2020; originally announced January 2020.

Comments: Accepted to MNRAS. 16 pages, 10 figures

Radiative cooling of swept up gas in AGN-driven galactic winds and its implications for molecular outflows

Authors: Alexander J. Richings, Claude-Andre Faucher-Giguere

Abstract: We recently used hydro-chemical simulations to demonstrate that molecular outflows observed in luminous quasars can be explained by molecule formation within the AGN wind. However, these simulations cover a limited parameter space, due to their computational cost. We have therefore developed an analytic model to follow cooling in the shocked ISM layer of an AGN wind. We explore different ambient d… ▽ More We recently used hydro-chemical simulations to demonstrate that molecular outflows observed in luminous quasars can be explained by molecule formation within the AGN wind. However, these simulations cover a limited parameter space, due to their computational cost. We have therefore developed an analytic model to follow cooling in the shocked ISM layer of an AGN wind. We explore different ambient densities ($1-10^{4} \, \rm{cm}^{-3}$), density profile slopes ($0-1.5$), AGN luminosities ($10^{44}-10^{47} \, \rm{erg} \, \rm{s}^{-1}$), and metallicities ($0.1-3 \rm{Z}_{\odot}$). The swept up gas mostly cools within ~1 Myr. Based on our previous simulations, we predict that this gas would produce observable molecular outflows. The instantaneous momentum boost initially increases as the outflow decelerates. However, it reaches a maximum of $\approx$20, due to work done against the gravitational potential. The predicted time-averaged observational estimate of the molecular outflow momentum boost reaches a maximum of $\approx1-2$, partly due to our assumed molecular fraction, 0.2, but also because the instantaneous and observational, time-averaged definitions are not equivalent. Thus recent observational estimates of order unity momentum boosts do not necessarily rule out energy-driven outflows. Finally, we find that dust grains are likely to re-form by accretion of metals after the shocked ISM layer has cooled, assuming that a small fraction of dust grains swept up after this layer has cooled are able to mix into the cool phase, and assuming that grain growth remains efficient in the presence of the strong AGN radiation field. This would enable rapid molecule formation, as assumed in our models. △ Less

Submitted 17 May, 2018; v1 submitted 25 October, 2017; originally announced October 2017.

Comments: 22 pages, 16 figures (including appendices). Accepted for publication in MNRAS

The multiphase circumgalactic medium traced by low metal ions in EAGLE zoom simulations

Authors: Benjamin D. Oppenheimer, Joop Schaye, Robert A. Crain, Jessica K. Werk, Alexander J. Richings

Abstract: We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including C II, Si II, Si III, Si IV, and Mg II. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of z~0.2 L* (M_200=10^11.7-10^12.3 Msol) haloes hosting star-forming gala… ▽ More We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including C II, Si II, Si III, Si IV, and Mg II. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of z~0.2 L* (M_200=10^11.7-10^12.3 Msol) haloes hosting star-forming galaxies and group-sized (M_200=10^12.7-10^13.3 Msol) haloes hosting mainly passive galaxies reproduce key trends observed by the COS-Halos survey-- low ion column densities show 1) little dependence on galaxy specific star formation rate, 2) a patchy covering fraction indicative of 10^4 K clouds with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20-160 kpc. Simulated Si II, Si III, Si IV, C II, and C III column densities show good agreement with observations, while Mg II is under-predicted. Low ions trace a significant metal reservoir, ~10^8 Msol, residing primarily at 10-100 kpc from star-forming and passive central galaxies. These clouds tend to flow inwards and most will accrete onto the central galaxy within the next several Gyr, while a small fraction are entrained in strong outflows. A two-phase structure describes the inner CGM (<0.5 R_200) with low-ion metal clouds surrounded by a hot, ambient medium. This cool phase is separate from the O VI observed by COS-Halos, which arises from the outer CGM (>0.5 R_200) tracing virial temperature gas around L* galaxies. Physical parameters derived from standard photo-ionization modelling of observed column densities (e.g. aligned Si II/Si III absorbers) are validated against our simulations. Our simulations therefore support previous ionization models indicating that cloud covering factors decline while densities and pressures show little variation with increasing impact parameter. △ Less

Submitted 21 September, 2017; originally announced September 2017.

