Showing 1–50 of 496 results for author: Lewis, G F

AGN Reverberation Mapping with LITMUS: Fundamental Limits on lag Recovery Rates

Authors: Hugh McDougall, Tamara M. Davis, Benjamin J. S. Pope, Paul Martini, Zhefu Yu, Chris Lidman, Geraint F. Lewis

Abstract: Reverberation mapping of active galactic nuclei provides one of the most direct probes of the geometry and kinematics of the broad-line region by measuring time delays between continuum and line variability. Modern RM surveys frequently suffer difficulties with lag measurements due to poor signal to noise and aliasing, whereby multimodal lag posterior distributions arise due to seasonal gaps in ou… ▽ More Reverberation mapping of active galactic nuclei provides one of the most direct probes of the geometry and kinematics of the broad-line region by measuring time delays between continuum and line variability. Modern RM surveys frequently suffer difficulties with lag measurements due to poor signal to noise and aliasing, whereby multimodal lag posterior distributions arise due to seasonal gaps in our data. These challenge the reliability of commonly used fitting tools such as JAVELIN, which can return a high rate of false positives. We implement a new lag measurement package, LITMUS, and introduce a new framework that uses Bayesian evidence to identify false positive lag measurements, as well as examine the question of how many AGN present detectable lags in high redshift industrial scale surveys like OzDES and SDSS. Our analysis differs from previous RM studies in six key respects: (i) our inference is robust to the previously under-diagnosed numerical component of aliasing, (ii) we use a consistent methodology for all sources, (iii) uncertainty in the underlying AGN variability is fully marginalised, (iv) lag significance is assessed via Bayesian model comparison rather than heuristic metrics, (v) false-positive rates are quantified by comparison against random-chance recoveries and (vi) we use marginal likelihoods to distinguish between sources where a lag is not detectable in our data and sources that show no evidence of reverberation. Applied to the OzDES sample, we find that previous RM studies are likely to have overestimated the confidence of recovered lags, and we find a stark contrast between a low reverberation percentage for the MgII line (3-28% depending on assumptions) and much higher percentages in the CIV and especially the H$β$ line, which is consistent with 100%. We also present a re-analysed set of lags from the OzDES sample with better quantified reliabilities △ Less

Submitted 2 August, 2026; originally announced August 2026.

Comments: 32 Pages, 21 Figures

Simulation-based tension quantification of the cosmic dipole

Authors: Mali Land-Strykowski, Harry T. J. Bevins, Oliver T. Oayda, Geraint F. Lewis

Abstract: The cosmic dipole measured in surveys of cosmologically distant sources consistently exceeds the expectation derived from the cosmic microwave background, posing a significant challenge to the standard $Λ$CDM cosmology. In the era of precision cosmology, quantifying robust tensions is constrained by our ability to model complex, non-linear effects that often result in intractable likelihood functi… ▽ More The cosmic dipole measured in surveys of cosmologically distant sources consistently exceeds the expectation derived from the cosmic microwave background, posing a significant challenge to the standard $Λ$CDM cosmology. In the era of precision cosmology, quantifying robust tensions is constrained by our ability to model complex, non-linear effects that often result in intractable likelihood functions. In this paper, we present a flexible simulation-based inference (SBI) architecture for measuring the cosmic dipole tension using Neural Ratio Estimators (NREs). We design and train an ensemble of NREs to evaluate the log Bayesian evidence ratio and measure $Nσ$ tension. We validate our approach against nested sampling, demonstrating that it accurately recovers the ground-truth. Under the kinematic interpretation of the dipole, we apply our approach to Planck, the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS), and the Wide-field Infrared Survey Explorer catalogue (CatWISE). Here, leveraging SBI, we measure a $\approx5.7σ$ tension between CatWISE and Planck. We demonstrate the extensibility of our approach by applying it to forward-modelled simulations of the CatWISE Eddington bias, revealing a $\approx6.7σ$ tension. The methodology proposed here enables robust tension quantification as we enter the era of LSST, Euclid, and the SKA. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Comments: 14 pages, 9 figures, accepted for publication in MNRAS

Journal ref: Mon Not R Astron Soc (2026)

Microlensing of Microlensing: Effects of Random Stars on the Double-Source-Plane Gravitational Lens

Authors: Nada Salama, Daniel J. Ballard, Huimin Qu, Geraint F. Lewis, Karl Glazebrook

Abstract: Microlensing, the influence of stars within a galactic gravitational lens, has emerged as a powerful probe of compact mass and, through differential magnification, sub-parsec scale sources at cosmological distances. The recent discovery of a double-source-plane gravitational lens system in which the most distant source is a quasar offers the prospect of compound microlensing, in which quasar light… ▽ More Microlensing, the influence of stars within a galactic gravitational lens, has emerged as a powerful probe of compact mass and, through differential magnification, sub-parsec scale sources at cosmological distances. The recent discovery of a double-source-plane gravitational lens system in which the most distant source is a quasar offers the prospect of compound microlensing, in which quasar light rays are influenced by compact masses within the two foreground lensing galaxies. Here, we present the first numerical simulations of this "microlensing of microlensing". We consider the recently discovered "Einstein zig-zag" lens, J1721+8842, as a fiducial case, and construct microlensing magnification maps for each of the six quasar images in this system. Due to the secondary microlensing effects of the myriad of initial microimages, the resulting maps contain more complex caustic features than seen in the case of single plane microlensing. This is reflected in the expected lightcurves seen for each of the images. △ Less

Submitted 19 July, 2026; originally announced July 2026.

Comments: 11 pages, 7 figures, accepted to MNRAS

Boötes III is a Tidally Disrupting Ultra-Faint Dwarf Galaxy on an Eccentric Polar Orbit

Authors: Ting S. Li, Denis Erkal, Andrew B. Pace, Jiaxun Yang, Sergey E. Koposov, Jo Bovy, Nathan R. Sandford, Andrew P. Li, Gustavo E. Medina, Lara R. Cullinane, Gary S. Da Costa, Alexander P. Ji, Kyler Kuehn, Geraint F. Lewis, Guilherme Limberg, Sarah L. Martell, Aldo Mura-Guzmán, Nora Shipp, Yong Yang, Daniel B. Zucker, Kaia R. Atzberger, Joss Bland-Hawthorn, John D. Dixon

Abstract: We present updated systemic properties of the ultra-faint dwarf galaxy Boötes III from the Southern Stellar Stream Spectroscopic Survey (S$^5$). We identify 21 high-probability members and measure a velocity dispersion of $σ_{v} = 1.69^{+1.03}_{-0.85}$ km s$^{-1}$, about six times smaller than the previously reported $10.7 \pm 3.5$ km s$^{-1}$, and a mean metallicity of [Fe/H] $= -2.34 \pm 0.11$.… ▽ More We present updated systemic properties of the ultra-faint dwarf galaxy Boötes III from the Southern Stellar Stream Spectroscopic Survey (S$^5$). We identify 21 high-probability members and measure a velocity dispersion of $σ_{v} = 1.69^{+1.03}_{-0.85}$ km s$^{-1}$, about six times smaller than the previously reported $10.7 \pm 3.5$ km s$^{-1}$, and a mean metallicity of [Fe/H] $= -2.34 \pm 0.11$. The revised dispersion brings Boötes III in line with other tidally disrupting dwarfs such as Antlia II and Crater II. Orbit integrations in a Milky Way (MW) + Large Magellanic Cloud (LMC) potential confirm a highly eccentric ($e \approx 0.8$), polar ($i \approx 89.5^\circ$) orbit with a recent pericentric passage $\sim 0.14$ Gyr ago at $r_{\rm peri} \approx 9.5$ kpc. Boötes III is thus likely actively tidally disrupting, as its tidal radius at pericenter, $r_t \approx 164$ pc, is only $\sim 0.35$ of its half-light radius. The unusually low dispersion also implies that Boötes III has either lost most of its dark matter to tides or hosts a cored inner density profile, making it a probe of the nature of dark matter. Simulated tidal streams are broadly consistent with the Styx stellar stream, though the predicted track and kinematics are sensitive to the MW halo mass, LMC mass, and solar velocity. Boötes III overlaps the Typhon stream in integrals-of-motion space but has a much lower mean metallicity, suggesting the two are not the same system but may have had a common group infall origin. Sagittarius-stream contamination prevents a direct tidal-tail detection, so deep spectroscopic follow-up remains essential, both to confirm Styx as a genuine stream and to establish it as Boötes III's tidal tail. △ Less

Submitted 8 July, 2026; originally announced July 2026.

Comments: 32 pages, 15 figures, 7 tables. To be submitted to the Open Journal of Astrophysics. Analysis code and data available at https://github.com/s5collab/boo3

The GALAH Survey: Neutron-Capture Elemental Abundances for 350,000 Gaia-RVS Spectra and the Chemodynamics of Accreted Structures

Authors: Pradosh Barun Das, Daniel B. Zucker, Aldo Mura-Guzmán, Nicholas W. Borsato, Gayandhi M. De Silva, Sven Buder, Diane Feuillet, Thomas Nordlander, Melissa K. Ness, Sarah L. Martell, Janez Kos, Joss Bland-Hawthorn, Ken C. Freeman, Andrew R. Casey, Geraint F. Lewis, Dennis Stello, Richard de Grijs, the GALAH Collaboration

Abstract: We present a comprehensive data-driven spectroscopic analysis of 357,415 red giant stars using Gaia DR3 Radial Velocity Spectrometer (RVS) spectra (8460-8700 A; $R\approx11,500$), aimed at deriving precise stellar parameters and elemental abundances (collectively referred to as stellar labels). We employ The Cannon, a generative model based on 2747 giants in common with GALAH DR4, adopting GALAH l… ▽ More We present a comprehensive data-driven spectroscopic analysis of 357,415 red giant stars using Gaia DR3 Radial Velocity Spectrometer (RVS) spectra (8460-8700 A; $R\approx11,500$), aimed at deriving precise stellar parameters and elemental abundances (collectively referred to as stellar labels). We employ The Cannon, a generative model based on 2747 giants in common with GALAH DR4, adopting GALAH labels ($R\approx28,000$) for training. The resulting model predicts eleven stellar labels for RVS giants: effective temperature ($T_{\rm eff}$), surface gravity ($\log g$), projected rotational velocity ($v\sin i$), and abundances of [Fe/H], [Ca/Fe], [Si/Fe], [Ni/Fe], [Ti/Fe], as well as the neutron-capture elements [Zr/Fe], [Ce/Fe], and [Nd/Fe]. Building on these results, we develop a probabilistic framework to chemically identify debris from the Gaia-Sausage-Enceladus (GSE) accretion event. A logistic regression classifier, optimised via Markov Chain Monte Carlo sampling and trained on a small reference sample of GSE members and comparison stars, identifies stars with high GSE membership probabilities based solely on their chemical abundances, with the resulting candidates exhibiting distinctive abundance-ratio patterns, including [Ca/Ti], [Ti/Ce], and [Nd/Zr]. Applying independent kinematic constraints yields a robust sample of GSE candidates, demonstrating that the characteristic chemical signatures remain consistent after applying these constraints. This work demonstrates the power of data-driven analysis techniques to extract detailed chemical information from medium-resolution spectra and establishes a framework for tracing Galactic accretion events using chemical abundances. △ Less

Submitted 3 June, 2026; originally announced June 2026.

Comments: Accepted for Publication in MNRAS; 21 Pages, 14 Figures, 8 Tables

Journal ref: Mon Not R Astron Soc (2026)

The Information Content of Quasar Variability Light Curves: How Well Can we Infer Stochastic Model Parameters?

Authors: Brendon J. Brewer, Geraint F. Lewis, Xiang Yu, Yuan Li

Abstract: Quasar variability, driven by multi-scale physical processing within a relativistic accretion disk, is commonly modelled with stochastic time series models. The simplest of these is the Damped Random Walk (DRW), also known as the Ornstein-Uhlenbeck (OU) process. Here, we demonstrate that, when fitting such a model to quasar light curve data, the mean of the light curve, $μ$, should not be fixed (w… ▽ More Quasar variability, driven by multi-scale physical processing within a relativistic accretion disk, is commonly modelled with stochastic time series models. The simplest of these is the Damped Random Walk (DRW), also known as the Ornstein-Uhlenbeck (OU) process. Here, we demonstrate that, when fitting such a model to quasar light curve data, the mean of the light curve, $μ$, should not be fixed (which is the typical approach), as this leads to overconfident inferences about the variability timescale $τ$, with substantially underestimated uncertainties. However, the short term volatility parameter $η$ is typically very well constrained from short light curves. Through simulations, we compute information theoretic quantities such as the conditional entropy and the mutual information, confirming that light curves provide much more information about $η$ than about $τ$. As a result, we recommend that future quasar variability studies focus on $η$ rather than $τ$. To demonstrate this approach, we fit a hierarchical Bayesian regression model for $η$ as a function of bolometric luminosity and rest wavelength to a dataset of 570 light curves measured over decades. We perform the fit using a likelihood function that uses the light curves directly, rather than using intermediate $η$ values from individual light curve fits. We find that volatility decreases as a function of both bolometric luminosity and rest wavelength. The volatility also decreases more steeply with redshift than time dilation alone would suggest, pointing to an increase in intrinsic volatility as quasars evolve over cosmic time. △ Less

Submitted 3 July, 2026; v1 submitted 31 May, 2026; originally announced June 2026.

