Showing 1–50 of 51 results for author: Lancaster, L

Illuminating M82: Simulating X-ray Emission from Galactic Winds in a Starburst Galaxy

Authors: Allison Lin, Lachlan Lancaster, Viraj Pandya, Drummond B. Fielding, Greg L. Bryan, John A. ZuHone, Sebastian Lopez, Laura A. Lopez, Christopher Carr

Abstract: We generate mock X-ray observations from a suite of idealized high-resolution ($\sim 4$ pc), tall-box ($\sim 2 \times 2 \times 8$ kpc$^3$) simulations of star formation driven galactic winds in an M82-like system, varying the spatial resolution as well as the strength and distribution of supernova (SN) energy injection. We compare our mock X-ray observations with deep Chandra observations of the h… ▽ More We generate mock X-ray observations from a suite of idealized high-resolution ($\sim 4$ pc), tall-box ($\sim 2 \times 2 \times 8$ kpc$^3$) simulations of star formation driven galactic winds in an M82-like system, varying the spatial resolution as well as the strength and distribution of supernova (SN) energy injection. We compare our mock X-ray observations with deep Chandra observations of the hot plasma around M82. While the simulated total X-ray luminosity, $L_X$, increases with resolution and when SNe feedback is spatially distributed, even in the best case scenario, our simulated $L_X$ is a factor of $\sim 50-100$ lower than observed and the surface brightness profiles of X-ray emission, $S_X(z)$, fall off too quickly with distance from the galaxy. Past results were able to reproduce these observables and we discuss potential simulation differences that could explain this discrepancy. We make the first comparison of the X-ray spectrum of our simulations to observations and find that our simulated spectrum is too soft, with a deficit of hard X-ray photons at $\gtrsim 1$ keV. We discuss how physical processes missing from our simulations and prior work (e.g., thermal conduction and cosmic rays) could help resolve this discrepancy. △ Less

Submitted 13 July, 2026; originally announced July 2026.

Comments: 16 pages, 7 figures, 2 tables. Submitted to The Astrophysical Journal

Clumps in a Cocoon: Geometry and Mixing Set the Universal X-ray to H$α$ Surface Brightness Ratio

Authors: Zirui Chen, S. Peng Oh, Drummond B. Fielding, Lachlan Lancaster, Yuan Li, Brent Tan

Abstract: Recent observations reveal a universal X-ray to H$α$ surface-brightness ratio, ${\rm SB}_{\rm X}/{\rm SB}_{\rm Hα}\sim 3$, in galactic winds, ram-pressure stripped tails, and cluster filaments. This is surprising because H$α$ traces cold ($\sim 10^4$ K) gas while X-rays trace much hotter ($\sim 10^{6}$--$10^{7}$ K) gas. Plane-parallel mixing-layer models do not recover this ratio, and can be off b… ▽ More Recent observations reveal a universal X-ray to H$α$ surface-brightness ratio, ${\rm SB}_{\rm X}/{\rm SB}_{\rm Hα}\sim 3$, in galactic winds, ram-pressure stripped tails, and cluster filaments. This is surprising because H$α$ traces cold ($\sim 10^4$ K) gas while X-rays trace much hotter ($\sim 10^{6}$--$10^{7}$ K) gas. Plane-parallel mixing-layer models do not recover this ratio, and can be off by orders of magnitude. Motivated by recent work showing that geometry controls the temperature PDF of multiphase gas (Chen & Oh 2026), we run 3D wind-tunnel simulations in the high density contrast ($χ\sim 10^3$) regime. In this limit, the cold phase shatters into many small H$α$-emitting clumps, while X-ray-emitting gas forms a volume-filling cocoon around them. After smoothing on the tail-width scale, the measured surface-brightness ratio converges to the observed value, which can be understood theoretically. The H$α$ luminosity fraction is set by atomic physics, whereas the X-ray luminosity fraction is set by the residence time of gas in the X-ray-emitting band. This residence time is much shorter than the cooling time at X-ray temperatures, but scales roughly inversely with pressure, suggesting that it is tied to the cooling time at a lower-temperature outlet of the mixing cascade. This framework naturally explains why the observed ratio is order unity, and robust to changes in gas pressure. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Comments: 6 pages, 3 figures. Submitted to MNRAS Letters

The Origin of Da Scaling: Suppressed Cooling in Fast-Cooling Mixing Layers

Authors: Lachlan Lancaster, Drummond B. Fielding, Rajsekhar Mohapatra, Greg L. Bryan

Abstract: In numerical experiments simulating Turbulent Radiative Mixing Layers (TRMLs) it is observed that as the cooling time in the mixed gas, $t_{\rm cool}$, becomes very short compared to the dynamical time of the turbulence, $t_{\rm eddy}/t_{\rm cool} \gg 1$, there is a change in the scaling behavior of the total energy radiated in the TRML as a function of this ratio, also known as the Damköhler numb… ▽ More In numerical experiments simulating Turbulent Radiative Mixing Layers (TRMLs) it is observed that as the cooling time in the mixed gas, $t_{\rm cool}$, becomes very short compared to the dynamical time of the turbulence, $t_{\rm eddy}/t_{\rm cool} \gg 1$, there is a change in the scaling behavior of the total energy radiated in the TRML as a function of this ratio, also known as the Damköhler number, ${\rm Da} \equiv t_{\rm eddy}/t_{\rm cool}$, from $\dot{E}_{\rm cool} \propto {\rm Da}^{1/2}$ to $\dot{E}_{\rm cool} \propto {\rm Da}^{1/4}$. The latter, so-called "fast-cooling," regime is of particular interest as many astrophysical mixing layers lie in this regime. We demonstrate that the origin of this change is the suppression of turbulent folding of the surface by the ram-pressure of the inflowing gas, which becomes much greater than the turbulent pressure in this regime. We present an argument that reproduces the $\dot{E}_{\rm cool} \propto {\rm Da}^{1/4}$ behavior by appealing to the suppression of the fractal structure of the interface by the ram-pressure of the inflowing gas. △ Less

Submitted 2 June, 2026; originally announced June 2026.

Comments: 11 pages, 5 figures, submitted to ApJ

Ceci n'est pas une Couche de Mélange: The Meaning of Resolved Turbulent Radiative Mixing

Authors: Lachlan Lancaster, Rajsekhar Mohapatra, Drummond B. Fielding, Greg L. Bryan

Abstract: Turbulent Radiative Mixing Layers (TRMLs) are of fundamental importance to the transport of energy and momentum in multi-phase, astrophysical fluids. We use measurements of the "micro" and "macro" properties of these layers in high-resolution \texttt{AthenaK} simulations to investigate when their properties can be considered \textit{well}-resolved. In particular, we demonstrate that the previously… ▽ More Turbulent Radiative Mixing Layers (TRMLs) are of fundamental importance to the transport of energy and momentum in multi-phase, astrophysical fluids. We use measurements of the "micro" and "macro" properties of these layers in high-resolution \texttt{AthenaK} simulations to investigate when their properties can be considered \textit{well}-resolved. In particular, we demonstrate that the previously noticed resolution independence of total cooling, $\dot{E}_{\rm cool}$, in these simulations is due to a remarkable, and perhaps fortuitous, cancellation of the countervailing effects of numerical dissipation and numerical viscosity. This calls into question the degree to which we can trust the results of these experiments, as there is no physical picture that explains this cancellation. We also demonstrate that in order to correctly resolve the phase structure in these layers, important for accurate predictions of their observable properties, one must resolve the scale on which turbulent diffusion acts on time-scales comparable to the cooling time. This "turbulent Field length", $λ_{\rm F,turb}$, is where the eddy turnover time is equal to the cooling time ($t_{\rm eddy}(λ_{\rm F,turb}) = t_{\rm cool}$). We demonstrate that resolving this scale results in converged phase-structure and spatially resolved transitions in the gas phases. △ Less

Submitted 2 June, 2026; originally announced June 2026.