Comments: 26 pages, 19 figures, submitted to MNRAS

The origin of fast molecular outflows in quasars: molecule formation in AGN-driven galactic winds

Authors: Alexander J. Richings, Claude-Andre Faucher-Giguere

Abstract: We explore the origin of fast molecular outflows that have been observed in Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is difficult to create such an outflow by accelerating existing molecular clouds in the host galaxy, as the clouds will be destroyed before they can reach the high velocities that are observed. In this work, we consider an alternative scenario wher… ▽ More We explore the origin of fast molecular outflows that have been observed in Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is difficult to create such an outflow by accelerating existing molecular clouds in the host galaxy, as the clouds will be destroyed before they can reach the high velocities that are observed. In this work, we consider an alternative scenario where molecules form in-situ within the AGN outflow. We present a series of hydro-chemical simulations of an isotropic AGN wind interacting with a uniform medium. We follow the time-dependent chemistry of 157 species, including 20 molecules, to determine whether molecules can form rapidly enough to produce the observed molecular outflows. We find H$_2$ outflow rates up to 140 M$_\odot$ yr$^{-1}$, which is sensitive to density, AGN luminosity, and metallicity. We compute emission and absorption lines of CO, OH and warm (a few hundred K) H$_2$ from the simulations in post-processing. The CO-derived outflow rates and OH absorption strengths at solar metallicity agree with observations, although the maximum line of sight velocities from the model CO spectra are a factor $\approx$2 lower than is observed. We derive a CO (1-0) to H$_2$ conversion factor of $α_{\rm{CO} (1-0)}$ = 0.13 M$_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$, 6 times lower than is commonly assumed in observations of such systems. We find strong emission from the mid-infrared lines of H$_2$. The mass of H$_2$ traced by this infrared emission is within a few per cent of the total H$_2$ mass. This H$_2$ emission may be observable by JWST. △ Less

Submitted 20 October, 2017; v1 submitted 12 June, 2017; originally announced June 2017.

Comments: 30 pages, 21 figures (including appendices), resubmitted to MNRAS following referee's report. Some results have changed from the previous version, in particular for warm H2 emission (see Figs. 5 and 13)

Flickering AGN can explain the strong circumgalactic O VI observed by COS-Halos

Authors: Benjamin D. Oppenheimer, Marijke Segers, Joop Schaye, Alexander J. Richings, Robert A. Crain

Abstract: Proximity zone fossils (PZFs) are ionization signatures around recently active galactic nuclei (AGN) where metal species in the circumgalactic medium remain over-ionized after the AGN has shut-off due to their long recombination timescales. We explore cosmological zoom hydrodynamic simulations using the EAGLE model paired with a non-equilibrium ionization and cooling module including time-variable… ▽ More Proximity zone fossils (PZFs) are ionization signatures around recently active galactic nuclei (AGN) where metal species in the circumgalactic medium remain over-ionized after the AGN has shut-off due to their long recombination timescales. We explore cosmological zoom hydrodynamic simulations using the EAGLE model paired with a non-equilibrium ionization and cooling module including time-variable AGN radiation to model PZFs around star-forming, disk galaxies in the z~0.2 Universe. Previous simulations typically under-estimated the O VI content of galactic haloes, but we show that plausible PZF models increase O VI column densities by 2-3x to achieve the levels observed around COS-Halos star-forming galaxies out to 150 kpc. Models with AGN bolometric luminosities >~10^43.6 erg s^-1, duty cycle fractions <~10%, and AGN lifetimes <~10^6 yr are the most promising, because their super-massive black holes grow at the cosmologically expected rate and they mostly appear as inactive AGN, consistent with COS-Halos. The central requirement is that the typical star-forming galaxy hosted an active AGN within a timescale comparable to the recombination time of a high metal ion, which for circumgalactic O VI is 10^7 years. H I, by contrast, returns to equilibrium much more rapidly due to its low neutral fraction and does not show a significant PZF effect. O VI absorption features originating from PZFs appear narrow, indicating photo-ionization, and are often well-aligned with lower metal ion species. PZFs are highly likely to affect the physical interpretation of circumgalactic high ionization metal lines if, as expected, normal galaxies host flickering AGN. △ Less

Submitted 11 April, 2018; v1 submitted 22 May, 2017; originally announced May 2017.