Comments: Published in the Open Journal of Astrophysics. 18 pages, 7 figures, 3 tables

Journal ref: The Open Journal of Astrophysics Vol. 9, 2026, July 07, 2026

The Ellis and Baldwin test of the Cosmic Dipole: Exploring the impact of multiple flux density cuts

Authors: Vasudev Mittal, Geraint F. Lewis

Abstract: The cosmic dipole tension - the discrepancy between the Cosmic Microwave Background kinematic dipole and the matter dipole inferred from all-sky surveys poses a significant challenge to the Cosmological Principle, which dictates that the universe is homogeneous and isotropic at the largest scales. Traditional measurement of the matter dipole requires selecting an appropriate limiting flux and calc… ▽ More The cosmic dipole tension - the discrepancy between the Cosmic Microwave Background kinematic dipole and the matter dipole inferred from all-sky surveys poses a significant challenge to the Cosmological Principle, which dictates that the universe is homogeneous and isotropic at the largest scales. Traditional measurement of the matter dipole requires selecting an appropriate limiting flux and calculating the dipolar modulation using sources brighter than the flux. This approach, however, ignores the shape of the source luminosity function (LF) and deprives the analysis of this crucial information. In this study, we present a new approach to calculate the matter dipole by integrating the source flux distribution into the analysis. We achieve this by dividing the catalogue into disjoint flux bins and simultaneously fitting the matter dipole across them. For non-power-law LFs, this method gives a higher Bayes factor - and hence a better description of the matter dipole - as compared to the traditional approach. The method works best when the flux cuts are selected in regions where the LF's shape changes significantly. We discuss the feasibility of this method for upcoming cosmological surveys and show that it has the potential to yield decisive results at both radio and infrared wavelengths. △ Less

Submitted 26 May, 2026; originally announced May 2026.

Comments: 26 pages, 14 figures, accepted for publication in Journal of Cosmology and Astroparticle Physics (JCAP)

The DECam MAGIC Survey: Investigating the Jet Stellar Stream with Photometric Metallicities

Authors: H. Q. Do, A. Chiti, P. S. Ferguson, A. P. Ji, G. Limberg, K. R. Atzberger, J. L. Carlin, W. Cerny, A. Drlica-Wagner, G. F. Lewis, T. S. Li, C. E. Martínez-Vázquez, S. L. Martell, G. E. Medina, N. E. D. Noël, A. B. Pace, V. M. Placco, A. H. Riley, D. J. Sand, G. S. Stringfellow, J. A. Carballo-Bello, L. R. Cullinane, D. Erkal, S. E. Koposov, B. Mutlu-Pakdil , et al. (5 additional authors not shown)

Abstract: Stellar streams are dynamically fragile structures formed by the tidal disruption of dwarf galaxies and stellar clusters. These objects are valuable tracers of the gravitational potential and accretion history of the Milky Way, and are key probes for the presence and interactions of starless dark matter subhalos. The Jet stream is a $\sim 30^\circ$-long stellar stream that is situated at 30.4 kpc… ▽ More Stellar streams are dynamically fragile structures formed by the tidal disruption of dwarf galaxies and stellar clusters. These objects are valuable tracers of the gravitational potential and accretion history of the Milky Way, and are key probes for the presence and interactions of starless dark matter subhalos. The Jet stream is a $\sim 30^\circ$-long stellar stream that is situated at 30.4 kpc and originates from a disrupted globular cluster. It consists of metal-poor stars that follow a retrograde orbit, reducing the impulse imparted from the Milky Way bar and making it especially sensitive to gravitational perturbations from dark matter subhalos. This paper investigates the known extent of the Jet stream by leveraging photometric metallicities derived from a narrowband filter centered on the Ca II H&K lines at $\sim$3950A on the Dark Energy Camera (DECam), as part of the Mapping the Ancient Galaxy in CaHK (MAGIC) survey. The wide field-of-view of DECam enables the efficient derivation of photometric metallicities for stars across the full extent of the stream, allowing for a metallicity-based selection to identify likely members. We demonstrate the efficacy of photometric metallicities in isolating stream members when used with Gaia DR3 proper motions, identifying a sample of 213 candidate Jet stream member stars. This then allows for the study of stream morphology, through which we identify a clear fanning of the stream toward the end farther from the Milky Way bar. We provide a list of candidate members, enabling spectroscopic follow-up of the Jet stream to facilitate further studies of its dynamics. △ Less

Submitted 16 April, 2026; v1 submitted 14 April, 2026; originally announced April 2026.

Comments: 18 pages, 7 figures, 3 tables, Submitted to AAS Journals

The Illusion of Morphology in Tidal Structures: Changes to Stellar Shells and Streams in Non-Spherical Haloes

Authors: Smrithi Gireesh Babu, Viraj Ekanayaka, William H. Oliver, Geraint F. Lewis

Abstract: We identify shell-like tidal structures in flattened haloes that appear stream-like under different projections. This projection dependence demonstrates how changes in the host halo directly impact the formation and classification of tidal debris, highlighting the challenges of relying solely on visual inspection. To address this, we employ our clustering-based classification framework to systemat… ▽ More We identify shell-like tidal structures in flattened haloes that appear stream-like under different projections. This projection dependence demonstrates how changes in the host halo directly impact the formation and classification of tidal debris, highlighting the challenges of relying solely on visual inspection. To address this, we employ our clustering-based classification framework to systematically categorise tidally disrupted satellites into stream-like and shell-like structures. Our host consists of a static three-component MW model with flattening introduced along the z-axis NFW dark halo. We consider three halo shapes: spherical q = 1, extremely oblate q = 0.5, and prolate q = 1.5. We evolve three subhalo types: a highly radial massive subhalo favouring shell formation, an eccentric orbit leading to stream formation, and an intermediate orbit. We first classify the tidal structures visually using face-on and edge-on density projections of the 3D position distribution. This reveals shell-like and stream-like formations across face-on projections, while edge-on views lead to contrary classifications in some cases. To resolve these ambiguities, we apply the classification method developed in our earlier work, analysing structures in ordered density, radial, and energy-angle space. We further investigate the spatial dispersion of stream-like structures and the rate at which core density reduces as the flattening parameter varies. Our results demonstrate that halo shape variations affect tidal debris formation and classification, as well as the spatial dispersion and core density evolution of streams. These findings offer new insights into the role of dark matter halo geometry in shaping tidal structure formation and its contribution to hierarchical galaxy formation and evolution. △ Less

Submitted 7 April, 2026; originally announced April 2026.

Comments: 14 pages, 10 figures. Accepted for publication in MNRAS

Epicyclic Density Variations in the Indus Stellar Stream

Authors: Yong Yang, Geraint F. Lewis, Ting S. Li, Sarah L. Martell, Denis Erkal, Alexander P. Ji, Sergey E. Koposov, Daniel B. Zucker, Andrew B. Pace, Lara R. Cullinane, Gary S. Da Costa, Kyler Kuehn, Guilherme Limberg, Gustavo E. Medina, S5 Collaboration

Abstract: Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynami… ▽ More Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning $\sim 90^\circ$ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo which results in steeper peaks. This arises from different instantaneous mass loss due to distinct central mass distributions of halos, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo. △ Less

Submitted 10 March, 2026; originally announced March 2026.

Comments: 18 pages, 16 figures, 3 supplementary movies, accepted by MNRAS

Journal ref: MNRAS Volume 547, Issue 4, April 2026, stag488

Searching for and characterizing halo substructures with the GALAH DR4 survey

Authors: Iryna Kushniruk, Kristopher Youakim, Karin Lind, Sven Buder, Janes Kos, Diane Feuillet, Sarah L. Martell, Richard de Grijs, Geraint F. Lewis, Joss Bland-Hawthorn, Gary Da Costa, Michael Hayden, Daniel Zucker, Tomaz Zwitter, Sanjib Sharma

Abstract: Recent studies show that the Milky Way stellar halo is composed of populations of different origins, shaped by multiple accretion events. To better understand the formation of the Milky Way and other spiral galaxies, we characterize the chemical and kinematic properties of halo substructures using GALAH DR4 and Gaia data. We apply wavelet transforms in the space of sqrt(J_r) and azimuthal action (… ▽ More Recent studies show that the Milky Way stellar halo is composed of populations of different origins, shaped by multiple accretion events. To better understand the formation of the Milky Way and other spiral galaxies, we characterize the chemical and kinematic properties of halo substructures using GALAH DR4 and Gaia data. We apply wavelet transforms in the space of sqrt(J_r) and azimuthal action (L_z) to identify kinematic overdensities. Stars in the detected structures are analyzed in elemental abundance space to determine their origin. We further assess contamination using the unsupervised machine-learning algorithm t-distributed stochastic neighbor embedding (t-SNE), performing chemical tagging with 15 elemental abundances. We recover five structures: the Galactic disk, the Splash, Gaia-Sausage-Enceladus (GSE), Thamnos1, and Thamnos2. GSE shows two peaks; one at sqrt(J_r) ~ 25 kpc km s^-1 is due to disk contamination, while the other above sqrt(J_r) ~ 40 kpc km s^-1 represents the cleanest GSE population. Thamnos exhibits three peaks linked to Thamnos1 and Thamnos2. Thamnos2 shows higher [alpha/Fe], iron-peak elements are enhanced in the Splash, and halo groups retain a stronger r-process signature. The multiply peaked structures suggest that the splashed disk extends beyond prograde orbits. The distinct chemo-dynamical properties of the halo groups support their extragalactic origin. △ Less

Submitted 23 February, 2026; originally announced February 2026.

Journal ref: A&A 709, A69 (2026)

A Bayesian Exploration of the Mass of Ursa Major III: Kinematics, Rotation and their influence on the Mass to Light Ratio

Authors: T. R. Adams, B. J. Brewer, G. F. Lewis

Abstract: We investigate the kinematics of the potential ultra-faint dwarf galaxy (UFD) UMa III/U1 using Bayesian inference to search for the signal of any potential intrinsic rotation. The magnitude of rotation is relevant to estimating the total mass of UMa III/U1, which is critical in determining whether or not UMa III/U1 is in fact a UFD, or possibly a star cluster home to a significant binary fraction.… ▽ More We investigate the kinematics of the potential ultra-faint dwarf galaxy (UFD) UMa III/U1 using Bayesian inference to search for the signal of any potential intrinsic rotation. The magnitude of rotation is relevant to estimating the total mass of UMa III/U1, which is critical in determining whether or not UMa III/U1 is in fact a UFD, or possibly a star cluster home to a significant binary fraction. A non-rotating model and a rotational model are fitted for the current total population of member stars of UMa III/U1, finding that a non-rotating model was preferred by a factor of $\sim 5-12 \times$. This was repeated on a reduced population of UMa III/U1, where potential contaminant stars were removed. A similar preference for non-rotation was found for these reduced populations. We calculate a lower-bound rotational mass estimate for UMa III/U1 and a corresponding lower bound mass-to-light ratio of $ 734.4^{+339.0}_{-176.2} \mathrm{M_\odot} / \mathrm{L_\odot} $ for the total population. We conclude that UMa III/U1 still remains an ambiguous object with viable arguments for both the UFD and self-gravitating star cluster scenarios, however under both, UMa III/U1 is unlikely to be supported by rotational pressure. △ Less

Submitted 19 February, 2026; originally announced February 2026.

Comments: Published in the Open Journal of Astrophysics

Wising up to CatWISE: using simulation-based inference to interpret the ecliptic bias and confirm the cosmic dipole excess

Authors: Oliver T. Oayda, Geraint F. Lewis

Abstract: We apply Simulation-Based Inference ('SBI') to the cosmic dipole problem for the first time, measuring the distribution of quasar counts over the sky in the CatWISE2020 ('CatWISE') sample. We show that the quadrupole anisotropy in CatWISE can be attributed to the correlation between WISE's scanning law and photometric uncertainty in the $W1$ and $W2$ magnitudes, inducing an Eddington bias which va… ▽ More We apply Simulation-Based Inference ('SBI') to the cosmic dipole problem for the first time, measuring the distribution of quasar counts over the sky in the CatWISE2020 ('CatWISE') sample. We show that the quadrupole anisotropy in CatWISE can be attributed to the correlation between WISE's scanning law and photometric uncertainty in the $W1$ and $W2$ magnitudes, inducing an Eddington bias which varies with sky position. After explicitly modelling this with SBI, we use a neural likelihood estimator to find the posterior distribution for CatWISE's dipole, confirming the presence of a dipole twice as large as the CMB expectation but more seriously misaligned with the CMB direction ($\approx 3 σ$). We also use our learned likelihood to infer the Bayesian evidence, learning that models which increase the scale of CatWISE's photometric errors are most favoured. This is strong evidence that the sample's errors are underestimated or that there is an additional, unresolved systematic producing the same effect as Eddington bias. While our results indicate that the cosmic dipole excess is a persistent issue for $Λ$CDM, we showcase that SBI can untangle the subtle and complex systematic issues affecting any sample derived from real astronomical data. △ Less

Submitted 4 February, 2026; originally announced February 2026.