Comments: 19 pages, 10 figure, Accepted in ApJ

Cosmic Ray Feedback in Galactic Disks: Star Formation, Cosmic Ray Transport, and Multiphase Outflows in TIGRESS++ Simulations

Authors: Chang-Goo Kim, Lucia Armillotta, Eve C. Ostriker, Sanghyuk Moon, Lachlan Lancaster, Jeong-Gyu Kim, Nora B. Linzer, Ronan N. Hix

Abstract: We present new simulations of local star-forming disks that self-consistently evolve cosmic rays (CRs) and multiphase gas using TIGRESS++. To isolate the role of CRs, we conduct paired simulations under solar-neighborhood conditions: a magnetohydrodynamics (MHD) model following the standard TIGRESS-classic framework with FUV heating and supernova (SN) feedback from star clusters formed via gravita… ▽ More We present new simulations of local star-forming disks that self-consistently evolve cosmic rays (CRs) and multiphase gas using TIGRESS++. To isolate the role of CRs, we conduct paired simulations under solar-neighborhood conditions: a magnetohydrodynamics (MHD) model following the standard TIGRESS-classic framework with FUV heating and supernova (SN) feedback from star clusters formed via gravitational collapse; and a CRMHD model in which an additional 10% of each SN's energy is injected as CRs. These CRs are transported anisotropically along magnetic field lines via a two-moment solver, with the CR scattering rate set by balancing Alfven-wave growth and damping based on the self-confinement paradigm. The CRMHD model develops a characteristic two-zone vertical CR profile: uniform pressure in the diffusion-dominated, high-density midplane gas, and an exponential atmosphere shaped primarily by advection and streaming in low-density extraplanar gas. The CR pressure is comparable to the total thermal gas pressure in the midplane, but is too uniform to affect gas dynamics, leaving SFRs unchanged. In contrast, the vertical CR pressure gradient at |z| > 1 kpc accelerates warm outflowing gas, resulting in an approximately 4 times higher mass loading factor than in the MHD model. CR-gas interactions increase CR energy near the midplane through compressional work, while CR streaming heats low-density warm-hot gas. TIGRESS++ opens a path toward investigating CR transport and CR-regulated ISM and outflow dynamics at high resolution across diverse galactic environments. △ Less

Submitted 31 May, 2026; v1 submitted 25 May, 2026; originally announced May 2026.

Comments: ApJ accepted

Blue Monsters and Dusty Descendants: Reconciling UV and IR Emission from Galaxies from z=7, up to z= 14

Authors: Laura Sommovigo, Lachlan Lancaster, Shyam H. Menon, Joseph A. O'Leary, Rachel S. Somerville, Greg L. Bryan

Abstract: Recent JWST observations reveal massive, UV-bright galaxies at $z > 10$ with little apparent dust attenuation, whereas ALMA detections at $z \simeq 7$ show similarly massive systems that are already dust-rich and IR-luminous. This raises a fundamental question: can a single physical model of star formation and dust production explain both populations across cosmic time? We address this using a min… ▽ More Recent JWST observations reveal massive, UV-bright galaxies at $z > 10$ with little apparent dust attenuation, whereas ALMA detections at $z \simeq 7$ show similarly massive systems that are already dust-rich and IR-luminous. This raises a fundamental question: can a single physical model of star formation and dust production explain both populations across cosmic time? We address this using a minimal framework with only two free parameters--the instantaneous star formation efficiency ($ε_\star$) and the dust yield per Type II supernova ($y_d$)--and predict the rest-frame UV and IR luminosity functions (LFs) from $z \simeq 14$ to 7. For a uniform ISM, we find a UV-IR tension at the bright end of the LFs at $z \ge 7$. The UV LF requires low dust yields ($y_d \lesssim 0.01\,M_\odot$), whereas the $z=7$ IR LF requires higher yields ($y_d \sim 0.1\,M_\odot$) unless the star formation efficiency is boosted above $ε_\star \approx 5$-10%. We show that incorporating a porous, turbulent ISM largely resolves this tension: turbulence opens low-column-density sightlines that enhance the UV escape fraction while leaving the total absorbed energy--and thus the IR luminosity--nearly unchanged once radiative-transfer--induced flattening of the attenuation curve is included. Large-grain dust distributions, while reducing UV opacity, play a secondary role once ISM porosity and radiative transfer are taken into account. At $z > 10$, however, even strong turbulence cannot reproduce the bright end of the UV LF at high dust yield. This could be resolved either by efficient dust removal in early massive systems or by substantial ISM dust growth by $z \simeq 7$. Our results highlight dust physics as a key lever for interpreting the rapidly growing UV and IR constraints within the broader context of early galaxy formation. △ Less

Submitted 20 February, 2026; originally announced February 2026.

Comments: 27 Pages, 12 figures. Submitted to ApJ. The manuscript has passed the first referee round. A public GitHub repository will be made available upon publication

Taming the Tarantula: How Stellar Wind Feedback Shapes Gas and Dust in 30 Doradus

Authors: Jennifer A. Rodriguez, Laura A. Lopez, Lachlan Lancaster, Anna L. Rosen, Omnarayani Nayak, Sebastian Lopez, Tyler Holland-Ashford, Trinity L. Webb

Abstract: Observations of massive star-forming regions show that classical stellar wind models over-predict the luminosity of the X-ray emitting gas, indicating a significant fraction of wind energy is lost. In this paper, we present a multi-wavelength analysis of the giant HII region 30 Doradus and its central star cluster R136 using 2 Ms of Chandra X-ray Observatory data, combined with James Webb Space Te… ▽ More Observations of massive star-forming regions show that classical stellar wind models over-predict the luminosity of the X-ray emitting gas, indicating a significant fraction of wind energy is lost. In this paper, we present a multi-wavelength analysis of the giant HII region 30 Doradus and its central star cluster R136 using 2 Ms of Chandra X-ray Observatory data, combined with James Webb Space Telescope and Hubble Space Telescope imaging and Spitzer spectral-energy distributions, to investigate how the hot gas energy is lost through turbulent mixing, radiative cooling, and physical leakage. We compare the spatial and spectral properties of the hot gas with those of the warm ionized gas and dust. We find no significant correlation between the dust and hot gas temperatures, suggesting they are not directly coupled and that the dust resides in the swept-up shells where it is heated radiatively. H$α$ and X-ray surface brightness profiles show that the X-rays peak interior to the H$α$ shells, demonstrating partial confinement of the hot gas. The fragmented shell structure and bright X-ray interior that declines near the H$α$ shell reflect efficient cooling from turbulent mixing at the hot-cold interface. We compare against recent simulations of stellar-feedback driven bubbles which have broad agreement with the morphology of the X-ray and H$α$ emission, but the simulations produce a dip in the interior X-ray surface brightness and a lack of hard X-rays compared to the observations. These differences may suggest thermal conduction is important as mass-loading of the hot bubble could reproduce the X-ray observables. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 17 pages, 12 figures

The Co-Evolution of Stellar Wind-blown Bubbles and Photoionized Gas II: 3D RMHD Simulations and Tests of Semi-Analytic Models

Authors: Lachlan Lancaster, Chang-Goo Kim, Jeong-Gyu Kim, Eve C. Ostriker, Greg L. Bryan

Abstract: In a companion paper (Paper I) we presented a Co-Evolution Model (CEM) in which to consider the evolution of feedback bubbles driven by massive stars through both stellar winds and ionizing radiation, outlining when either of these effects is dominant and providing a model for how they evolve together. Here we present results from three-dimensional radiation magneto-hydrodynamical (RMHD) simulatio… ▽ More In a companion paper (Paper I) we presented a Co-Evolution Model (CEM) in which to consider the evolution of feedback bubbles driven by massive stars through both stellar winds and ionizing radiation, outlining when either of these effects is dominant and providing a model for how they evolve together. Here we present results from three-dimensional radiation magneto-hydrodynamical (RMHD) simulations of this scenario for parameters typical of massive star-forming clouds in the Milky Way: precisely the regime where we expect both feedback mechanisms to matter. While we find that the CEM agrees with the simulations to within 25% for key parameters and modestly outperforms previous idealized models, disagreements remain. We show that these deviations originate mainly from the CEM's lack of (i) background inhomogeneity caused by turbulence and (ii) time-variable momentum enhancements in the wind-blown bubble (WBB). Additionally, we find that photoionized gas acts similarly to magnetic fields ([as in Lancaster et al. 2024a) by decreasing the WBB's surface area. This causes a decrease in the amount of cooling at the WBB's interface, resulting in an enhanced WBB dynamical impact. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 33 pages, 11 Figures. Submitted to ApJ. Comments welcome

The Co-Evolution of Stellar Wind-blown Bubbles and Photoionized Gas I: Physical Principles and a Semi-Analytic Model

Authors: Lachlan Lancaster, Jeong-Gyu Kim, Greg L. Bryan, Shyam H. Menon, Eve C. Ostriker, Chang-Goo Kim