Comments: 17 pages, 12 figures, accepted in MNRAS, companion website: http://noneq.strw.leidenuniv.nl/PZF/

Metals in the circumgalactic medium are out of ionization equilibrium due to fluctuating active galactic nuclei

Authors: Marijke C. Segers, Benjamin D. Oppenheimer, Joop Schaye, Alexander J. Richings

Abstract: We study the effect of a fluctuating active galactic nucleus (AGN) on the abundance of circumgalactic OVI in galaxies selected from the EAGLE simulations. We follow the time-variable OVI abundance in post-processing around four galaxies - two at $z=0.1$ with stellar masses of $M_{\ast} \sim 10^{10}$ M$_{\odot}$ and $M_{\ast} \sim 10^{11}$ M$_{\odot}$, and two at $z=3$ with similar stellar masses -… ▽ More We study the effect of a fluctuating active galactic nucleus (AGN) on the abundance of circumgalactic OVI in galaxies selected from the EAGLE simulations. We follow the time-variable OVI abundance in post-processing around four galaxies - two at $z=0.1$ with stellar masses of $M_{\ast} \sim 10^{10}$ M$_{\odot}$ and $M_{\ast} \sim 10^{11}$ M$_{\odot}$, and two at $z=3$ with similar stellar masses - out to impact parameters of twice their virial radii, implementing a fluctuating central source of ionizing radiation. Due to delayed recombination, the AGN leave significant `AGN proximity zone fossils' around all four galaxies, where OVI and other metal ions are out of ionization equilibrium for several megayears after the AGN fade. The column density of OVI is typically enhanced by $\approx 0.3-1.0$ dex at impact parameters within $0.3R_{\rm vir}$, and by $\approx 0.06-0.2$ dex at $2R_{\rm vir}$, thereby also enhancing the covering fraction of OVI above a given column density threshold. The fossil effect tends to increase with increasing AGN luminosity, and towards shorter AGN lifetimes and larger AGN duty cycle fractions. In the limit of short AGN lifetimes, the effect converges to that of a continuous AGN with a luminosity of $(f_{\rm duty}/100\%)$ times the AGN luminosity. We also find significant fossil effects for other metal ions, where low-ionization state ions are decreased (SiIV, CIV at $z=3$) and high-ionization state ions are increased (CIV at $z=0.1$, NeVIII, MgX). Using observationally motivated AGN parameters, we predict AGN proximity zone fossils to be ubiquitous around $M_{\ast} \sim 10^{10-11}$ M$_{\odot}$ galaxies, and to affect observations of metals in the circumgalactic medium at both low and high redshifts. △ Less

Submitted 27 June, 2017; v1 submitted 18 April, 2017; originally announced April 2017.

Comments: 17 pages, 11 figures (not including appendix), accepted for publication in MNRAS

Chemical evolution of giant molecular clouds in simulations of galaxies

Authors: Alexander J. Richings, Joop Schaye

Abstract: We present an analysis of Giant Molecular Clouds (GMCs) within hydrodynamic simulations of isolated, low-mass (M* ~ 10^9 M_sol) disc galaxies. We study the evolution of molecular abundances and the implications for CO emission and the X_CO conversion factor in individual clouds. We define clouds either as regions above a density threshold n_H,min = 10 cm^-3, or using an observationally motivated C… ▽ More We present an analysis of Giant Molecular Clouds (GMCs) within hydrodynamic simulations of isolated, low-mass (M* ~ 10^9 M_sol) disc galaxies. We study the evolution of molecular abundances and the implications for CO emission and the X_CO conversion factor in individual clouds. We define clouds either as regions above a density threshold n_H,min = 10 cm^-3, or using an observationally motivated CO intensity threshold of 0.25 K km s^-1. Our simulations include a non-equilibrium chemical model with 157 species, including 20 molecules. We also investigate the effects of resolution and pressure floors (i.e. Jeans limiters). We find cloud lifetimes up to ~40 Myr, with a median of 13 Myr, in agreement with observations. At one tenth solar metallicity, young clouds (<10-15 Myr) are underabundant in H2 and CO compared to chemical equilibrium, by factors of ~3 and 1-2 orders of magnitude, respectively. At solar metallicity, GMCs reach chemical equilibrium faster (within ~1 Myr). We also compute CO emission from individual clouds. The mean CO intensity, I_CO, is strongly suppressed at low dust extinction, A_v, and possibly saturates towards high A_v, in agreement with observations. The I_CO - A_v relation shifts towards higher A_v for higher metallicities and, to a lesser extent, for stronger UV radiation. At one tenth solar metallicity, CO emission is weaker in young clouds (<10-15 Myr), consistent with the underabundance of CO. Consequently, X_CO decreases by an order of magnitude from 0 to 15 Myr, albeit with a large scatter. △ Less

Submitted 11 May, 2016; v1 submitted 4 April, 2016; originally announced April 2016.

Comments: 28 pages, 29 figures (including appendices), Accepted for publication in MNRAS. Minor changes relative to v1