Comments: Accepted for publication in MNRAS; 15 pages, 10 figures

Local environmental dependence on weak-lensing shear statistics

Authors: Sonia Akter Ema, Md Rasel Hossen, Krzysztof Bolejko, Geraint F. Lewis

Abstract: Despite the assumption that an ideal FLRW observer is not dependent on the local environment, observations are biased by the positions of the observers due to the matter correlations in the large-scale structure (LSS) of the universe. The variation of the mass distribution of the LSS of the universe implies that observers residing in different locations may suffer bias in their measurements when t… ▽ More Despite the assumption that an ideal FLRW observer is not dependent on the local environment, observations are biased by the positions of the observers due to the matter correlations in the large-scale structure (LSS) of the universe. The variation of the mass distribution of the LSS of the universe implies that observers residing in different locations may suffer bias in their measurements when they look at the images of distant galaxies. Here, we assess the influence of the local environment on weak gravitational lensing (WL) shear statistics in the context of relativistic $N$-body code, \texttt{gevolution}. We derive numerical constraints on the cosmological parameters from the WL shear angular power spectrum and comment on the local environment's influence on WL shear. We find tighter constraints on the parameter $Ω_\mathrm{m}$ above redshift $z$ = 0.2, which implies over this redshift the local environment's impact is minor. We also investigate the bispectrum and conclude that on average the impact of the local environment on $f_{\rm NL}$ (a measure of non-Gaussianities) is minimal and consistent with zero effect. However, we find that within the assembly of all possible observers/locations, there will also be a few that could infer the parameter $f_{\rm NL}$ of the order 10. These results could thus be used to estimate the uncertainty in the inference of cosmological parameters such as $f_{\rm NL}$ based on WL shear bispectrum and thus may have implications for future surveys requiring precision at the percent level. △ Less

Submitted 27 January, 2026; originally announced January 2026.

Comments: 16 pages, 14 figures, Accepted for publication in Physical Review D

Journal ref: Physical Review D 113, 043540 (2026)

Multi-band Reconstruction of Sixteen Gravitational Lens Systems using PISCO data

Authors: Huimin Qu, Daniel J. Ballard, Geraint F. Lewis, Karl Glazebrook, Antony Stark, Sarah M. Sweet, Colin Jacobs, Kim-Vy Tran, Brian Stalder, Tania M. Barone, Tucker Jones, Keerthi Vasan G. C., Thomas E. Collett, Glenn G. Kacprzak, Dorota Bayer

Abstract: Next-generation surveys such as the Euclid survey, the Legacy Survey of Space and Time (LSST), and the China Space Station Telescope (CSST) survey are expected to discover ~10^5 galaxy-galaxy scale strong gravitational lenses. This motivates the development of scalable and robust lens modeling approaches that can efficiently and reliably learn from wide-field survey datasets before high-resolution… ▽ More Next-generation surveys such as the Euclid survey, the Legacy Survey of Space and Time (LSST), and the China Space Station Telescope (CSST) survey are expected to discover ~10^5 galaxy-galaxy scale strong gravitational lenses. This motivates the development of scalable and robust lens modeling approaches that can efficiently and reliably learn from wide-field survey datasets before high-resolution follow-up. We design a scalable, Bayesian, Lenstronomy-based pipeline and apply it to a sample of sixteen lens candidates observed with the Parallel Imager for Southern Cosmology Observations (PISCO) on the Magellan telescope. PISCO provides four-band imaging (z, i, r, g) with colours, depth and seeing conditions comparable to LSST. To fully exploit the constraining power of this dataset, our pipeline performs simultaneous multi-band modeling, using a common mass profile across all four bands while allowing independent light profiles in each. This approach leverages color information to provide joint constraints on the lens mass and yields reduced uncertainties compared to single-band analyses. Fifteen out of sixteen PISCO lens candidates are successfully recovered with interpretable lensing configurations, including DESJ0533-2536, the first reported hyperbolic-umbilic galaxy-galaxy scale strong lensing candidate. We further assess how much model complexity can be reliably constrained given the resolution and seeing of PISCO-like data. Overall, our results demonstrate that scalable, multi-band lens modeling of ground-based data can extract meaningful constraints on mass and source morphology, providing a practical pathway to maximize the scientific return from large samples in upcoming surveys. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Comments: Accepted by MNRAS. 22 pages, 16 figures, 4 tables

Rotational Kinematics in the Globular Cluster System of M31: Insights from Bayesian Inference

Authors: Yuan Li, Brendon J. Brewer, Geraint F. Lewis, Dougal Mackey

Abstract: As ancient stellar systems, globular clusters (GCs) offer valuable insights into the dynamical histories of large galaxies. Previous studies of GC populations in the inner and outer regions of the Andromeda Galaxy (M31) have revealed intriguing subpopulations with distinct kinematic properties. Here, we build upon earlier studies by employing Bayesian modelling to investigate the kinematics of the… ▽ More As ancient stellar systems, globular clusters (GCs) offer valuable insights into the dynamical histories of large galaxies. Previous studies of GC populations in the inner and outer regions of the Andromeda Galaxy (M31) have revealed intriguing subpopulations with distinct kinematic properties. Here, we build upon earlier studies by employing Bayesian modelling to investigate the kinematics of the combined inner and outer GC populations of M31. Given the heterogeneous nature of the data, we examine subpopulations defined by GCs' metallicity and by associations with substructure, in order to characterise possible relationships between the inner and outer GC populations. We find that lower-metallicity GCs and those linked to substructures exhibit a common, more rapid rotation, whose alignment is distinct from that of higher-metallicity and non-substructure GCs. Furthermore, the higher-metallicity GCs rotate in alignment with Andromeda's stellar disk. These pronounced kinematic differences reinforce the idea that different subgroups of GCs were accreted to M31 at distinct epochs, shedding light on the complex assembly history of the galaxy. △ Less

Submitted 13 January, 2026; v1 submitted 8 January, 2026; originally announced January 2026.

Comments: Published in the Open Journal of Astrophysics. 13 pages, 10 figures

Journal ref: The Open Journal of Astrophysics Vol. 9, January 12, 2026

Detailed Chemical Abundance Analysis of the Brightest Stars in the Turranburra and Willka Yaku Stellar Streams

Authors: Kaitlin B. Webber, Terese T. Hansen, Jennifer L. Marshall, Alexander P. Ji, Ting S. Li, Gary S. Da Costa, Lara R. Cullinane, Denis Erkal, Sergey E. Koposov, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Sarah L. Martell, Andrew B. Pace, Nora Shipp, Jeffrey D. Simpson, Zhen Wan, Daniel B. Zucker, Victor A. Alvarado, Joss Bland-Hawthorn, Guilherme Limberg, Gustavo E. Medina, Sam A. Usman

Abstract: We present a detailed chemical abundance analysis of the three brightest known stars from each of the Turranburra and Willka Yaku stellar streams using high-resolution Magellan/MIKE spectra. Abundances for 27 elements, ranging from carbon to dysprosium, were derived. Our results support the original classification that Turranburra, with a low average metallicity of… ▽ More We present a detailed chemical abundance analysis of the three brightest known stars from each of the Turranburra and Willka Yaku stellar streams using high-resolution Magellan/MIKE spectra. Abundances for 27 elements, ranging from carbon to dysprosium, were derived. Our results support the original classification that Turranburra, with a low average metallicity of $\mathrm{[Fe/H]=-2.45} \pm 0.07$, likely originates from a dwarf-galaxy progenitor. Willka Yaku has a low average metallicity of $\mathrm{[Fe/H]=-2.35 \pm 0.03}$ with a small scatter in the abundances, consistent with a globular cluster progenitor as suggested by previous studies. Both streams exhibit mild enhancements in neutron-capture elements, with averages of $\mathrm{[Eu II/Fe]}=$ $0.47 \pm{0.09}$ for Turranburra and $0.44 \pm{0.05}$ for Willka Yaku, consistent with enrichment from an $r$-process event. A similar enrichment is observed in other stellar streams, and we further discuss this signature as it relates to the potential enrichment histories of these two streams. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Comments: 21 pages, 10 tables, 6 figures

$S^5$: Tidal Disruption in Crater 2 and Formation of Diffuse Dwarf Galaxies in the Local Group

Authors: Guilherme Limberg, Alexander P. Ji, Ting S. Li, Denis Erkal, Sergey E. Koposov, Andrew B. Pace, Andrew P. Li, Petra Awad, Alexandra Senkevich, Joss Bland-Hawthorn, Lara Cullinane, Gary Da Costa, Alex Drlica-Wagner, Raphaël Errani, Peter S. Ferguson, Kyler Kuehn, Geraint F. Lewis, Sarah L. Martell, Jorge Peñarrubia, Nora Shipp, Yong Yang, Daniel B. Zucker

Abstract: We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ($S^5$). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identi… ▽ More We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ($S^5$). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identify 143 Cra2 members, of which 114 belong to the galaxy's main body and 29 are deemed part of its stellar stream. We confirm that Cra2 is dynamically cold (central velocity dispersion $2.51^{+0.33}_{-0.30}\,{\rm km\,s^{-1}}$) and also discover a $\approx$7$σ$ velocity gradient consistent with its tidal debris track. We separately estimate the stream velocity dispersion to be $5.74^{+0.98}_{-0.83}\,{\rm km\,s^{-1}}$. We develop a suite of $N$-body simulations with both cuspy and cored density profiles on a realistic Cra2 orbit to compare with $S^5$ observations. We find that the velocity dispersion ratio between Cra2 stream and galaxy ($2.30^{+0.41}_{-0.35}$) is difficult to reconcile with a cuspy halo with fiducial concentration and an initial mass predicted by standard stellar mass$-$halo mass relationships. Instead, either a cored halo with relatively small core radius or a low-concentration cuspy model can reproduce this ratio. Despite tidal mass loss, Cra2 is metal-poor ($\langle \rm[Fe/H]\rangle=-2.16\pm0.04$) compared to the stellar mass$-$metallicity relation for its luminosity. Other diffuse dwarf galaxies similar to Cra2 in the Local Group (Antlia 2 and Andromeda 19) also challenge galaxy formation models. Finally, we discuss possible formation scenarios for Cra2, including ram-pressure stripping of a gas-rich progenitor combined with tides. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: Submitted to AAS journals

OzDES Reverberation Mapping of Active Galactic Nuclei: Final Data Release, Black-Hole Mass Results, & Scaling Relations

Authors: H. McDougall, T. M. Davis, Z. Yu, P. Martini, C. Lidman, U. Malik, A. Penton, G. F. Lewis, B. E. Tucker, B. J. S. Pope, S. Allam, F. Andrade-Oliveira, J. Asorey, D. Bacon, S. Bocquet, D. Brooks, A. Carnero Rosell, D. Carollo, A. Carr, J. Carretero, T. Y. Cheng, L. N. da Costa, M. E. da Silva Pereira, J. De Vicente, H. T. Diehl , et al. (31 additional authors not shown)

Abstract: Over the last decade, the Australian Dark Energy (OzDES) collaboration has used Reverberation Mapping to measure the masses of high redshift supermassive black holes. Here we present the final review and analysis of this OzDES reverberation mapping campaign. These observations use 6-7 years of photometric and spectroscopic observations of 735 Active Galactic Nuclei (AGN) in the redshift range 0.13… ▽ More Over the last decade, the Australian Dark Energy (OzDES) collaboration has used Reverberation Mapping to measure the masses of high redshift supermassive black holes. Here we present the final review and analysis of this OzDES reverberation mapping campaign. These observations use 6-7 years of photometric and spectroscopic observations of 735 Active Galactic Nuclei (AGN) in the redshift range 0.13-3.85 and bolometric luminosity range 44.3 - 47.5 erg/s. Both photometry and spectra are observed in visible wavelengths, allowing for the physical scale of the AGN broad line region to be estimated from reverberations of the H\b{eta}, MgII and CIV emission lines. We successfully use reverberation mapping to constrain the masses of 62 super-massive black holes, and combine with existing data to fit a power law to the lag-luminosity relation for the H\b{eta} and MgII lines with a scatter of ~0.25 dex, the tightest yet identified, fit specifically for consistency with high redshift AGN. We fit a similarly constrained relation for CIV, resolving a tension with the low luminosity literature AGN by accounting for selection effects arising from finite survey length. We also examine the impact of emission line width and luminosity (related to accretion rate) in reducing the scatter of these scaling relationships and find no significant improvement over the lag-only approach for any of the three lines. Using these relations, we further estimate the masses and accretion rates of 246 AGN with single epoch methods. We also use these relations to estimate the relative sizes of the H\b{eta}, MgII and CIV emitting regions. In short, we provide a comprehensive benchmark of high redshift AGN reverberation mapping at the close of this most recent generation of surveys, including light curves, time-delays, and a set of significantly improved radius-luminosity relations for use with high-redshift populations. △ Less

Submitted 24 June, 2026; v1 submitted 30 November, 2025; originally announced December 2025.