Abstract: We propose a new framework for the simultaneous feedback of stellar winds and photo-ionizing radiation from massive stars, distinguishing the locations where forces are applied, and consequences for internal spatio-temporal evolution of the whole feedback bubble (FB). We quantify the relative dynamical importance of wind-blown bubbles (WBB) versus the photoionized region (PIR) by the ratio of the… ▽ More We propose a new framework for the simultaneous feedback of stellar winds and photo-ionizing radiation from massive stars, distinguishing the locations where forces are applied, and consequences for internal spatio-temporal evolution of the whole feedback bubble (FB). We quantify the relative dynamical importance of wind-blown bubbles (WBB) versus the photoionized region (PIR) by the ratio of the radius at which the WBB is in pressure equilibrium with the PIR, $R_{\rm eq}$, to the Strömgren radius, $R_{\rm St}$. $ζ\equiv R_{\rm eq}/R_{\rm St}$ quantifies the dynamical dominance of WBBs ($ζ> 1$) or the PIR ($ζ< 1$). We calculate $ζ$ and find that, for momentum-driven winds, $0.1 \lesssim ζ\lesssim 1$ for the star-forming regions in (i) typical Milky Way-like giant molecular clouds (GMCs), (ii) the most massive of individual OB stars, and (iii) dense, low-metallicity environments, relevant in the early universe. In this regime, both WBBs and the PIR are dynamically important to the expansion of the FB. We develop a semi-analytic Co-Evolution Model (CEM) that takes into account the spatial distribution of forces and the back reactions of both the WBB and PIR. In the $ζ<1$ regime where the CEM is most relevant, the model differs in the total FB momentum by up to 25% compared to naive predictions. In the weak-wind limit of $ζ\ll 1$, applicable to individual OB stars or low-mass clusters, the CEM has factors $\gtrsim 2$ differences in WBB properties. In a companion paper we compare these models to three-dimensional, turbulent hydro-dynamical simulations. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 33 pages, 6 figures, Accepted for Publication in ApJ. Comments welcome

Suppression of Shock X-ray Emission in Novae from Turbulent Mixing with Cool Gas

Authors: Brian D. Metzger, Lachlan Lancaster, Rebecca Diesing

Abstract: Shock interaction in classical novae occurs when a fast outflow from the white dwarf > 1000 km s/s collides with a slower, cooler shell of gas released earlier in the outburst. The shocks radiate across the electromagnetic spectrum, from radio synchrotron to GeV gamma-rays. The hot shocked gas also emits >~ keV thermal X-rays, typically peaking weeks after the eruption, once the ejecta becomes tra… ▽ More Shock interaction in classical novae occurs when a fast outflow from the white dwarf > 1000 km s/s collides with a slower, cooler shell of gas released earlier in the outburst. The shocks radiate across the electromagnetic spectrum, from radio synchrotron to GeV gamma-rays. The hot shocked gas also emits >~ keV thermal X-rays, typically peaking weeks after the eruption, once the ejecta becomes transparent to photoelectric absorption. However, the observed hard X-ray luminosities are typically >4 orders of magnitude smaller than would be naively expected given the powerful shocks implied by the gamma-rays. We argue that a key missing piece to this puzzle is turbulence behind the shock, driven, e.g., by thin-shell and/or thermal instabilities. Turbulence efficiently mixes the hot X-ray emitting gas with cooler gas, sapping the hot gas of energy faster than it can directly radiate. Using analytic arguments motivated by numerical simulations, we show that energy losses due to turbulent mixing can easily balance shock heating, greatly reducing the volume of the hot gas and suppressing the X-ray luminosity. Equating the characteristic thickness of the X-ray emitting region to the minimum outer length scale of the turbulence capable of cooling the hot gas through mixing, we obtain X-ray luminosities consistent with nova observations if only ~1% of the shock's kinetic power goes into turbulent motions. A similar process may act to suppress thermal X-rays from other shock powered transients, such as interacting supernovae. △ Less

Submitted 10 June, 2025; v1 submitted 13 May, 2025; originally announced May 2025.

Comments: 11 pages, 2 figures, resubmitted to ApJ after reviewers comments incorporated

Density modulated star formation efficiency: implications for the observed abundance of ultra-violet luminous galaxies at z>10

Authors: Rachel S. Somerville, L. Y. Aaron Yung, Lachlan Lancaster, Shyam Menon, Laura Sommovigo, Steven L. Finkelstein

Abstract: The number density of UV luminous galaxies discovered by the James Webb Space Telescope at ultra high redshift ($z \gtrsim 10$) is higher, and declines much more slowly with increasing redshift, than expected from extrapolations of lower redshift observations or pre-launch physics-based models. Most of these models assume star formation efficiencies (SFE) of only a few percent, motivated by observ… ▽ More The number density of UV luminous galaxies discovered by the James Webb Space Telescope at ultra high redshift ($z \gtrsim 10$) is higher, and declines much more slowly with increasing redshift, than expected from extrapolations of lower redshift observations or pre-launch physics-based models. Most of these models assume star formation efficiencies (SFE) of only a few percent, motivated by observations of nearby galaxies. In this work, we incorporate a scaling of SFE with gas surface density (which we refer to as Density Modulated SFE; DMSFE), motivated by cloud-scale simulations and theory, into a semi-analytic cosmological model (SAM) of galaxy formation which is calibrated to match the observed rest-UV sizes of high redshift galaxies. We also model the impact of dust and bursty star formation on the SAM-predicted properties of observed galaxies. We show that with plausible values of the main parameters, such as the fraction of gas in dense clouds $f_{\rm dense}$, our new models easily reproduce or even exceed the observed galaxy number densities at $z\sim 6$-17. While no single value of $f_{\rm dense}$ is able to reproduce the very shallow observed decline of the galaxy number density at $z\gtrsim 12$, it is plausible and even expected for $f_{\rm dense}$ to have some effective dependence on cosmic time, which could bring these models into closer agreement with the data. We show that the combined effects of DMSFE, decreasing dust attenuation, and increasingly bursty star formation at earlier cosmic epochs could conspire to reproduce the observed evolution. △ Less

Submitted 20 October, 2025; v1 submitted 8 May, 2025; originally announced May 2025.

Comments: Accepted version, MNRAS in press

Journal ref: Mon Not R Astron Soc (2025) 3774-3798

Geometry, Dissipation, Cooling, and the Dynamical Evolution of Wind-Blown Bubbles

Authors: Lachlan Lancaster, Eve C. Ostriker, Chang-Goo Kim, Jeong-Gyu Kim, Greg L. Bryan

Abstract: Bubbles driven by energy and mass injection from small scales are ubiquitous in astrophysical fluid systems and essential to feedback across multiple scales. In particular, O stars in young clusters produce high velocity winds that create hot bubbles in the surrounding gas. We demonstrate that the dynamical evolution of these bubbles is critically dependent upon the geometry of their interfaces wi… ▽ More Bubbles driven by energy and mass injection from small scales are ubiquitous in astrophysical fluid systems and essential to feedback across multiple scales. In particular, O stars in young clusters produce high velocity winds that create hot bubbles in the surrounding gas. We demonstrate that the dynamical evolution of these bubbles is critically dependent upon the geometry of their interfaces with their surroundings and the nature of heat transport across these interfaces. These factors together determine the amount of energy that can be lost from the interior through cooling at the interface, which in turn determines the ability of the bubble to do work on its surroundings. We further demonstrate that the scales relevant to physical dissipation across this interface are extremely difficult to resolve in global numerical simulations of bubbles for parameter values of interest. This means the dissipation driving evolution of these bubbles in numerical simulations is often of a numerical nature. We describe the physical and numerical principles that determine the level of dissipation in these simulations; we use this, along with a fractal model for the geometry of the interfaces, to explain differences in convergence behavior between hydrodynamical and magneto-hydrodynamical simulations presented here. We additionally derive an expression for momentum as a function of bubble radius expected when the relevant dissipative scales are resolved and show that it still results in efficiently-cooled solutions as postulated in previous work. △ Less

Submitted 3 May, 2024; originally announced May 2024.