Comments: 36 Pages, 15 Figures; see also companion paper Penton et. al. 2025

Report number: DES-2025-860 FERMILAB-PUB-25-0824-PPD

Journal ref: PAS-2026-10926

OzDES Reverberation Mapping Program: CIV lags from six years of data

Authors: A. Penton, H. McDougall, T. M. Davis, Z. Yu, U. Malik, P. Martini, B. E. Tucker, C. Lidman, G. F. Lewis, R. Sharp, M. Aguena, S. Allam, F. Andrade-Oliveira, J. Asorey, D. Bacon, S. Bocquet, D. Brooks, R. Camilleri, A. Carnero Rosell, D. Carollo, A. Carr, J. Carretero, T. Y. Cheng, L. N. da Costa, M. E. da Silva Pereira , et al. (30 additional authors not shown)

Abstract: We present 29 successfully recovered CIV time lags in Active Galactic Nuclei from the complete Dark Energy Survey Reverberation Mapping campaign. The AGN in this sample span a redshift range of 1.9<z<3.5. We successfully measure the velocity dispersion from the CIV spectral linewidth for 25 of these 29 sources, and use these to calculate new high-redshift black hole mass estimates, finding masses… ▽ More We present 29 successfully recovered CIV time lags in Active Galactic Nuclei from the complete Dark Energy Survey Reverberation Mapping campaign. The AGN in this sample span a redshift range of 1.9<z<3.5. We successfully measure the velocity dispersion from the CIV spectral linewidth for 25 of these 29 sources, and use these to calculate new high-redshift black hole mass estimates, finding masses between 0.8 and 1.3 billion solar masses. We also identify a selection effect due to the duration of the survey that can impact the radius-luminosity relation derived from this and other (high-redshift) data. This paper represents the culmination of the OzDES CIV campaign. △ Less

Submitted 15 July, 2026; v1 submitted 30 November, 2025; originally announced December 2025.

Comments: 12 pages, 8 figures; see also companion paper McDougall et al; v2 is the version accepted for publication in PASA including some figure updates and data links available

Report number: FERMILAB-PUB-25-0823-PPD, DES-2025-891

Cosmic Multipoles in Galaxy Surveys II: Comparing Different Methods in Assessing the Cosmic Dipole

Authors: Vasudev Mittal, Oliver T. Oayda, Geraint F. Lewis

Abstract: We present a comparative analysis of estimators and Bayesian methods for determining the number count dipole from cosmological surveys. The increase in discordance between the number count dipole and the CMB's kinematic dipole has presented a challenge to the assumption of an isotropic and homogeneous universe. The level of discordance has depended on the choice of method to determine the dipole;… ▽ More We present a comparative analysis of estimators and Bayesian methods for determining the number count dipole from cosmological surveys. The increase in discordance between the number count dipole and the CMB's kinematic dipole has presented a challenge to the assumption of an isotropic and homogeneous universe. The level of discordance has depended on the choice of method to determine the dipole; hence, there is a need to compare them to determine the better approach. We select the healpy fit_dipole algorithm as our estimator and show that it gives unbiased results regardless of the noise levels and sky coverage in the data. However, for low sky coverage, the estimator's results have a large variance, which indicates that the estimator is not reliable in that regime. We then compare the estimator's outputs with the Bayesian results. If the sky coverage is sufficient, the Bayesian posterior probabilities agree with both the estimator's outcomes and the true parameter values. But if the sky coverage is low, Bayesian analysis is often inconclusive, which safeguards against incorrect conclusions. Both methods provide the ability to analyse multiple samples: on one hand, we need to select a different estimator, while in Bayesian inference, we need to extend our likelihood function to incorporate additional parameters. Our study emphasises the need to perform a principled statistical analysis of sky surveys for dipole determination. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Comments: 13 pages, 7 figures, Published in OJAp

Cosmic dipole tensions: confronting the cosmic microwave background with infrared and radio populations of cosmological sources

Authors: Mali Land-Strykowski, Geraint F. Lewis, Tara Murphy

Abstract: The cosmic dipole measured in surveys of cosmologically distant sources is generally found to be in disagreement with the kinematic expectation of the Cosmic Microwave Background (CMB). This discrepancy represents severe tension with the Cosmological Principle and challenges the standard model of cosmology. Here, we present a Bayesian analysis of the tension between datasets used to measure the co… ▽ More The cosmic dipole measured in surveys of cosmologically distant sources is generally found to be in disagreement with the kinematic expectation of the Cosmic Microwave Background (CMB). This discrepancy represents severe tension with the Cosmological Principle and challenges the standard model of cosmology. Here, we present a Bayesian analysis of the tension between datasets used to measure the cosmic dipole. We examine the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS) and the Wide-field Infrared Survey Explorer catalogue (CatWISE), and jointly analyse them with the Planck observations of the CMB. Under the kinematic interpretation, we find that Planck is in severe tension with CatWISE above 5$σ$, strong tension with RACS, and moderate tension with NVSS. Moreover, the strong concordance between CatWISE and NVSS suggests that their dipoles arise from a common astrophysical signal. Conversely, the high discordance between RACS and both CatWISE and NVSS indicates a possible systematic difference in the RACS catalogue itself. Whilst the tension between Planck and infrared-selected quasars is already significant, the question of whether or not the dipole in individual radio surveys adds to the challenge against the standard model is yet to be seen. We estimate that $\mathcal{O}(10^6)$ radio sources are required to measure the tension to a significance of 5$σ$. Therefore, in light of the upcoming SKA radio surveys, we are on the cusp of disentangling the anomaly of the cosmic dipole. △ Less

Submitted 1 May, 2026; v1 submitted 23 September, 2025; originally announced September 2025.

Comments: 13 pages, 5 figures, accepted for publication in MNRAS; updated to include published correction, conclusions unchanged (3 pages, 3 figures)

Journal ref: MNRAS, Volume 543, Issue 4, November 2025, Pages 3229-3241; Correction: MNRAS, Volume 548, Issue 3, May 2026, stag483

Constraining Cosmology with Double-source-plane Strong Gravitational Lenses from the AGEL Survey

Authors: Duncan J. Bowden, Nandini Sahu, Anowar J. Shajib, Kim-Vy Tran, Tania M. Barone, Keerthi Vasan G. C., Daniel J. Ballard, Thomas E. Collett, Faith Dalessandro, Giovanni Ferrami, Karl Glazebrook, William J. Gottemoller, Leena Iwamoto, Tucker Jones, Glenn G. Kacprzak, Geraint F. Lewis, Haven McIntosh-Lombardo, Hannah Skobe, Sherry H. Suyu, Sarah M. Sweet

Abstract: Double-source-plane strong gravitational lenses (DSPLs), with two sources at different redshifts, are independent cosmological probes of the dark energy equation of state parameter $w$ and the matter density parameter $Ω_{\rm m}$. We present the lens model for the DSPL AGEL035346$-$170639 and infer cosmological constraints from this system for flat $Λ$ cold dark matter and flat $w$CDM cosmologies.… ▽ More Double-source-plane strong gravitational lenses (DSPLs), with two sources at different redshifts, are independent cosmological probes of the dark energy equation of state parameter $w$ and the matter density parameter $Ω_{\rm m}$. We present the lens model for the DSPL AGEL035346$-$170639 and infer cosmological constraints from this system for flat $Λ$ cold dark matter and flat $w$CDM cosmologies. From the joint posterior of $w$ and $Ω_{\rm m}$ in the flat $w$CDM cosmology, we extract the following median values and 1$σ$ uncertainties: $w = -1.52^{+0.49}_{-0.33}$ and $Ω_{\rm m} = 0.192^{+0.305}_{-0.131}$ from AGEL0353 alone. Combining our measurements with two previously analyzed DSPLs, we present the joint constraint on these parameters from a sample of three, the largest galaxy-scale DSPL sample used for cosmological measurement to date. The combined precision of $w$ from three DSPLs is higher by 15% over AGEL0353 alone. Combining DSPL and cosmic microwave background (CMB) measurements improves the precision of $w$ from CMB-only constraints by 39%, demonstrating the complementarity of DSPLs with the CMB. Despite their promising constraining power, DSPLs are limited by sample size, with only a handful discovered so far. Although ongoing and near-future wide-area sky surveys will increase the number of known DSPLs by up to two orders of magnitude, these systems will still require dedicated high-resolution imaging and spectroscopic follow-ups like those presented in this paper. Our ASTRO 3D Galaxy Evolution with Lenses collaboration is undertaking such follow-up campaigns for several newly discovered DSPLs and will provide cosmological measurements from larger samples of DSPLs in the future. △ Less

Submitted 4 November, 2025; v1 submitted 18 September, 2025; originally announced September 2025.

Comments: 14 pages, 10 figures, Updated final version published in the Astrophysical Journal (ApJ)

Journal ref: Duncan J. Bowden et al 2025 ApJ 993 124

Streams and Shells Decoded: A Density-Driven Approach to Stellar Clustering in Galactic Halos with AstroLink

Authors: Viraj Ekanayaka, Smrithi Gireesh Babu, William H. Oliver, Geraint F. Lewis

Abstract: We present a novel method to differentiate stream-like and shell-like tidal remnants of stellar systems in galactic halos using the density-based approach of the clustering algorithm AstroLink. While previous studies lean on observation, phase-space, and action-space based criteria for stream and shell determination, we introduce AstroLink's ordered-density plot and cluster identification as a via… ▽ More We present a novel method to differentiate stream-like and shell-like tidal remnants of stellar systems in galactic halos using the density-based approach of the clustering algorithm AstroLink. While previous studies lean on observation, phase-space, and action-space based criteria for stream and shell determination, we introduce AstroLink's ordered-density plot and cluster identification as a viable tool for classification. For a given data set, the AstroLink ordered-density plot reveals the density-based hierarchical clustering structure from which the resultant clusters are identified as being statistically significant overdensities. Using simulations of sub-halo disruptions in an external potential to generate samples of tidal structures, we find that the curvature of the ordered-density plot is positive for stream-like structures and negative for shell-like structures. Comparisons with more standard classification techniques reveal strong agreement on which structures typically fit into stream-like and shell-like categories. Furthermore, we investigate the properties of clustered stream and shell samples in radial phase space and energy-angle space. Given the sensitivity of stellar tidal structures to their host dark matter halos, the identification and subsequent classification of these structures provide exciting avenues of investigation in galactic evolution dynamics and dark matter structure formation. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Comments: This paper has been accepted for publication in PASA

Journal ref: Publ. Astron. Soc. Aust. 42 (2025) e115

Unveiling M33's Hidden Merger History: A Potential Population of Star Clusters on Retrograde Orbits

Authors: Borja Anguiano, Geraint F. Lewis, Steven R. Majewski

Abstract: We report the discovery of a possible sub-population of stellar clusters that appear to follow retrograde orbits around the third largest galaxy in the Local Group, M33 (Triangulum). This spiral disk galaxy has apparently had a mostly quiescent existence, although recent discoveries, particularly of a pronounced warp in the gas and stellar disk, suggest that M33's relatively quiet past was interru… ▽ More We report the discovery of a possible sub-population of stellar clusters that appear to follow retrograde orbits around the third largest galaxy in the Local Group, M33 (Triangulum). This spiral disk galaxy has apparently had a mostly quiescent existence, although recent discoveries, particularly of a pronounced warp in the gas and stellar disk, suggest that M33's relatively quiet past was interrupted at least once by a dynamical interaction with another galaxy. We suggest that this sub population provides evidence of accretion of one or more dwarf galaxies in M33's history. We estimate a lower limit for the accreted halo virial mass of $M_{\rm vir} \sim (7 \pm 3) \times 10^{10}$\msun, accounting for about 10\% of the virial mass in the halo of M33 that has an accretion origin. We propose one of these accretion events as the source of the observed warp in M33's disk. △ Less

Submitted 17 July, 2025; originally announced July 2025.

Comments: Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS). The manuscript consists of 11 pages and includes 8 figures

Sifting for a Stream: The Morphology of the $300S$ Stellar Stream

Authors: Benjamin Cohen, Alexander P. Ji, Peter S. Ferguson, Sergey E. Koposov, Alex Drlica-Wagner, Andrew P. Li, Ting S. Li, Lara R. Cullinane, Gary S. Da Costa, Denis Erkal, Kyler Kuehn, Geraint F. Lewis, Sarah L. Martell, Andrew B. Pace, Daniel B. Zucker, Petra Awad, Joss Bland-Hawthorn, Guilherme Limberg, Clara E. Martínez-Vázquez, Joshua D. Simon, Yong Yang

Abstract: Stellar streams are sensitive laboratories for understanding the small-scale structure in our Galaxy's gravitational field. Here, we analyze the morphology of the $300S$ stellar stream, which has an eccentric, retrograde orbit and thus could be an especially powerful probe of both baryonic and dark substructures within the Milky Way. Due to extensive background contamination from the Sagittarius s… ▽ More Stellar streams are sensitive laboratories for understanding the small-scale structure in our Galaxy's gravitational field. Here, we analyze the morphology of the $300S$ stellar stream, which has an eccentric, retrograde orbit and thus could be an especially powerful probe of both baryonic and dark substructures within the Milky Way. Due to extensive background contamination from the Sagittarius stream (Sgr), we perform an analysis combining Dark Energy Camera Legacy Survey photometry, $\textit{Gaia}$ DR3 proper motions, and spectroscopy from the Southern Stellar Stream Spectroscopic Survey ($\textit{S}^5$). We redetermine the stream coordinate system and distance gradient, then apply two approaches to describe $300S$'s morphology. In the first, we analyze stars from $\textit{Gaia}$ using proper motions to remove Sgr. In the second, we generate a simultaneous model of $300S$ and Sgr based purely on photometric information. Both approaches agree within their respective domains and describe the stream over a region spanning $33^\circ$. Overall, $300S$ has three well-defined density peaks and smooth variations in stream width. Furthermore, $300S$ has a possible gap of $\sim 4.7^\circ$ and a kink. Dynamical modeling of the kink implies that $300S$ was dramatically influenced by the Large Magellanic Cloud. This is the first model of $300S$'s morphology across its entire known footprint, opening the door for deeper analysis to constrain the structures of the Milky Way. △ Less

Submitted 20 April, 2026; v1 submitted 26 June, 2025; originally announced June 2025.

Comments: 33 Pages, 22 Figures, 13 Tables, and 3 Appendices. Published in the Open Journal of Astrophysics (OJAp). Associated Zenodo Repository: https://zenodo.org/records/15391938

Accreted Globular Clusters and Horizontal Branch Morphology in the Outer Halo of M31

Authors: Gracie McGill, Annette M. N. Ferguson, Dougal Mackey, Avon P. Huxor, Geraint F. Lewis, Nicolas F. Martin, Alan W. McConnachie, Charli M. Sakari, Nial R. Tanvir, Kim A. Venn

Abstract: M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (… ▽ More M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (CMDs), we quantify the horizontal branch (HB) morphologies and employ new empirical relationships, calibrated on Milky Way (MW) GCs, to consistently derive metallicities and line-of-sight extinctions. We find a remarkable correlation between HB morphology and the presence of substructure: GCs with very red HBs are almost exclusively associated with substructure, while non-substructure GCs have extended blue HBs. This provides the first direct evidence that red HB halo clusters originate from satellite accretion, a notion introduced nearly 50 years ago from MW studies which has remained unconfirmed until now. In addition to a more metal-rich tail, the substructure GC sample also contains a few clusters with very low metallicities and red HBs, unlike any objects known in the MW. We suggest these are recently accreted young clusters, supporting the growing evidence that M31 has experienced a more prolonged accretion history than the MW. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 7 pages, 4 figures. Accepted for publication in MNRAS

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

Authors: Eleonora Di Valentino, Jackson Levi Said, Adam Riess, Agnieszka Pollo, Vivian Poulin, Adrià Gómez-Valent, Amanda Weltman, Antonella Palmese, Caroline D. Huang, Carsten van de Bruck, Chandra Shekhar Saraf, Cheng-Yu Kuo, Cora Uhlemann, Daniela Grandón, Dante Paz, Dominique Eckert, Elsa M. Teixeira, Emmanuel N. Saridakis, Eoin Ó Colgáin, Florian Beutler, Florian Niedermann, Francesco Bajardi, Gabriela Barenboim, Giulia Gubitosi, Ilaria Musella , et al. (516 additional authors not shown)

Abstract: The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-t… ▽ More The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged] △ Less

Submitted 4 August, 2025; v1 submitted 2 April, 2025; originally announced April 2025.

Comments: 416 pages, 81 figures

Journal ref: Phys. Dark Univ. 49 (2025) 101965

Cosmography with the Double Source Plane Strong Gravitational Lens AGEL150745+052256

Authors: Nandini Sahu, Anowar J. Shajib, Kim-Vy Tran, Hannah Skobe, Sunny Rhoades, Tucker Jones, Karl Glazebrook, Thomas E. Collett, Sherry H. Suyu, Tania M. Barone, Keerthi Vasan G. C., Duncan J. Bowden, Daniel Ballard, Glenn G. Kacprzak, Sarah M. Sweet, Geraint F. Lewis, Themiya Nanayakkara

Abstract: Strong gravitational lenses with two background sources at widely separated redshifts are a powerful and independent probe of cosmological parameters. We can use these systems, known as Double-Source-Plane Lenses (DSPLs), to measure the ratio ($β$) of angular-diameter distances of the sources, which is sensitive to the matter density ($Ω_m$) and the equation-of-state parameter for dark-energy (… ▽ More Strong gravitational lenses with two background sources at widely separated redshifts are a powerful and independent probe of cosmological parameters. We can use these systems, known as Double-Source-Plane Lenses (DSPLs), to measure the ratio ($β$) of angular-diameter distances of the sources, which is sensitive to the matter density ($Ω_m$) and the equation-of-state parameter for dark-energy ($w$). However, DSPLs are rare and require high-resolution imaging and spectroscopy for detection, lens modeling, and measuring $β$. Here we report only the second DSPL ever used to measure cosmological parameters. We model the DSPL AGEL150745+052256 from the ASTRO 3D Galaxy Evolution with Lenses (AGEL) survey using HST/WFC3 imaging and Keck/KCWI spectroscopy. The spectroscopic redshifts for the deflector and two sources in AGEL1507 are $z_{\rm defl}=0.594$, $z_{\rm S1}=2.163$, and $z_{\rm S2}=2.591$. We measure a stellar velocity dispersion of $σ_{\rm obs}=109 \pm 27$ km s$^{-1}$ for the nearer source. Using $σ_{\rm obs}$ for the main deflector (from literature) and S1, we test the robustness of our DSPL model. We measure $β=0.953^{+0.008}_{-0.010}$ for AGEL1507 and infer $Ω_{\rm m}=0.33^{+0.38}_{-0.23}$ for $Λ$CDM cosmology. Combining AGEL1507 with the published model of the Jackpot lens improves the precision on $Ω_{\rm m}$ ($Λ$CDM) and w (wCDM) by $\sim 10 \%$. The inclusion of DSPLs significantly improves the constraints when combined with Plancks cosmic microwave background observations, enhancing precision on w by $30 \%$. This paper demonstrates the constraining power of DSPLs and their complementarity to other standard cosmological probes. Tighter future constraints from larger DSPL samples discovered from ongoing and forthcoming large-area sky surveys would provide insights into the nature of dark energy. △ Less

Submitted 1 April, 2025; originally announced April 2025.

Comments: 19 pages, 10 figures, Submitted for publication to the Astrophysical Journal (ApJ)

Flipping of the tidal tails of the Ophiuchus stream due to the decelerating Galactic bar

Authors: Yong Yang, Geraint F. Lewis, Denis Erkal, Ting S. Li, Andrew P. Li, Sarah L. Martell, Lara R. Cullinane, Guilherme Limberg, Daniel B. Zucker, Joss Bland-Hawthorn, Andrew B. Pace, Gary S. Da Costa, Alexander P. Ji, Sergey E. Koposov, Kyler Kuehn, Nora Shipp, Miles Pearson, Sam A. Usman, S5 Collaboration

Abstract: The Ophiuchus stellar stream presents a puzzle due to its complicated morphology, with a substructure perpendicular to the main track (spur), a broadened tail (fanning), and a shorter than expected angular extent given its old stellar population and short orbital period. The location of the stream approaches the Galactic center, implying a possible connection between its orbit and its unusual morp… ▽ More The Ophiuchus stellar stream presents a puzzle due to its complicated morphology, with a substructure perpendicular to the main track (spur), a broadened tail (fanning), and a shorter than expected angular extent given its old stellar population and short orbital period. The location of the stream approaches the Galactic center, implying a possible connection between its orbit and its unusual morphology. Here we demonstrate that the morphology of Ophiuchus can be attributed to its interaction with the decelerating Galactic bar, which leads to the flipping or transposition of its tidal tails. The short length of the stream is the result of stars stripped in the ancient past still remaining concentrated, and the spur, as well as the fanning, are composed of either leading or trailing tails built up of stars released at different time intervals. Our new spectroscopic data, obtained as part of the Southern Stellar Stream Spectroscopic Survey $(S^5)$, and modeling of Ophiuchus indicate that, in the presence of the bar, an initial leading tail can be redistributed to the trailing side and vice versa, and the morphology of a stream can be reshaped. This result confirms that the Galactic bar plays a vital role in reconstructing the orbital behavior of streams passing close to the central region of the Milky Way. △ Less

Submitted 24 March, 2025; originally announced March 2025.

Comments: Accepted by ApJ, 20 pages, 12 figures, 1 machine readable table, 4 videos

Journal ref: 2025 ApJ 984 189

The AGEL Survey Data Release 2: A Gravitational Lens Sample for Galaxy Evolution and Cosmology

Authors: Tania M. Barone, Keerthi Vasan G. C., Kim-Vy Tran, Glenn G. Kacprzak, Karl Glazebrook, Tucker Jones, Duncan J. Bowden, Faith Dalessandro, Nandini Sahu, Hannah Skobe, Rebecca J. Allen, A. Makai Baker, Daniel J. Ballard, Yuguang Chen, Thomas E. Collett, Giovanni Ferrami, Jimena Gonzalez, William Gottemoller, Anishya Harshan, Xiaosheng Huang, Leena Iwamoto, Colin Jacobs, Tesla E. Jeltema, Kaustubh Rajesh Gupta, Geraint F. Lewis , et al. (9 additional authors not shown)

Abstract: The ASTRO 3D Galaxy Evolution with Lenses (AGEL) Survey is an ongoing effort to spectroscopically confirm a diverse sample of gravitational lenses with high spatial resolution imaging, to facilitate a broad range of science outcomes. The AGEL systems span single galaxy-scale deflectors to groups and clusters, and include rare targets such as galaxy-scale lenses with multiple sources, lensed quiesc… ▽ More The ASTRO 3D Galaxy Evolution with Lenses (AGEL) Survey is an ongoing effort to spectroscopically confirm a diverse sample of gravitational lenses with high spatial resolution imaging, to facilitate a broad range of science outcomes. The AGEL systems span single galaxy-scale deflectors to groups and clusters, and include rare targets such as galaxy-scale lenses with multiple sources, lensed quiescent galaxies, and Einstein rings. We build on the 77 systems presented in Tran et al. 2022 (AGEL data release 1) to present a total 138 lenses, and high resolution F140W and F200LP Hubble Space Telescope images for 71 lenses from a completed HST SNAP program. Lens candidates were originally identified by convolutional neural networks in the DES and DECaLS imaging fields, and of the targets with follow-up spectroscopy we find a high (96%) success rate. Compared with other spectroscopic lens samples, AGEL lenses tend to have both higher redshift deflectors and sources. We briefly discuss the common causes of false-positive candidates, and strategies for mitigating false-positives in next generation lens searches. Lastly, we present 6 galaxy-scale double-source plane lenses useful for cosmological analyses. With next-generation telescopes and surveys such as Euclid, Vera Rubin's Legacy Survey of Space and Time, Keck Observatory's KAPA program, and 4MOST's 4SLSLS surveys on the horizon, the AGEL survey represents a pathfinder for refining automated candidate search methods and identifying and triaging candidates for followup based on scientific potential. △ Less

Submitted 11 March, 2025; originally announced March 2025.

Comments: Submitted to AJ. 14 figures, 5 tables

The Hubble Space Telescope Survey of M31 Satellite Galaxies IV. Survey Overview and Lifetime Star Formation Histories

Authors: A. Savino, D. R. Weisz, A. E. Dolphin, M. J. Durbin, N. Kallivayalil, A. Wetzel, J. Anderson, G. Besla, M. Boylan-Kolchin, T. M. Brown, J. S. Bullock, A. A. Cole, M. L. M. Collins, M. C. Cooper, A. J. Deason, A. L. Dotter, M. Fardal, A. M. N. Ferguson, T. K. Fritz, M. C. Geha, K. M. Gilbert, P. Guhathakurta, R. Ibata, M. J. Irwin, M. Jeon , et al. (13 additional authors not shown)

Abstract: From $>1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and… ▽ More From $>1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within $1.8$ Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. iii) $\sim50$\% of our M31 satellites show prominent ancient star formation ($>12$ Gyr ago) followed by delayed quenching ($8-10$ Gyr ago), which is not commonly observed among the MW satellites. iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts show that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST. △ Less

Submitted 22 January, 2025; originally announced January 2025.