Comments: 41 pages, 20 figures, submitted to ApJ, comments welcome

The Interplay between the IMF and Star Formation Efficiency through Radiative Feedback at High Stellar Surface Densities

Authors: Shyam H. Menon, Lachlan Lancaster, Blakesley Burkhart, Rachel S. Somerville, Avishai Dekel, Mark R. Krumholz

Abstract: The observed rest-UV luminosity function at cosmic dawn ($z \sim 8-14$) measured by JWST revealed an excess of UV-luminous galaxies relative to many pre-launch theoretical predictions. A high star-formation efficiency (SFE) and a top-heavy initial mass function (IMF) are among the mechanisms proposed for explaining this excess. Although a top-heavy IMF has been proposed for its ability to increase… ▽ More The observed rest-UV luminosity function at cosmic dawn ($z \sim 8-14$) measured by JWST revealed an excess of UV-luminous galaxies relative to many pre-launch theoretical predictions. A high star-formation efficiency (SFE) and a top-heavy initial mass function (IMF) are among the mechanisms proposed for explaining this excess. Although a top-heavy IMF has been proposed for its ability to increase the light-to-mass ratio (\(Ψ_{\mathrm{UV}}\)), the resulting enhanced radiative pressure from young stars could decrease the star formation efficiency (SFE), potentially driving galaxy luminosities back down. In this Letter, we use idealized radiation hydrodynamic simulations of star cluster formation to explore the effects of a top-heavy IMF on the SFE of clouds typical of the high pressure conditions found at these redshifts. We find that the SFE in star clusters with solar neighbourhood-like dust abundance decreases with increasingly top-heavy IMF's -- by $\sim 20 \%$ for an increase of factor 4 in $Ψ_{\mathrm{UV}}$, and by $50 \%$ for a factor $ \sim 10$ in $Ψ_{\mathrm{UV}}$. However, we find that an expected decrease in the dust-to-gas ratio ($\sim 0.01 \times \mathrm{Solar}$) at these redshifts can completely compensate for the enhanced light output. This leads to a (cloud-scale; $\sim 10 \, \mathrm{pc}$) SFE that is $\gtrsim 70\%$ even for a factor 10 increase in $Ψ_{\mathrm{UV}}$, implying that highly efficient star formation is unavoidable for high surface density and low metallicity conditions. Our results suggest that a top-heavy IMF, if present, likely coexists with efficient star formation in these galaxies. △ Less

Submitted 1 May, 2024; originally announced May 2024.

Comments: 12 pages, 4 figures. Accepted to the Astrophysical Journal Letters

Detection of Diffuse Hot Gas Around the Young, Potential Superstar Cluster H72.97-69.39

Authors: Trinity L. Webb, Jennifer A. Rodriguez, Laura A. Lopez, Anna L. Rosen, Lachlan Lancaster, Omnarayani Nayak, Anna F. McLeod, Paarmita Pandey, Grace M. Olivier

Abstract: We present the first Chandra X-ray observations of H72.97-69.39, a highly-embedded, potential super-star cluster (SSC) in its infancy located in the star-forming complex N79 of the Large Magellanic Cloud. We detect particularly hard, diffuse X-ray emission that is coincident with the young stellar objects (YSOs) identified with JWST, and the hot gas fills cavities in the dense gas mapped by ALMA.… ▽ More We present the first Chandra X-ray observations of H72.97-69.39, a highly-embedded, potential super-star cluster (SSC) in its infancy located in the star-forming complex N79 of the Large Magellanic Cloud. We detect particularly hard, diffuse X-ray emission that is coincident with the young stellar objects (YSOs) identified with JWST, and the hot gas fills cavities in the dense gas mapped by ALMA. The X-ray spectra are best fit with either a thermal plasma or power-law model, and assuming the former, we show that the X-ray luminosity of L_X = (1.0 +- 0.3)e34 erg/s is a factor of ~20 below the expectation for a fully-confined wind bubble. Our results suggest that stellar wind feedback produces diffuse hot gas in the earliest stages of massive star cluster formation and that wind energy can be lost quickly via either turbulent mixing followed by radiative cooling or by physical leakage. △ Less

Submitted 11 December, 2024; v1 submitted 21 February, 2024; originally announced February 2024.

Comments: 13 pages, 7 figures, 3 tables, accepted to ApJ

Journal ref: ApJ 977 45 (2024)

Structure, Kinematics, and Observability of the Large Magellanic Cloud's Dynamical Friction Wake in Cold vs. Fuzzy Dark Matter

Authors: Hayden R. Foote, Gurtina Besla, Philip Mocz, Nicolás Garavito-Camargo, Lachlan Lancaster, Martin Sparre, Emily C. Cunningham, Mark Vogelsberger, Facundo A. Gómez, Chervin F. P. Laporte

Abstract: The Large Magellanic Cloud (LMC) will induce a dynamical friction (DF) wake on infall to the Milky Way (MW). The MW's stellar halo will respond to the gravity of the LMC and the dark matter (DM) wake, forming a stellar counterpart to the DM wake. This provides a novel opportunity to constrain the properties of the DM particle. We present a suite of high-resolution, windtunnel-style simulations of… ▽ More The Large Magellanic Cloud (LMC) will induce a dynamical friction (DF) wake on infall to the Milky Way (MW). The MW's stellar halo will respond to the gravity of the LMC and the dark matter (DM) wake, forming a stellar counterpart to the DM wake. This provides a novel opportunity to constrain the properties of the DM particle. We present a suite of high-resolution, windtunnel-style simulations of the LMC's DF wake that compare the structure, kinematics, and stellar tracer response of the DM wake in cold DM (CDM), with and without self-gravity, vs. fuzzy DM (FDM) with $m_a = 10^{-23}$ eV. We conclude that the self-gravity of the DM wake cannot be ignored. Its inclusion raises the wake's density by $\sim 10\%$, and holds the wake together over larger distances ($\sim$ 50 kpc) than if self-gravity is ignored. The DM wake's mass is comparable to the LMC's infall mass, meaning the DM wake is a significant perturber to the dynamics of MW halo tracers. An FDM wake is more granular in structure and is $\sim 20\%$ dynamically colder than a CDM wake, but with comparable density. The granularity of an FDM wake increases the stars' kinematic response at the percent level compared to CDM, providing a possible avenue of distinguishing a CDM vs. FDM wake. This underscores the need for kinematic measurements of stars in the stellar halo at distances of 70-100 kpc. △ Less

Submitted 8 September, 2023; v1 submitted 30 June, 2023; originally announced July 2023.

Comments: 38 pages, 31 figures. Updated to published version

Journal ref: ApJ 954 163 (2023)

Cosmological Structure Formation and Soliton Phase Transition in Fuzzy Dark Matter with Axion Self-Interactions

Authors: Philip Mocz, Anastasia Fialkov, Mark Vogelsberger, Michael Boylan-Kolchin, Pierre-Henri Chavanis, Mustafa A. Amin, Sownak Bose, Tibor Dome, Lars Hernquist, Lachlan Lancaster, Matthew Notis, Connor Painter, Victor H. Robles, Jesus Zavala

Abstract: We investigate cosmological structure formation in Fuzzy Dark Matter (FDM) with an attractive self-interaction (SI) with numerical simulations. Such a SI would arise if the FDM boson were an ultra-light axion, which has a strong CP symmetry-breaking scale (decay constant). Although weak, the attractive SI may be strong enough to counteract the quantum 'pressure' and alter structure formation. We f… ▽ More We investigate cosmological structure formation in Fuzzy Dark Matter (FDM) with an attractive self-interaction (SI) with numerical simulations. Such a SI would arise if the FDM boson were an ultra-light axion, which has a strong CP symmetry-breaking scale (decay constant). Although weak, the attractive SI may be strong enough to counteract the quantum 'pressure' and alter structure formation. We find in our simulations that the SI can enhance small-scale structure formation, and soliton cores above a critical mass undergo a phase transition, transforming from dilute to dense solitons. △ Less

Submitted 24 January, 2023; originally announced January 2023.

Comments: 10 pages, 3 figures, submitted to mnras

Ultra-Diffuse Galaxies as Extreme Star-forming Environments II: Star Formation and Pressure Balance in HI-Rich UDGs

Authors: Erin Kado-Fong, Chang-Goo Kim, Jenny E. Greene, Lachlan Lancaster

Abstract: In addition to occupying the extreme, diffuse tail of the dwarf galaxy population, Ultra-Diffuse Galaxies (UDGs) are themselves a key laboratory in which to study star formation in extreme low-density environments. In the second paper of this series, we compare the spatially resolved star formation activity of 22 HI-selected UDGs and 21 "normal" dwarf galaxies within 120 Mpc to predictions within… ▽ More In addition to occupying the extreme, diffuse tail of the dwarf galaxy population, Ultra-Diffuse Galaxies (UDGs) are themselves a key laboratory in which to study star formation in extreme low-density environments. In the second paper of this series, we compare the spatially resolved star formation activity of 22 HI-selected UDGs and 21 "normal" dwarf galaxies within 120 Mpc to predictions within the pressure-regulated, feedback-modulated (PRFM) theory of star formation. To do so, we employ a joint SED fitting method that allows us to estimate star formation rate and stellar mass surface density from UV-optical imaging. We find that the PRFM framework extends successfully to the UDG regime - although the UDGs in our sample show unusually low star formation rate surface densities given their HI content, this low star formation efficiency can be naturally explained by the diffuse structure of the UDGs. In fact, when cast in the PRFM framework, the relationship between midplane pressure and star formation in the UDG sample is in good agreement not only with the "normal" dwarf reference sample, but also with measurements from more massive galaxies. Our results suggest that despite their low star formation efficiencies, the HI-rich UDGs need not be forming stars in an exotic manner. We also find that the UDGs are likely H$_2$-poor compared even to the overall dwarf population. △ Less

Submitted 24 October, 2022; v1 submitted 12 September, 2022; originally announced September 2022.