Comments: Accepted for publication on ApJ. 47 pages, 24 figures, 12 tables. Corresponding HLSP data can be retrieved at: https://archive.stsci.edu/hlsp/m31-satellites

The GALAH survey: Improving chemical abundances using star clusters

Authors: Janez Kos, Sven Buder, Kevin L. Beeson, Joss Bland-Hawthorn, Gayandhi M. De Silva, Valentina D'Orazi, Ken Freeman, Michael Hayden, Geraint F. Lewis, Karin Lind, Sarah L. Martell, Sanjib Sharma, Daniel B. Zucker, Tomaž Zwitter, Gary S. Da Costa, Richard de Grijs, Madeline Howell, Madeleine McKenzie, Thomas Nordlander, Dennis Stello, Gregor Traven

Abstract: Large spectroscopic surveys aim to consistently compute stellar parameters of very diverse stars while minimizing systematic errors. We explore the use of stellar clusters as benchmarks to verify the precision of spectroscopic parameters in the 4. data release (DR4) of the GALAH survey. We examine 58 open and globular clusters and associations to validate measurements of temperature, gravity, chem… ▽ More Large spectroscopic surveys aim to consistently compute stellar parameters of very diverse stars while minimizing systematic errors. We explore the use of stellar clusters as benchmarks to verify the precision of spectroscopic parameters in the 4. data release (DR4) of the GALAH survey. We examine 58 open and globular clusters and associations to validate measurements of temperature, gravity, chemical abundances, and stellar ages. We focus on identifying systematic errors and understanding trends between stellar parameters, particularly temperature and chemical abundances. We identify trends by stacking measurements of chemical abundances against effective temperature and modelling them with splines. We also refit spectra in three clusters with the Spectroscopy Made Easy and Korg packages to reproduce the trends in DR4 and to search for their origin by varying temperature and gravity priors, linelists, and spectral continuum. Trends are consistent between clusters of different ages and metallicities, can reach amplitudes of ~0.5 dex and differ for dwarfs and giants. We use the derived trends to correct the DR4 abundances of 24 and 31 chemical elements for dwarfs and giants, and publish a detrended catalogue. While the origin of the trends could not be pinpointed, we found that: i) photometric priors affect derived abundances, ii) temperature, metallicity, and continuum levels are degenerate in spectral fitting, and it is hard to break the degeneracy even by using independent measurements, iii) the completeness of the linelist used in spectral synthesis is essential for cool stars, and iv) different spectral fitting codes produce significantly different iron abundances for stars of all temperatures. We conclude that clusters can be used to characterise the systematic errors of parameters produced in large surveys, but further research is needed to explain the origin of the trends. △ Less

Submitted 14 February, 2025; v1 submitted 10 January, 2025; originally announced January 2025.

Journal ref: A&A 703, A104 (2025)

Revisiting The Cosmological Time Dilation of Distant Quasars: Influence of Source Properties and Evolution

Authors: Brendon J. Brewer, Geraint F. Lewis, Yuan Li

Abstract: After decades of searching, cosmological time dilation was recently identified in the timescale of variability seen in distant quasars. Here, we expand on the previous analysis to disentangle this cosmological signal from the influence of the properties of the source population, specifically the quasar bolometric luminosity and the rest-frame emission wavelength at which the variability was observ… ▽ More After decades of searching, cosmological time dilation was recently identified in the timescale of variability seen in distant quasars. Here, we expand on the previous analysis to disentangle this cosmological signal from the influence of the properties of the source population, specifically the quasar bolometric luminosity and the rest-frame emission wavelength at which the variability was observed. Furthermore, we consider the potential influence of the evolution of the quasar population over cosmic time. We find that a significant intrinsic scatter of 0.288 +- 0.021 dex in the variability timescales, which was not considered in the previous analysis, is favoured by the data. This slightly increases the uncertainty in the results. However, the expected cosmological dependence of the variability timescales is confirmed to be robust to changes in the underlying assumptions. We find that the variability timescales increase smoothly with both wavelength and bolometric luminosity, and that black hole mass has no effect on the variability timescale once rest wavelength and bolometric luminosity are accounted for. Moreover, if the standard cosmological model is correct, governed by relativistic expansion, we also find very little cosmological evolution in the intrinsic variability timescales of distant quasars. △ Less

Submitted 13 January, 2026; v1 submitted 7 January, 2025; originally announced January 2025.

Comments: 8 pages, 5 figures. Accepted for publication in MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 537, Issue 2, February 2025, Pages 809-816

Cosmic Multipoles in Galaxy Surveys Part I: How Inferences Depend on Source Counts and Masks

Authors: Oliver T. Oayda, Vasudev Mittal, Geraint F. Lewis

Abstract: We present a new approach to constructing and fitting dipoles and higher-order multipoles in synthetic galaxy samples over the sky. Within our Bayesian paradigm, we illustrate that this technique is robust to masked skies, allowing us to make credible inferences about the relative contributions of each multipole. We also show that dipoles can be recovered in surveys with small footprints, determin… ▽ More We present a new approach to constructing and fitting dipoles and higher-order multipoles in synthetic galaxy samples over the sky. Within our Bayesian paradigm, we illustrate that this technique is robust to masked skies, allowing us to make credible inferences about the relative contributions of each multipole. We also show that dipoles can be recovered in surveys with small footprints, determining the requisite source counts required for concrete estimation of the dipole parameters. This work is motivated by recent probes of the cosmic dipole in galaxy catalogues. Namely, the kinematic dipole of the Cosmic Microwave Background, as arising from the motion of our heliocentric frame at $\approx 370\ \text{km}\,\text{s}^{-1}$, implies that an analogous dipole should be observed in the number counts of galaxies in flux-density-limited samples. Recent studies have reported a dipole aligning with the kinematic dipole but with an anomalously large amplitude. Accordingly, our new technique will be important as forthcoming galaxy surveys are made available and for revisiting previous data. △ Less

Submitted 17 December, 2024; originally announced December 2024.

Comments: 20 pages, 21 figures, accepted for publication in MNRAS

$S^5$: New insights from deep spectroscopic observations of the tidal tails of the globular clusters NGC 1261 and NGC 1904

Authors: Petra Awad, Ting S. Li, Denis Erkal, Reynier F. Peletier, Kerstin Bunte, Sergey E. Koposov, Andrew Li, Eduardo Balbinot, Rory Smith, Marco Canducci, Peter Tino, Alexandra M. Senkevich, Lara R. Cullinane, Gary S. Da Costa, Alexander P. Ji, Kyler Kuehn, Geraint F. Lewis, Andrew B. Pace, Daniel B. Zucker, Joss Bland-Hawthorn, Guilherme Limberg, Sarah L. Martell, Madeleine McKenzie, Yong Yang, Sam A. Usman

Abstract: As globular clusters (GCs) orbit the Milky Way, their stars are tidally stripped forming tidal tails that follow the orbit of the clusters around the Galaxy. The morphology of these tails is complex and shows correlations with the phase of the orbit and the orbital angular velocity, especially for GCs on eccentric orbits. Here, we focus on two GCs, NGC 1261 and NGC 1904, that have potentially been… ▽ More As globular clusters (GCs) orbit the Milky Way, their stars are tidally stripped forming tidal tails that follow the orbit of the clusters around the Galaxy. The morphology of these tails is complex and shows correlations with the phase of the orbit and the orbital angular velocity, especially for GCs on eccentric orbits. Here, we focus on two GCs, NGC 1261 and NGC 1904, that have potentially been accreted alongside Gaia-Enceladus and that have shown signatures of having, in addition of tidal tails, structures formed by distributions of extra-tidal stars that are misaligned with the general direction of the clusters' respective orbits. To provide an explanation for the formation of these structures, we make use of spectroscopic measurements from the Southern Stellar Stream Spectroscopic Survey ($S^5$) as well as proper motion measurements from Gaia's third data release (DR3), and apply a Bayesian mixture modeling approach to isolate high-probability member stars. We recover extra-tidal features similar to those found in Shipp et al. (2018) surrounding each cluster. We conduct N-body simulations and compare the expected distribution and variation in the dynamical parameters along the orbit with those of our potential member sample. Furthermore, we use Dark Energy Camera (DECam) photometry to inspect the distribution of the member stars in the color-magnitude diagram (CMD). We find that the potential members agree reasonably with the N-body simulations and that the majority of them follow a simple stellar population-like distribution in the CMD which is characteristic of GCs. In the case of NGC 1904, we clearly detect the tidal debris escaping the inner and outer Lagrange points which are expected to be prominent when at or close to the apocenter of its orbit. Our analysis allows for further exploration of other GCs in the Milky Way that exhibit similar extra-tidal features. △ Less

Submitted 13 November, 2024; originally announced November 2024.

Journal ref: A&A 693, A69 (2025)

Chemical Abundances in the Leiptr Stellar Stream: A Disrupted Ultra-faint Dwarf Galaxy?

Authors: Kaia R. Atzberger, Sam A. Usman, Alexander P. Ji, Lara R. Cullinane, Denis Erkal, Terese T. Hansen, Geraint F. Lewis, Ting S. Li, Guilherme Limberg, Alice Luna, Sarah L. Martell, Madeleine McKenzie, Andrew B. Pace, Daniel B. Zucker

Abstract: Chemical abundances of stellar streams can be used to determine the nature of a stream's progenitor. Here we study the progenitor of the recently discovered Leiptr stellar stream, which was previously suggested to be a tidally disrupted halo globular cluster. We obtain high-resolution spectra of five red giant branch stars selected from the Gaia DR2 STREAMFINDER catalog with Magellan/MIKE. One sta… ▽ More Chemical abundances of stellar streams can be used to determine the nature of a stream's progenitor. Here we study the progenitor of the recently discovered Leiptr stellar stream, which was previously suggested to be a tidally disrupted halo globular cluster. We obtain high-resolution spectra of five red giant branch stars selected from the Gaia DR2 STREAMFINDER catalog with Magellan/MIKE. One star is a clear non-member. The remaining four stars display chemical abundances consistent with those of a low-mass dwarf galaxy: they have a low mean metallicity, $\langle{\rm[Fe/H]}\rangle = -2.2$; they do not all have identical metallicities; and they display low [$α$/Fe] $\sim 0$ and [Sr/Fe] and [Ba/Fe] $\sim -1$. This pattern of low $α$ and neutron-capture element abundances is only found in intact dwarf galaxies with stellar mass $\lesssim 10^5 M_\odot$. Although more data are needed to be certain, Leiptr's chemistry is consistent with being the lowest-mass dwarf galaxy stream without a known intact progenitor, possibly in the mass range of ultra-faint dwarf galaxies. Leiptr thus preserves a record of one of the lowest-mass early accretion events into the Milky Way. △ Less

Submitted 29 May, 2025; v1 submitted 22 October, 2024; originally announced October 2024.

Comments: 19 pages, 12 figures, 8 tables, Final version accepted for publication in The Open Journal of Astrophysics

Journal ref: OJA 8 (June 2025)

The GALAH Survey: Stellar parameters and abundances for 800,000 Gaia RVS spectra using GALAH DR4 and The Cannon

Authors: Pradosh Barun Das, Daniel B. Zucker, Gayandhi M. De Silva, Nicholas W. Borsato, Aldo Mura-Guzmán, Sven Buder, Melissa Ness, Thomas Nordlander, Andrew R. Casey, Sarah L. Martell, Joss Bland-Hawthorn, Richard de Grijs, Ken C. Freeman, Janez Kos, Dennis Stello, Geraint F. Lewis, Michael R. Hayden, Sanjib Sharma

Abstract: Analysing stellar parameters and abundances from nearly one million Gaia Data Release 3 (DR3) Radial Velocity Spectrometer (RVS) spectra poses challenges due to the limited spectral coverage (restricted to the infrared Ca II triplet) and variable signal-to-noise ratios of the data. To address this, we use The Cannon, a data-driven method, to transfer stellar parameters and abundances from the GALA… ▽ More Analysing stellar parameters and abundances from nearly one million Gaia Data Release 3 (DR3) Radial Velocity Spectrometer (RVS) spectra poses challenges due to the limited spectral coverage (restricted to the infrared Ca II triplet) and variable signal-to-noise ratios of the data. To address this, we use The Cannon, a data-driven method, to transfer stellar parameters and abundances from the GALAH (GALactic Archaeology with HERMES) DR4 (R ~ 28,000) catalogue to the lower resolution Gaia DR3 RVS spectra (R ~ 11,500). Our model, trained on 14,484 common targets, predicts parameters such as $T_{\text{eff}}$, $\log g$, and $[\text{Fe/H}]$, along with several other elements across approximately 800,000 Gaia RVS spectra. We utilise stars from open and globular clusters present in the Gaia RVS catalogue to validate our predicted mean $[\text{Fe/H}]$ with high precision (~0.02-0.10 dex). Additionally, we recover the bimodal distribution of $[\text{Ti/Fe}]$ versus $[\text{Fe/H}]$, reflecting the high and low $α$-components of Milky Way disc stars, demonstrating The Cannon's capability for accurate stellar abundance determination from medium-resolution Gaia RVS spectra. The methodologies and resultant catalogue presented in this work highlight the remarkable potential of the RVS dataset, which by the end of the Gaia mission will comprise spectra of over 200 million stars. △ Less

Submitted 12 March, 2025; v1 submitted 16 October, 2024; originally announced October 2024.

Comments: Published in MNRAS, 17 pages, 15 figures. The resulting catalogue is available at CDS via anonymous ftp to https://cdsarc.u-strasbg.fr or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/538/605

Journal ref: MNRAS 538, 605-621 (2025)

The GALAH Survey: Data Release 4

Authors: S. Buder, J. Kos, E. X. Wang, M. McKenzie, M. Howell, S. L. Martell, M. R. Hayden, D. B. Zucker, T. Nordlander, B. T. Montet, G. Traven, J. Bland-Hawthorn, G. M. De Silva, K. C. Freeman, G. F. Lewis, K. Lind, S. Sharma, J. D. Simpson, D. Stello, T. Zwitter, A. M. Amarsi, J. J. Armstrong, K. Banks, M. A. Beavis, K. Beeson , et al. (14 additional authors not shown)

Abstract: The stars of the Milky Way carry the chemical history of our Galaxy in their atmospheres as they journey through its vast expanse. Like barcodes, we can extract the chemical fingerprints of stars from high-resolution spectroscopy. The fourth data release (DR4) of the Galactic Archaeology with HERMES (GALAH) Survey, based on a decade of observations, provides the chemical abundances of up to 32 ele… ▽ More The stars of the Milky Way carry the chemical history of our Galaxy in their atmospheres as they journey through its vast expanse. Like barcodes, we can extract the chemical fingerprints of stars from high-resolution spectroscopy. The fourth data release (DR4) of the Galactic Archaeology with HERMES (GALAH) Survey, based on a decade of observations, provides the chemical abundances of up to 32 elements for 917 588 stars that also have exquisite astrometric data from the $Gaia$ satellite. For the first time, these elements include life-essential nitrogen to complement carbon, and oxygen as well as more measurements of rare-earth elements critical to modern-life electronics, offering unparalleled insights into the chemical composition of the Milky Way. For this release, we use neural networks to simultaneously fit stellar parameters and abundances across the whole wavelength range, leveraging synthetic grids computed with Spectroscopy Made Easy. These grids account for atomic line formation in non-local thermodynamic equilibrium for 14 elements. In a two-iteration process, we first fit stellar labels to all 1 085 520 spectra, then co-add repeated observations and refine these labels using astrometric data from $Gaia$ and 2MASS photometry, improving the accuracy and precision of stellar parameters and abundances. Our validation thoroughly assesses the reliability of spectroscopic measurements and highlights key caveats. GALAH DR4 represents yet another milestone in Galactic archaeology, combining detailed chemical compositions from multiple nucleosynthetic channels with kinematic information and age estimates. The resulting dataset, covering nearly a million stars, opens new avenues for understanding not only the chemical and dynamical history of the Milky Way, but also the broader questions of the origin of elements and the evolution of planets, stars, and galaxies. △ Less

Submitted 11 March, 2025; v1 submitted 29 September, 2024; originally announced September 2024.

Comments: 45 pages, 40 figures. Accepted for publication in PASA. Accompanying the GALAH Data Release 4, see https://www.galah-survey.org and https://cloud.datacentral.org.au/teamdata/GALAH/public/GALAH_DR4/. All code available on https://github.com/svenbuder/GALAH_DR4/ and https://github.com/svenbuder/galah_dr4_paper. Comments welcome

Gravitational Lensing Reveals Cool Gas within 10-20 kpc around a Quiescent Galaxy

Authors: Tania M. Barone, Glenn G. Kacprzak, James W. Nightingale, Nikole M. Nielsen, Karl Glazebrook, Kim-Vy H. Tran, Tucker Jones, Hasti Nateghi, Keerthi Vasan G. C., Nandini Sahu, Themiya Nanayakkara, Hannah Skobe, Jesse van de Sande, Sebastian Lopez, Geraint F. Lewis

Abstract: While quiescent galaxies have comparable amounts of cool gas in their outer circumgalactic medium (CGM) compared to star-forming galaxies, they have significantly less interstellar gas. However, open questions remain on the processes causing galaxies to stop forming stars and stay quiescent . Theories suggest dynamical interactions with the hot corona prevent cool gas from reaching the galaxy, the… ▽ More While quiescent galaxies have comparable amounts of cool gas in their outer circumgalactic medium (CGM) compared to star-forming galaxies, they have significantly less interstellar gas. However, open questions remain on the processes causing galaxies to stop forming stars and stay quiescent . Theories suggest dynamical interactions with the hot corona prevent cool gas from reaching the galaxy, therefore predicting the inner regions of quiescent galaxy CGMs are devoid of cool gas. However, there is a lack of understanding of the inner regions of CGMs due to the lack of spatial information in quasar-sightline methods. We present integral-field spectroscopy probing 10--20~kpc (2.4--4.8 R\textsubscript{e}) around a massive quiescent galaxy using a gravitationally lensed star-forming galaxy. We detect absorption from Magnesium (MgII) implying large amounts of cool atomic gas (10\textsuperscript{8.4} -- 10\textsuperscript{9.3} M\textsubscript{$\odot$} with T$\sim$10\textsuperscript{4} Kelvin), in comparable amounts to star-forming galaxies. Lens modeling of Hubble imaging also reveals a diffuse asymmetric component of significant mass consistent with the spatial extent of the MgII absorption, and offset from the galaxy light profile. This study demonstrates the power of galaxy-scale gravitational lenses to not only probe the gas around galaxies, but to also independently probe the mass of the CGM due to it's gravitational effect. △ Less

Submitted 15 August, 2024; originally announced August 2024.

Comments: accepted nature communications physics

Modelling the impact of host galaxy dust on type Ia supernova distance measurements

Authors: B. Popovic, P. Wiseman, M. Sullivan, M. Smith, S. González-Gaitán, D. Scolnic, J. Duarte, P. Armstrong, J. Asorey, D. Brout, D. Carollo, L. Galbany, K. Glazebrook, L. Kelsey, R. Kessler, C. Lidman, J. Lee, G. F. Lewis, A. Möller, R. C. Nichol, B. O. Sánchez, M. Toy, B. E. Tucker, M. Vincenzi, T. M. C. Abbott , et al. (43 additional authors not shown)

Abstract: Type Ia Supernovae (SNe Ia) are a critical tool in measuring the accelerating expansion of the universe. Recent efforts to improve these standard candles have focused on incorporating the effects of dust on distance measurements with SNe Ia. In this paper, we use the state-of-the-art Dark Energy Survey 5 year sample to evaluate two different families of dust models: empirical extinction models der… ▽ More Type Ia Supernovae (SNe Ia) are a critical tool in measuring the accelerating expansion of the universe. Recent efforts to improve these standard candles have focused on incorporating the effects of dust on distance measurements with SNe Ia. In this paper, we use the state-of-the-art Dark Energy Survey 5 year sample to evaluate two different families of dust models: empirical extinction models derived from SNe Ia data, and physical attenuation models from the spectra of galaxies. Among the SNe Ia-derived models, we find that a logistic function of the total-to-selective extinction RV best recreates the correlations between supernova distance measurements and host galaxy properties, though an additional 0.02 magnitudes of grey scatter are needed to fully explain the scatter in SNIa brightness in all cases. These empirically-derived extinction distributions are highly incompatible with the physical attenuation models from galactic spectral measurements. From these results, we conclude that SNe Ia must either preferentially select extreme ends of galactic dust distributions, or that the characterisation of dust along the SNe Ia line-of-sight is incompatible with that of galactic dust distributions. △ Less

Submitted 7 June, 2024; originally announced June 2024.

The Dark Energy Survey Supernova Program: Slow supernovae show cosmological time dilation out to $z \sim 1$

Authors: R. M. T. White, T. M. Davis, G. F. Lewis, D. Brout, L. Galbany, K. Glazebrook, S. R. Hinton, J. Lee, C. Lidman, A. Möller, M. Sako, D. Scolnic, M. Smith, M. Sullivan, B. O. Sánchez, P. Shah, M. Vincenzi, P. Wiseman, T. M. C. Abbott, M. Aguena, S. Allam, F. Andrade-Oliveira, J. Asorey, D. Bacon, S. Bocquet , et al. (45 additional authors not shown)

Abstract: We present a precise measurement of cosmological time dilation using the light curves of 1504 type Ia supernovae from the Dark Energy Survey spanning a redshift range $0.1\lesssim z\lesssim 1.2$. We find that the width of supernova light curves is proportional to $(1+z)$, as expected for time dilation due to the expansion of the Universe. Assuming type Ia supernovae light curves are emitted with a… ▽ More We present a precise measurement of cosmological time dilation using the light curves of 1504 type Ia supernovae from the Dark Energy Survey spanning a redshift range $0.1\lesssim z\lesssim 1.2$. We find that the width of supernova light curves is proportional to $(1+z)$, as expected for time dilation due to the expansion of the Universe. Assuming type Ia supernovae light curves are emitted with a consistent duration $Δt_{\rm em}$, and parameterising the observed duration as $Δt_{\rm obs}=Δt_{\rm em}(1+z)^b$, we fit for the form of time dilation using two methods. Firstly, we find that a power of $b \approx 1$ minimises the flux scatter in stacked subsamples of light curves across different redshifts. Secondly, we fit each target supernova to a stacked light curve (stacking all supernovae with observed bandpasses matching that of the target light curve) and find $b=1.003\pm0.005$ (stat) $\pm\,0.010$ (sys). Thanks to the large number of supernovae and large redshift-range of the sample, this analysis gives the most precise measurement of cosmological time dilation to date, ruling out any non-time-dilating cosmological models at very high significance. △ Less

Submitted 20 August, 2024; v1 submitted 7 June, 2024; originally announced June 2024.

Comments: 14 pages, 13 figures. Updated in response to reviewer feedback. Accepted for publication in the Monthly Notices of the Royal Astronomical Society (MNRAS)

Report number: FERMILAB-PUB-24-0293-PPD, DES-2024-0831

The Dark Energy Survey Supernova Program: An updated measurement of the Hubble constant using the Inverse Distance Ladder

Authors: R. Camilleri, T. M. Davis, S. R. Hinton, P. Armstrong, D. Brout, L. Galbany, K. Glazebrook, J. Lee, C. Lidman, R. C. Nichol, M. Sako, D. Scolnic, P. Shah, M. Smith, M. Sullivan, B. O. Sánchez, M. Vincenzi, P. Wiseman, S. Allam, T. M. C. Abbott, M. Aguena, F. Andrade-Oliveira, J. Asorey, S. Avila, D. Bacon , et al. (55 additional authors not shown)

Abstract: We measure the current expansion rate of the Universe, Hubble's constant $H_0$, by calibrating the absolute magnitudes of supernovae to distances measured by Baryon Acoustic Oscillations. This `inverse distance ladder' technique provides an alternative to calibrating supernovae using nearby absolute distance measurements, replacing the calibration with a high-redshift anchor. We use the recent rel… ▽ More We measure the current expansion rate of the Universe, Hubble's constant $H_0$, by calibrating the absolute magnitudes of supernovae to distances measured by Baryon Acoustic Oscillations. This `inverse distance ladder' technique provides an alternative to calibrating supernovae using nearby absolute distance measurements, replacing the calibration with a high-redshift anchor. We use the recent release of 1829 supernovae from the Dark Energy Survey spanning $0.01\lt z \lt1.13$ anchored to the recent Baryon Acoustic Oscillation measurements from DESI spanning $0.30 \lt z_{\mathrm{eff}} \lt 2.33$. To trace cosmology to $z=0$, we use the third-, fourth- and fifth-order cosmographic models, which, by design, are agnostic about the energy content and expansion history of the universe. With the inclusion of the higher-redshift DESI-BAO data, the third-order model is a poor fit to both data sets, with the fourth-order model being preferred by the Akaike Information Criterion. Using the fourth-order cosmographic model, we find $H_0=67.19^{+0.66}_{-0.64}\mathrm{~km} \mathrm{~s}^{-1} \mathrm{~Mpc}^{-1}$, in agreement with the value found by Planck without the need to assume Flat-$Λ$CDM. However the best-fitting expansion history differs from that of Planck, providing continued motivation to investigate these tensions. △ Less

Submitted 7 June, 2024; originally announced June 2024.