Comments: Accepted to ApJ, 22 pages, 11 figures

Simulating spin dynamics with quantum computers

Authors: Jarrett L. Lancaster, D. Brysen Allen

Abstract: IBM quantum computers are used to simulate the dynamics of small systems of interacting quantum spins. For time-independent systems with fewer than three spins, we compute the exact time evolution at arbitrary times and measure spin expectation values and energy. It is demonstrated that even in such small systems, one can observe the connection between conservation laws and symmetries in the model… ▽ More IBM quantum computers are used to simulate the dynamics of small systems of interacting quantum spins. For time-independent systems with fewer than three spins, we compute the exact time evolution at arbitrary times and measure spin expectation values and energy. It is demonstrated that even in such small systems, one can observe the connection between conservation laws and symmetries in the model. Larger systems require approximating the time-evolution operator, and we investigate the case of $N=3$ spins explicitly. While it is shown to be unfeasible to use such devices to probe such larger systems without more advanced algorithms or reliable error correction, we demonstrate that the quantum circuit simulator is an easy-to-use method for studying spin dynamics in systems with $N\sim\mathcal{O}(10)$) spins. The computations presented provide an interesting experimental component to the standard treatment of quantum spin in an undergraduate quantum mechanics course. △ Less

Submitted 30 July, 2024; v1 submitted 29 June, 2022; originally announced July 2022.

Comments: 13 pages, 9 figures

Journal ref: Am. J. Phys. 93, 98-109 (2025)

walter: A Tool for Predicting Resolved Stellar Population Observations with Applications to the Roman Space Telescope

Authors: Lachlan Lancaster, Sarah Pearson, Benjamin F. Williams, Kathryn V. Johnston, Tjitske K. Starkenburg, Erin Kado-Fong, Anil C. Seth, Eric F. Bell

Abstract: Studies of resolved stellar populations in the Milky Way and nearby galaxies reveal an amazingly detailed and clear picture of galaxy evolution. Within the Local Group, the ability to probe the stellar populations of small and large galaxies opens up the possibility of exploring key questions such as the nature of dark matter, the detailed formation history of different galaxy components, and the… ▽ More Studies of resolved stellar populations in the Milky Way and nearby galaxies reveal an amazingly detailed and clear picture of galaxy evolution. Within the Local Group, the ability to probe the stellar populations of small and large galaxies opens up the possibility of exploring key questions such as the nature of dark matter, the detailed formation history of different galaxy components, and the role of accretion in galactic formation. Upcoming wide-field surveys promise to extend this ability to all galaxies within 10~Mpc, drastically increasing our capability to decipher galaxy evolution and enabling statistical studies of galaxies' stellar populations. To facilitate the optimum use of these upcoming capabilities we develop a simple formalism to predict the density of resolved stars for an observation of a stellar population at fixed surface brightness and population parameters. We provide an interface to calculate all quantities of interest to this formalism via a public release of the code: \texttt{walter}. This code enables calculation of (i) the expected number density of detected stars, (ii) the exposure time needed to reach certain population features, such as the horizontal branch, and (iii) an estimate of the crowding limit, among other features. These calculations will be very useful for planning surveys with NASA's upcoming Nancy Grace Roman Space Telescope (Roman, formerly WFIRST), which we use for example calculations throughout this work. △ Less

Submitted 5 July, 2022; originally announced July 2022.

Comments: 16 pages, 6 figures, submitted to AJ. Comments welcome

Testing the necessity of complex numbers in traditional quantum theory with quantum computers

Authors: Jarrett L. Lancaster, Nicholas M. Palladino

Abstract: A recent experiment testing the necessity of complex numbers in the standard formulation of quantum theory is recreated using IBM quantum computers. To motivate the experiment, we present a basic construction for real-valued quantum theory. The real-valued description is shown to predict correlations identical to those of complex-valued quantum mechanics for two types of Bell tests based on the Cl… ▽ More A recent experiment testing the necessity of complex numbers in the standard formulation of quantum theory is recreated using IBM quantum computers. To motivate the experiment, we present a basic construction for real-valued quantum theory. The real-valued description is shown to predict correlations identical to those of complex-valued quantum mechanics for two types of Bell tests based on the Clauser-Horne-Shimony-Holt (CHSH) inequality. A slight modification to one test, however, results in different predictions for the real- and complex-valued constructions. While noisier devices are incapable of delivering convincing results, it is shown that certain devices possess sufficiently small error rates to falsify real-valued formulations of quantum theory for composite states. The results obtained with quantum computers are consistent with published experiments. This work demonstrates the feasibility of using freely-available quantum devices to explore foundational features of quantum mechanics with minimal technical expertise. Accordingly, this treatment could inspire novel projects for undergraduate students taking a course on quantum mechanics. △ Less

Submitted 10 December, 2024; v1 submitted 2 May, 2022; originally announced May 2022.

Comments: 12 pages, 4 figures

Journal ref: Am. J. Phys. 93, 110-120 (2025)

A Method for Controlling Extrapolation when Visualizing and Optimizing the Prediction Profiles of Statistical and Machine Learning Models

Authors: Jeremy Ash, Laura Lancaster, Chris Gotwalt

Abstract: We present a novel method for controlling extrapolation in the prediction profiler in the JMP software. The prediction profiler is a graphical tool for exploring high dimensional prediction surfaces for statistical and machine learning models. The profiler contains interactive cross-sectional views, or profile traces, of the prediction surface of a model. Our method helps users avoid exploring pre… ▽ More We present a novel method for controlling extrapolation in the prediction profiler in the JMP software. The prediction profiler is a graphical tool for exploring high dimensional prediction surfaces for statistical and machine learning models. The profiler contains interactive cross-sectional views, or profile traces, of the prediction surface of a model. Our method helps users avoid exploring predictions that should be considered extrapolation. It also performs optimization over a constrained factor region that avoids extrapolation using a genetic algorithm. In simulations and real world examples, we demonstrate how optimal factor settings without constraint in the profiler are frequently extrapolated, and how extrapolation control helps avoid these solutions with invalid factor settings that may not be useful to the user. △ Less

Submitted 13 January, 2022; originally announced January 2022.

Comments: 29 pages, 8 figures, 7 supplementary figures

Absolute keV X-ray yield and conversion efficiency in over dense Si petawatt laser plasma

Authors: Sergey N. Ryazantsev, Artem S. Martynenko, Maksim V. Sedov, Igor Yu. Skobelev, Mikhail D. Mishchenko, Yaroslav S. Lavrinenko, Christopher D. Baird, Nicola Booth, Phil Durey, Leonard N. K. DÖhl, Damon Farley, Kathryn L. Lancaster, Paul Mckenna, Christopher D. Murphy, Tatiana A. Pikuz, Christopher Spindloe, Nigel Woolsey, Sergey A. Pikuz

Abstract: Laser-produced plasmas are bright, short sources of X-rays often used for time-resolved imaging and spectroscopy. Absolute measurement requires accurate knowledge of laser-to-x-ray conversion efficiencies, spectrum, photon yield and angular distribution. Here we report on soft X-ray emission from a thin Si foil irradiated by a sub-PW picosecond laser pulse. These absolute measurements cover a cont… ▽ More Laser-produced plasmas are bright, short sources of X-rays often used for time-resolved imaging and spectroscopy. Absolute measurement requires accurate knowledge of laser-to-x-ray conversion efficiencies, spectrum, photon yield and angular distribution. Here we report on soft X-ray emission from a thin Si foil irradiated by a sub-PW picosecond laser pulse. These absolute measurements cover a continuous and broad spectral range that extends from 4.75 to 7.5 Angstroms (1.7-2.6 keV). The X-ray spectrum consists of spectral line transitions from highly charged ions and broadband emission with contributions from recombination, and free-free processes that occur as electrons decelerate in plasma electromagnetic fields. These quantitative measurements are compared to particle-in-cell simulations allowing us to distinguish bremsstrahlung and synchrotron contributions to the free-free emission. We found that experiment and simulation estimations of laser-to-bremsstrahlung conversion efficiency are in a good agreement. This agreement illustrates the accuracy of experiment and physical interpretation of the measurements. △ Less

Submitted 20 December, 2021; originally announced December 2021.