The Dark Energy Survey Supernova Program: Light curves and 5-Year data release

Authors: B. O. Sánchez, D. Brout, M. Vincenzi, M. Sako, K. Herner, R. Kessler, T. M. Davis, D. Scolnic, M. Acevedo, J. Lee, A. Möller, H. Qu, L. Kelsey, P. Wiseman, P. Armstrong, B. Rose, R. Camilleri, R. Chen, L. Galbany, E. Kovacs, C. Lidman, B. Popovic, M. Smith, M. Sullivan, M. Toy , et al. (60 additional authors not shown)

Abstract: We present $griz$ photometric light curves for the full 5 years of the Dark Energy Survey Supernova program (DES-SN), obtained with both forced Point Spread Function (PSF) photometry on Difference Images (DIFFIMG) performed during survey operations, and Scene Modelling Photometry (SMP) on search images processed after the survey. This release contains $31,636$ DIFFIMG and $19,706$ high-quality SMP… ▽ More We present $griz$ photometric light curves for the full 5 years of the Dark Energy Survey Supernova program (DES-SN), obtained with both forced Point Spread Function (PSF) photometry on Difference Images (DIFFIMG) performed during survey operations, and Scene Modelling Photometry (SMP) on search images processed after the survey. This release contains $31,636$ DIFFIMG and $19,706$ high-quality SMP light curves, the latter of which contains $1635$ photometrically-classified supernovae that pass cosmology quality cuts. This sample spans the largest redshift ($z$) range ever covered by a single SN survey ($0.1<z<1.13$) and is the largest single sample from a single instrument of SNe ever used for cosmological constraints. We describe in detail the improvements made to obtain the final DES-SN photometry and provide a comparison to what was used in the DES-SN3YR spectroscopically-confirmed SN Ia sample. We also include a comparative analysis of the performance of the SMP photometry with respect to the real-time DIFFIMG forced photometry and find that SMP photometry is more precise, more accurate, and less sensitive to the host-galaxy surface brightness anomaly. The public release of the light curves and ancillary data can be found at https://github.com/des-science/DES-SN5YR. Finally, we discuss implications for future transient surveys, such as the forthcoming Vera Rubin Observatory Legacy Survey of Space and Time (LSST). △ Less

Submitted 7 June, 2024; originally announced June 2024.

A Bayesian approach to the cosmic dipole in radio galaxy surveys: Joint analysis of NVSS & RACS

Authors: Oliver T. Oayda, Vasudev Mittal, Geraint F. Lewis, Tara Murphy

Abstract: We examine the sky distribution of radio galaxies in the NRAO VLA Sky Survey (NVSS) and the Rapid ASKAP Continuum Survey (RACS). Analyses of these samples have reported tension between their inferred dipoles and the kinematic dipole of the Cosmic Microwave Background (CMB). This represents a challenge to the traditional assumption that the Universe is homogeneous and isotropic on large scales: the… ▽ More We examine the sky distribution of radio galaxies in the NRAO VLA Sky Survey (NVSS) and the Rapid ASKAP Continuum Survey (RACS). Analyses of these samples have reported tension between their inferred dipoles and the kinematic dipole of the Cosmic Microwave Background (CMB). This represents a challenge to the traditional assumption that the Universe is homogeneous and isotropic on large scales: the cosmological principle. We find that NVSS and RACS contain local radio sources which give a non-negligible contribution to the overall dipole signal. These need to be adequately accounted for since the aim is to probe the composition of the Universe at large scales. By appropriately considering these sources, the inferred dipole amplitude in either sample is reduced. Nonetheless, we find support for a dipole aligning with that of the CMB but larger in amplitude, especially in the joint analysis. However, the 'clustering dipole' - the contribution of local sources to the net inferred dipole - appears to align with the direction of the CMB dipole, and its magnitude increases as deeper nearby sources are considered up to a comoving distance of $\approx 130$ Mpc ($h=0.7$). The significance of this observation in the context of the cosmological principle is unclear and prompts further inquiry. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Comments: 15 pages, 13 figures, accepted for publication in MNRAS

AGEL: Is the Conflict Real? Investigating Galaxy Evolution Models using Strong Lensing at 0.3 < z < 0.9

Authors: Nandini Sahu, Kim-Vy Tran, Sherry H. Suyu, Anowar J. Shajib, Sebastian Ertl, Glenn G. Kacprzak, Karl Glazebrook, Tucker Jones, Keerthi Vasan G. C., Tania M. Barone, A. Makai Baker, Hannah Skobe, Caro Derkenne, Geraint F. Lewis, Sarah M. Sweet, Sebastian Lopez

Abstract: Observed evolution of the total mass distribution with redshift is crucial to testing galaxy evolution theories. To measure the total mass distribution, strong gravitational lenses complement the resolved dynamical observations currently limited to $z \lesssim 0.5$. Here we present the lens models for a pilot sample of seven galaxy-scale lenses from the ASTRO3D Galaxy Evolution with Lenses (AGEL)… ▽ More Observed evolution of the total mass distribution with redshift is crucial to testing galaxy evolution theories. To measure the total mass distribution, strong gravitational lenses complement the resolved dynamical observations currently limited to $z \lesssim 0.5$. Here we present the lens models for a pilot sample of seven galaxy-scale lenses from the ASTRO3D Galaxy Evolution with Lenses (AGEL) survey. The AGEL lenses, modeled using HST/WFC3-F140W images with Gravitational Lens Efficient Explorer (GLEE) software, have deflector redshifts between $0.3 < z_{\rm defl} < 0.9$. Assuming a power-law density profile with slope $γ$, we measure the total density profile for the deflector galaxies via lens modeling. We also measure the stellar velocity dispersions ($σ_{\rm obs}$) for four lenses and obtain $σ_{\rm obs}$ from SDSS-BOSS for the remaining lenses to test our lens models by comparing observed and model-predicted velocity dispersions. For the seven AGEL lenses, we measure an average density profile slope of $-1.95 \pm 0.09$ and a $γ$--$z$ relation that does not evolve with redshift at $z<1$. Although our result is consistent with some observations and simulations, it differs from other studies at $z<1$ that suggest the $γ$--$z$ relation evolves with redshift. The apparent conflicts among observations and simulations may be due to a combination of 1) systematics in the lensing and dynamical modeling; 2) challenges in comparing observations with simulations; and 3) assuming a simple power-law for the total mass distribution. By providing more lenses at $z_{\rm defl} > 0.5$, the AGEL survey will provide stronger constraints on whether the mass profiles evolve with redshift as predicted by current theoretical models. △ Less

Submitted 21 June, 2024; v1 submitted 24 May, 2024; originally announced May 2024.

Comments: Accepted for publication in the Astrophysical Journal (ApJ) 24 Pages, 8 Figures, 3 Tables

Rotation of the Globular Cluster Population of the Dark Matter Deficient Galaxy NGC 1052-DF4: Implication for the Total Mass

Authors: Yuan Li, Brendon J. Brewer, Geraint F. Lewis

Abstract: We explore the globular cluster population of NGC 1052-DF4, a dark matter deficient galaxy, using Bayesian inference to search for the presence of rotation. The existence of such a rotating component is relevant to the estimation of the mass of the galaxy, and therefore the question of whether NGC 1052-DF4 is truly deficient of dark matter, similar to NGC 1052-DF2 another galaxy in the same group.… ▽ More We explore the globular cluster population of NGC 1052-DF4, a dark matter deficient galaxy, using Bayesian inference to search for the presence of rotation. The existence of such a rotating component is relevant to the estimation of the mass of the galaxy, and therefore the question of whether NGC 1052-DF4 is truly deficient of dark matter, similar to NGC 1052-DF2 another galaxy in the same group. The rotational characteristics of seven globular clusters in NGC 1052-DF4 were investigated, finding that a non-rotating kinematic model has a higher Bayesian evidence than a rotating model, by a factor of approximately 2.5. In addition, we find that under the assumption of rotation, its amplitude must be small. This distinct lack of rotation strengthens the case that, based on its intrinsic velocity dispersion, NGC 1052-DF4 is a truly dark matter deficient galaxy. △ Less

Submitted 13 January, 2026; v1 submitted 16 May, 2024; originally announced May 2024.

Comments: 9 pages 6 figures. Accepted for publication in PASA

Journal ref: Publications of the Astronomical Society of Australia , Volume 41 , 2024 , e073

OzDES Reverberation Mapping Program: Stacking analysis with H$β$, Mg II and C IV

Authors: Umang Malik, Rob Sharp, A. Penton, Z. Yu, P. Martini, B. E. Tucker, T. M. Davis, G. F. Lewis, C. Lidman, M. Aguena, O. Alves, J. Annis, J. Asorey, D. Bacon, D. Brooks, A. Carnero Rosell, J. Carretero, T. -Y. Cheng, L. N. da Costa, M. E. S. Pereira, J. De Vicente, P. Doel, I. Ferrero, J. Frieman, G. Giannini , et al. (25 additional authors not shown)

Abstract: Reverberation mapping is the leading technique used to measure direct black hole masses outside of the local Universe. Additionally, reverberation measurements calibrate secondary mass-scaling relations used to estimate single-epoch virial black hole masses. The Australian Dark Energy Survey (OzDES) conducted one of the first multi-object reverberation mapping surveys, monitoring 735 AGN up to… ▽ More Reverberation mapping is the leading technique used to measure direct black hole masses outside of the local Universe. Additionally, reverberation measurements calibrate secondary mass-scaling relations used to estimate single-epoch virial black hole masses. The Australian Dark Energy Survey (OzDES) conducted one of the first multi-object reverberation mapping surveys, monitoring 735 AGN up to $z\sim4$, over 6 years. The limited temporal coverage of the OzDES data has hindered recovery of individual measurements for some classes of sources, particularly those with shorter reverberation lags or lags that fall within campaign season gaps. To alleviate this limitation, we perform a stacking analysis of the cross-correlation functions of sources with similar intrinsic properties to recover average composite reverberation lags. This analysis leads to the recovery of average lags in each redshift-luminosity bin across our sample. We present the average lags recovered for the H$β$, Mg II and C IV samples, as well as multi-line measurements for redshift bins where two lines are accessible. The stacking analysis is consistent with the Radius-Luminosity relations for each line. Our results for the H$β$ sample demonstrate that stacking has the potential to improve upon constraints on the $R-L$ relation, which have been derived only from individual source measurements until now. △ Less

Submitted 9 May, 2024; originally announced May 2024.

Comments: 20 pages, 15 figures. Accepted by MNRAS

Report number: FERMILAB-PUB-23-381-PPD

The GALAH survey: Tracing the Milky Way's formation and evolution through RR Lyrae stars

Authors: Valentina D'Orazi, Nicholas Storm, Andrew R. Casey, Vittorio F. Braga, Alice Zocchi, Giuseppe Bono, Michele Fabrizio, Christopher Sneden, Davide Massari, Riano E. Giribaldi, Maria Bergemann, Simon W. Campbell, Luca Casagrande, Richard de Grijs, Gayandhi De Silva, Maria Lugaro, Daniel B. Zucker, Angela Bragaglia, Diane Feuillet, Giuliana Fiorentino, Brian Chaboyer, Massimo Dall'Ora, Massimo Marengo, Clara E. Martínez-Vázquez, Noriyuki Matsunaga , et al. (17 additional authors not shown)

Abstract: Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicat… ▽ More Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H]$\approx -$1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from $-$1 to approximately solar metallicity. This sub-group includes almost ~1/4 of the sample, it is characterised by thin disc kinematics and displays sub-solar $α$-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams. △ Less

Submitted 7 May, 2024; originally announced May 2024.

Comments: Accepted for publication in MNRAS. 29 pages, 20 figures

Inferring dark matter subhalo properties from simulated subhalo-stream encounters

Authors: Tariq Hilmi, Denis Erkal, Sergey E. Koposov, Ting S. Li, Sophia Lilleengen, Alexander P. Ji, Geraint F. Lewis, Nora Shipp, Andrew B. Pace, Daniel B. Zucker, Guilherme Limberg, Sam A. Usman

Abstract: In the cold dark matter paradigm, our Galaxy is predicted to contain >10000 dark matter subhaloes in the $10^5-10^8M_\odot$ range which should be completely devoid of stars. Stellar streams are sensitive to the presence of these subhaloes, which can create small-scale features in streams if they pass closely enough. Modelling these encounters can therefore, potentially recover the subhalo's proper… ▽ More In the cold dark matter paradigm, our Galaxy is predicted to contain >10000 dark matter subhaloes in the $10^5-10^8M_\odot$ range which should be completely devoid of stars. Stellar streams are sensitive to the presence of these subhaloes, which can create small-scale features in streams if they pass closely enough. Modelling these encounters can therefore, potentially recover the subhalo's properties. In this work, we demonstrate this for streams generated in numerical simulations, modelled on eccentric orbits in a realistic Milky Way potential, which includes the Large Magellanic Cloud and the subhalo itself. We focus on a mock model of the ATLAS-Aliqa Uma stream and inject a $10^7 M_\odot$ subhalo, creating a similar discontinuous morphology to current observations. We then explore how well subhalo properties are recovered using mock stream observations, consisting of no observational errors, as well as assuming realistic observational setups. These setups include present day style observations, and what will be possible with 4MOST and Gaia DR5 in the future. We show that we can recover all parameters describing the impact even with uncertainties matching existing data, including subhalo positions, velocities, mass and scale radius. Modelling the subhalo on an orbit instead of assuming an impulse approximation, we greatly reduce the degeneracy between subhalo mass and velocity seen in previous works. However, we find a slight bias in the subhalo mass (~0.1 dex). This demonstrates that we should be able to reliably extract the properties of subhaloes with stellar streams in the near future. △ Less

Submitted 3 April, 2024; originally announced April 2024.

Comments: 13 pages, 14 figures. Submitted to MMRAS. Comments welcome!