Star Formation Regulation and Self-Pollution by Stellar Wind Feedback

Authors: Lachlan Lancaster, Eve C. Ostriker, Jeong-Gyu Kim, Chang-Goo Kim

Abstract: Stellar winds contain enough energy to easily disrupt the parent cloud surrounding a nascent star cluster, and for this reason have been considered candidates for regulating star formation. However, direct observations suggest most wind power is lost, and Lancaster21a,b recently proposed that this is due to efficient mixing and cooling processes. Here, we simulate star formation with wind feedback… ▽ More Stellar winds contain enough energy to easily disrupt the parent cloud surrounding a nascent star cluster, and for this reason have been considered candidates for regulating star formation. However, direct observations suggest most wind power is lost, and Lancaster21a,b recently proposed that this is due to efficient mixing and cooling processes. Here, we simulate star formation with wind feedback in turbulent, self-gravitating clouds, extending our previous work. Our simulations cover clouds with initial surface density $10^2-10^4$ $M_{\odot} \, {\rm pc}^{-2}$, and show that star formation and residual gas dispersal is complete within 2 - 8 initial cloud free-fall times. The "Efficiently Cooled" model for stellar wind bubble evolution predicts enough energy is lost for the bubbles to become momentum-driven, we find this is satisfied in our simulations. We also find that wind energy losses from turbulent, radiative mixing layers dominate losses by "cloud leakage" over the timescales relevant for star formation. We show that the net star formation efficiency (SFE) in our simulations can be explained by theories that apply wind momentum to disperse cloud gas, allowing for highly inhomogeneous internal cloud structure. For very dense clouds, the SFE is similar to those observed in extreme star-forming environments. Finally, we find that, while self-pollution by wind material is insignificant in cloud conditions with moderate density (only $\lesssim 10^{-4}$ of the stellar mass originated in winds), our simulations with conditions more typical of a super star cluster have star particles that form with as much as 1\% of their mass in wind material. △ Less

Submitted 11 October, 2021; originally announced October 2021.

Comments: 20 pages, 5 figures, submitted to ApJL, comments welcome

A Search for Wandering Black Holes in the Milky Way with Gaia and DECaLS

Authors: Jenny E. Greene, Lachlan Lancaster, Yuan-Sen Ting, Sergey E. Koposov, Shany Danieli, Song Huang, Fangzhou Jiang, Johnny P. Greco, Jay Strader

Abstract: We present a search for "hyper-compact" star clusters in the Milky Way using a combination of Gaia and the Dark Energy Camera Legacy Survey (DECaLS). Such putative clusters, with sizes of ~1 pc and containing 500-5000 stars, are expected to remain bound to intermediate-mass black holes (Mbh~10^3-10^5 M-sun) that may be accreted into the Milky Way halo within dwarf satellites. Using the semi-analyt… ▽ More We present a search for "hyper-compact" star clusters in the Milky Way using a combination of Gaia and the Dark Energy Camera Legacy Survey (DECaLS). Such putative clusters, with sizes of ~1 pc and containing 500-5000 stars, are expected to remain bound to intermediate-mass black holes (Mbh~10^3-10^5 M-sun) that may be accreted into the Milky Way halo within dwarf satellites. Using the semi-analytic model SatGen we find an expected ~100 wandering intermediate-mass black holes with if every infalling satellite hosts a black hole. We do not find any such clusters in our search. Our upper limits rule out 100% occupancy, but do not put stringent constraints on the occupation fraction. Of course, we need stronger constraints on the properties of the putative star clusters, including their assumed sizes as well as the fraction of stars that would be compact remnants. △ Less

Submitted 10 May, 2021; originally announced May 2021.

Comments: 20 pages, 8 figures, under review at ApJ

Efficiently Cooled Stellar Wind Bubbles in Turbulent Clouds II. Validation of Theory with Hydrodynamic Simulations

Authors: Lachlan Lancaster, Eve C. Ostriker, Jeong-Gyu Kim, Chang-Goo Kim

Abstract: In a companion paper, we develop a theory for the evolution of stellar wind driven bubbles in dense, turbulent clouds. This theory proposes that turbulent mixing at a fractal bubble-shell interface leads to highly efficient cooling, in which the vast majority of the input wind energy is radiated away. This energy loss renders the majority of the bubble evolution momentum-driven rather than energy-… ▽ More In a companion paper, we develop a theory for the evolution of stellar wind driven bubbles in dense, turbulent clouds. This theory proposes that turbulent mixing at a fractal bubble-shell interface leads to highly efficient cooling, in which the vast majority of the input wind energy is radiated away. This energy loss renders the majority of the bubble evolution momentum-driven rather than energy-driven, with expansion velocities and pressures orders of magnitude lower than in the classical Weaver77 solution. In this paper, we validate our theory with three-dimensional, hydrodynamic simulations. We show that extreme cooling is not only possible, but is generic to star formation in turbulent clouds over more than three orders of magnitude in density. We quantify the few free parameters in our theory, and show that the momentum exceeds the wind input rate by only a factor ~ 1.2-4. We verify that the bubble/cloud interface is a fractal with dimension ~ 2.5-2.7. The measured turbulent amplitude (v_t ~ 200-400 km/s) in the hot gas near the interface is shown to be consistent with theoretical requirements for turbulent diffusion to efficiently mix and radiate away most of the wind energy. The fraction of energy remaining after cooling is only 1-Θ~ 0.1-0.01, decreasing with time, explaining observations that indicate low hot-gas content and weak dynamical effects of stellar winds. △ Less

Submitted 15 April, 2021; originally announced April 2021.

Comments: Accepted for publication in ApJ, 36 pages, 25 figures, but short summary and conclusion. Comments welcome

Efficiently Cooled Stellar Wind Bubbles in Turbulent Clouds I. Fractal Theory and Application to Star-Forming Clouds

Authors: Lachlan Lancaster, Eve C. Ostriker, Jeong-Gyu Kim, Chang-Goo Kim

Abstract: Winds from massive stars have velocities of 1000 km/s or more, and produce hot, high pressure gas when they shock. We develop a theory for the evolution of bubbles driven by the collective winds from star clusters early in their lifetimes, which involves interaction with the turbulent, dense interstellar medium of the surrounding natal molecular cloud. A key feature is the fractal nature of the ho… ▽ More Winds from massive stars have velocities of 1000 km/s or more, and produce hot, high pressure gas when they shock. We develop a theory for the evolution of bubbles driven by the collective winds from star clusters early in their lifetimes, which involves interaction with the turbulent, dense interstellar medium of the surrounding natal molecular cloud. A key feature is the fractal nature of the hot bubble's surface. The large area of this interface with surrounding denser gas strongly enhances energy losses from the hot interior, enabled by turbulent mixing and subsequent cooling at temperatures T = 10^4-10^5 K where radiation is maximally efficient. Due to the extreme cooling, the bubble radius scales differently (R ~ t^1/2) from the classical Weaver77 solution, and has expansion velocity and momentum lower by factors of 10-10^2 at given R, with pressure lower by factors of 10^2 - 10^3. Our theory explains the weak X-ray emission and low shell expansion velocities of observed sources. We discuss further implications of our theory for observations of the hot bubbles and cooled expanding shells created by stellar winds, and for predictions of feedback-regulated star formation in a range of environments. In a companion paper, we validate our theory with a suite of hydrodynamic simulations. △ Less

Submitted 15 April, 2021; originally announced April 2021.

Comments: Accepted for publication in ApJ, comments welcome, 23 pages 4 figures

deep21: a Deep Learning Method for 21cm Foreground Removal

Authors: T. Lucas Makinen, Lachlan Lancaster, Francisco Villaescusa-Navarro, Peter Melchior, Shirley Ho, Laurence Perreault-Levasseur, David N. Spergel

Abstract: We seek to remove foreground contaminants from 21cm intensity mapping observations. We demonstrate that a deep convolutional neural network (CNN) with a UNet architecture and three-dimensional convolutions, trained on simulated observations, can effectively separate frequency and spatial patterns of the cosmic neutral hydrogen (HI) signal from foregrounds in the presence of noise. Cleaned maps rec… ▽ More We seek to remove foreground contaminants from 21cm intensity mapping observations. We demonstrate that a deep convolutional neural network (CNN) with a UNet architecture and three-dimensional convolutions, trained on simulated observations, can effectively separate frequency and spatial patterns of the cosmic neutral hydrogen (HI) signal from foregrounds in the presence of noise. Cleaned maps recover cosmological clustering statistics within 10% at all relevant angular scales and frequencies. This amounts to a reduction in prediction variance of over an order of magnitude on small angular scales ($\ell > 300$), and improved accuracy for small radial scales ($k_{\parallel} > 0.17\ \rm h\ Mpc^{-1})$ compared to standard Principal Component Analysis (PCA) methods. We estimate posterior confidence intervals for the network's prediction by training an ensemble of UNets. Our approach demonstrates the feasibility of analyzing 21cm intensity maps, as opposed to derived summary statistics, for upcoming radio experiments, as long as the simulated foreground model is sufficiently realistic. We provide the code used for this analysis on Github https://github.com/tlmakinen/deep21 as well as a browser-based tutorial for the experiment and UNet model via the accompanying http://bit.ly/deep21-colab Colab notebook. △ Less

Submitted 1 June, 2021; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: Published in JCAP 30 April 2021. 30 pages, 11 figures

Hyper Suprime-Cam Low Surface Brightness Galaxies II: A Hubble Space Telescope Study of the Globular Cluster Systems of Ultra-Diffuse Galaxies in Groups

Authors: Jean J. Somalwar, Jenny E. Greene, Johnny P. Greco, Song Huang, Rachael L. Beaton, Andy D. Goulding, Lachlan Lancaster

Abstract: We increase the sample of ultra diffuse galaxies (UDGs) in lower density environments with characterized globular cluster (GC) populations using new Hubble Space Telescope observations of nine UDGs in group environments. While the bulk of our UDGs have GC abundances consistent with normal dwarf galaxies, two of these UDGs have excess GC populations. These two UDGs both have GC luminosity functions… ▽ More We increase the sample of ultra diffuse galaxies (UDGs) in lower density environments with characterized globular cluster (GC) populations using new Hubble Space Telescope observations of nine UDGs in group environments. While the bulk of our UDGs have GC abundances consistent with normal dwarf galaxies, two of these UDGs have excess GC populations. These two UDGs both have GC luminosity functions consistent with higher surface brightness galaxies and cluster UDGs. We then combine our nine objects with previous studies to create a catalog of UDGs with analyzed GC populations that spans a uniquely diverse range of environments. We use this catalog to examine broader trends in the GC populations of low stellar mass galaxies. The highest GC abundances are found in cluster UDGs, but whether cluster UDGs are actually more extreme requires study of many more UDGs in groups. We find a possible positive correlation between GC abundance and stellar mass, and between GC abundance and galaxy size at fixed stellar mass. However, we see no significant stellar-mass galaxy-size relation, over our limited stellar mass range. We consider possible origins of the correlation between GC abundance and galaxy size, including the possibility that these two galaxy properties are both dependent on the galaxy dark matter halo, or that they are related through baryonic processes like internal feedback. △ Less

Submitted 6 August, 2020; originally announced August 2020.

Comments: 22 pages, 12 figures, submitted to ApJ, posted after referee's comments

A Mystery in Chamaeleon: Serendipitous Discovery of a Galactic Symbiotic Nova

Authors: Lachlan Lancaster, Jenny Greene, Yuan-Sen Ting, Sergey E. Koposov, Benjamin J. S. Pope, Rachael L. Beaton

Abstract: We present the serendipitous discovery of a low luminosity nova occurring in a symbiotic binary star system in the Milky Way. We lay out the extensive archival data alongside new follow-up observations related to the stellar object V$^*$ CN Cha in the constellation of Chamaeleon. The object had long period ($\sim\! 250\,$day), high amplitude ($\sim\! 3\,$mag) optical variability in its recent past… ▽ More We present the serendipitous discovery of a low luminosity nova occurring in a symbiotic binary star system in the Milky Way. We lay out the extensive archival data alongside new follow-up observations related to the stellar object V$^*$ CN Cha in the constellation of Chamaeleon. The object had long period ($\sim\! 250\,$day), high amplitude ($\sim\! 3\,$mag) optical variability in its recent past, preceding an increase in optical brightness by $\sim\! 8\,$magnitudes and a persistence at this luminosity for about 3 years, followed by a period of $\sim\! 1.4\,{\rm mag}\,{\rm yr}^{-1}$ dimming. The object's current optical luminosity seems to be dominated by H$α$ emission, which also exhibits blue-shifted absorption (a P-Cygni-like profile). After consideration of a number of theories to explain these myriad observations, we determine that V$^*$ CN Cha is most likely a symbiotic (an evolved star-white dwarf binary) system which has undergone a long-duration, low luminosity, nova. Interpreted in this way, the outburst in V$^*$ CN Cha is among the lowest luminosity novae ever observed. △ Less

Submitted 18 February, 2020; originally announced February 2020.

Comments: Submitted to AJ, 19 pages, 9 figure

Galaxy Formation with BECDM -- II. Cosmic Filaments and First Galaxies

Authors: Philip Mocz, Anastasia Fialkov, Mark Vogelsberger, Fernando Becerra, Xuejian Shen, Victor H. Robles, Mustafa A. Amin, Jesús Zavala, Michael Boylan-Kolchin, Sownak Bose, Federico Marinacci, Pierre-Henri Chavanis, Lachlan Lancaster, Lars Hernquist

Abstract: Bose-Einstein Condensate Dark Matter (BECDM; also known as Fuzzy Dark Matter) is motivated by fundamental physics and has recently received significant attention as a serious alternative to the established Cold Dark Matter (CDM) model. We perform cosmological simulations of BECDM gravitationally coupled to baryons and investigate structure formation at high redshifts ($z \gtrsim 5$) for a boson ma… ▽ More Bose-Einstein Condensate Dark Matter (BECDM; also known as Fuzzy Dark Matter) is motivated by fundamental physics and has recently received significant attention as a serious alternative to the established Cold Dark Matter (CDM) model. We perform cosmological simulations of BECDM gravitationally coupled to baryons and investigate structure formation at high redshifts ($z \gtrsim 5$) for a boson mass $m=2.5\cdot 10^{-22}~{\rm eV}$, exploring the dynamical effects of its wavelike nature on the cosmic web and the formation of first galaxies. Our BECDM simulations are directly compared to CDM as well as to simulations where the dynamical quantum potential is ignored and only the initial suppression of the power spectrum is considered -- a Warm Dark Matter-like ("WDM") model often used as a proxy for BECDM. Our simulations confirm that "WDM" is a good approximation to BECDM on large cosmological scales even in the presence of the baryonic feedback. Similarities also exist on small scales, with primordial star formation happening both in isolated haloes and continuously along cosmic filaments; the latter effect is not present in CDM. Global star formation and metal enrichment in these first galaxies are delayed in BECDM/"WDM" compared to the CDM case: in BECDM/"WDM" first stars form at $z\sim 13$/$13.5$ while in CDM star formation starts at $z\sim 35$. The signature of BECDM interference, not present in "WDM", is seen in the evolved dark matter power spectrum: although the small scale structure is initially suppressed, power on kpc scales is added at lower redshifts. Our simulations lay the groundwork for realistic simulations of galaxy formation in BECDM. △ Less

Submitted 13 November, 2019; originally announced November 2019.

Comments: 19 pages, 12 figures, submitted to MNRAS

First star-forming structures in fuzzy cosmic filaments

Authors: Philip Mocz, Anastasia Fialkov, Mark Vogelsberger, Fernando Becerra, Mustafa A. Amin, Sownak Bose, Michael Boylan-Kolchin, Pierre-Henri Chavanis, Lars Hernquist, Lachlan Lancaster, Federico Marinacci, Victor Robles, Jesús Zavala

Abstract: In hierarchical models of structure formation, the first galaxies form in low-mass dark matter potential wells, probing the behavior of dark matter on kiloparsec (kpc) scales. Even though these objects are below the detection threshold of current telescopes, future missions will open an observational window into this emergent world. In this Letter we investigate how the first galaxies are assemble… ▽ More In hierarchical models of structure formation, the first galaxies form in low-mass dark matter potential wells, probing the behavior of dark matter on kiloparsec (kpc) scales. Even though these objects are below the detection threshold of current telescopes, future missions will open an observational window into this emergent world. In this Letter we investigate how the first galaxies are assembled in a `fuzzy' dark matter (FDM) cosmology where dark matter is an ultralight $\sim 10^{-22}$~eV boson and the primordial stars are expected to form along dense dark matter filaments. Using a first-of-its-kind cosmological hydrodynamical simulation, we explore the interplay between baryonic physics and unique wavelike features inherent to FDM. In our simulation, the dark matter filaments show coherent interference patterns on the boson de Broglie scale and develop cylindrical soliton-like cores which are unstable under gravity and collapse into kpc-scale spherical solitons. Features of the dark matter distribution are largely unaffected by the baryonic feedback. On the contrary, the distributions of gas and stars, which do form along the entire filament, exhibit central cores imprinted by dark matter -- a smoking gun signature of FDM. △ Less

Submitted 7 October, 2019; v1 submitted 3 October, 2019; originally announced October 2019.

Comments: 6 pages, 2 figures, accepted to Phys. Rev. Lett

Dynamical Friction in a Fuzzy Dark Matter Universe

Authors: Lachlan Lancaster, Cara Giovanetti, Philip Mocz, Yonatan Kahn, Mariangela Lisanti, David N. Spergel

Abstract: We present an in-depth exploration of the phenomenon of dynamical friction in a universe where the dark matter is composed entirely of so-called Fuzzy Dark Matter (FDM), ultralight bosons of mass $m\sim\mathcal{O}(10^{-22})\,$eV. We review the classical treatment of dynamical friction before presenting analytic results in the case of FDM for point masses, extended mass distributions, and FDM backg… ▽ More We present an in-depth exploration of the phenomenon of dynamical friction in a universe where the dark matter is composed entirely of so-called Fuzzy Dark Matter (FDM), ultralight bosons of mass $m\sim\mathcal{O}(10^{-22})\,$eV. We review the classical treatment of dynamical friction before presenting analytic results in the case of FDM for point masses, extended mass distributions, and FDM backgrounds with finite velocity dispersion. We then test these results against a large suite of fully non-linear simulations that allow us to assess the regime of applicability of the analytic results. We apply these results to a variety of astrophysical problems of interest, including infalling satellites in a galactic dark matter background, and determine that \emph{(1)}~for FDM masses $m\gtrsim 10^{-21}\, {\rm eV}\, c^{-2}$, the timing problem of the Fornax dwarf spheroidal's globular clusters is no longer solved and \emph{(2)}~the effects of FDM on the process of dynamical friction for satellites of total mass $M$ and relative velocity $v_{\rm rel}$ should require detailed numerical simulations for $\left(M/10^9~M_{\odot}\right) \left(m/10^{-22}~{\rm eV}\right)\left(100~{\rm km}~{\rm s}^{-1}/v_{\rm rel}\right) \sim 1$, parameters which would lie outside the validated range of applicability of any currently developed analytic theory, due to transient wave structures in the time-dependent regime. △ Less

Submitted 29 November, 2019; v1 submitted 13 September, 2019; originally announced September 2019.

Comments: 41 pages, 11 figures

Persistence of power-law correlations in nonequilibrium steady states of gapped quantum spin chains

Authors: Jarrett L. Lancaster, Joseph P. Godoy

Abstract: The existence of quasi-long range order is demonstrated in nonequilibrium steady states in isotropic $XY$ spin chains including of two types of additional terms that each generate a gap in the energy spectrum. The system is driven out of equilibrium by initializing a domain-wall magnetization profile through application of an external magnetic field and switching off the magnetic field at the same… ▽ More The existence of quasi-long range order is demonstrated in nonequilibrium steady states in isotropic $XY$ spin chains including of two types of additional terms that each generate a gap in the energy spectrum. The system is driven out of equilibrium by initializing a domain-wall magnetization profile through application of an external magnetic field and switching off the magnetic field at the same time the energy gap is activated. An energy gap is produced by either applying a staggered magnetic field in the $z$ direction or introducing a modulation to the $XY$ coupling. The magnetization, spin current, and spin-spin correlation functions are computed analytically in the thermodynamic limit at long times after the quench. For both types of systems, we find the persistence of power-law correlations despite the ground-state correlation functions exhibiting exponential decay. △ Less

Submitted 15 November, 2019; v1 submitted 29 July, 2019; originally announced July 2019.

Comments: 19 pages, 9 figures, published version

Journal ref: Phys. Rev. Research 1, 033104 (2019)

Introducing SU(3) color charge in undergraduate quantum mechanics

Authors: Brandon L. Inscoe, Jarrett L. Lancaster

Abstract: We present a framework for investigating effective dynamics of SU(3) color charge. Two- and three-body effective interaction terms inspired by the Heisenberg spin model are considered. In particular, a toy model for a three-source "baryon" is constructed and investigated analytically and numerically for various choices of interactions. VPython is used to visualize the nontrivial color charge dynam… ▽ More We present a framework for investigating effective dynamics of SU(3) color charge. Two- and three-body effective interaction terms inspired by the Heisenberg spin model are considered. In particular, a toy model for a three-source "baryon" is constructed and investigated analytically and numerically for various choices of interactions. VPython is used to visualize the nontrivial color charge dynamics. The treatment should be accessible to undergraduate students who have taken a first course in quantum mechanics, and suggestions for independent student projects are proposed. △ Less

Submitted 23 September, 2020; v1 submitted 29 July, 2019; originally announced July 2019.

Comments: 14 pages, 6 figures

Journal ref: Am. J. Phys. 89, 172-184 (2021)

Slow-dissipation limit of the harmonic oscillator with general power-law damping

Authors: Jarrett L. Lancaster

Abstract: An approximate solution is presented for simple harmonic motion in the presence of damping by a force which is a general power-law function of the velocity. The approximation is shown to be quite robust, allowing for a simple way to investigate amplitude decay in the presence of general types of weak, nonlinear damping. An approximate solution is presented for simple harmonic motion in the presence of damping by a force which is a general power-law function of the velocity. The approximation is shown to be quite robust, allowing for a simple way to investigate amplitude decay in the presence of general types of weak, nonlinear damping. △ Less

Submitted 23 September, 2020; v1 submitted 7 November, 2018; originally announced November 2018.

Comments: 6 pages, 3 figures

Journal ref: Am. J. Phys. 88, 1151-1155 (2020)

The Halo's Ancient Metal-Rich Progenitor Revealed with BHB Stars

Authors: Lachlan Lancaster, Sergey E. Koposov, Vasily Belokurov, N. Wyn Evans, Alis J. Deason

Abstract: Using the data from the Sloan Digital Sky Survey and the Gaia satellite, we assemble a pure sample of $\sim$3000 Blue Horizontal Branch (BHB) stars with 7-D information, including positions, velocities and metallicities. We demonstrate that, as traced with BHBs, the Milky Way's stellar halo is largely unmixed and can not be well represented with a conventional Gaussian velocity distribution. A sin… ▽ More Using the data from the Sloan Digital Sky Survey and the Gaia satellite, we assemble a pure sample of $\sim$3000 Blue Horizontal Branch (BHB) stars with 7-D information, including positions, velocities and metallicities. We demonstrate that, as traced with BHBs, the Milky Way's stellar halo is largely unmixed and can not be well represented with a conventional Gaussian velocity distribution. A single-component model fails because the inner portions of the halo are swamped with metal-rich tidal debris from an ancient, head-on collision, known as the "Gaia Sausage". Motivated by the data, we build a flexible mixture model which allows us to track the evolution of the halo make-up across a wide range of radii. It is built from two components, one representing the radially anisotropic Sausage stars with their lobed velocity distribution, the other representing a more metal-poor and more isotropic component built up from minor mergers. We show that inside 25 kpc the "Sausage" contributes at least 50 % of the Galactic halo. The fraction of "Sausage" stars diminishes sharply beyond 30 kpc, which is the long-established break radius of the classical stellar halo. △ Less

Submitted 14 February, 2020; v1 submitted 11 July, 2018; originally announced July 2018.

Comments: 11 pages, 8 figures

Apocenter Pile-Up: Origin of the Stellar Halo Density Break

Authors: Alis J. Deason, Vasily Belokurov, Sergey E. Koposov, Lachlan Lancaster

Abstract: We measure the orbital properties of halo stars using 7-dimensional information provided by Gaia and the Sloan Digital Sky Survey. A metal-rich population of stars, present in both local main sequence stars and more distant blue horizontal branch stars, have very radial orbits (eccentricity ~ 0.9) and apocenters that coincide with the stellar halo "break radius" at galactocentric distance r ~ 20 k… ▽ More We measure the orbital properties of halo stars using 7-dimensional information provided by Gaia and the Sloan Digital Sky Survey. A metal-rich population of stars, present in both local main sequence stars and more distant blue horizontal branch stars, have very radial orbits (eccentricity ~ 0.9) and apocenters that coincide with the stellar halo "break radius" at galactocentric distance r ~ 20 kpc. Previous work has shown that the stellar halo density falls off much more rapidly beyond this break radius. We argue that the correspondence between the apocenters of high metallicity, high eccentricity stars and the broken density profile is caused by the build-up of stars at the apocenter of a common dwarf progenitor. Although the radially biased stars are likely present down to metallicities of [Fe/H] ~ -2 the increasing dominance at higher metallicities suggests a massive dwarf progenitor, which is at least as massive as the Fornax and Sagittarius dwarf galaxies, and is likely the dominant progenitor of the inner stellar halo. △ Less

Submitted 3 July, 2018; v1 submitted 25 May, 2018; originally announced May 2018.

Comments: 5 pages, 5 figures. Accepted for publication in ApJ Letters

Quantifying the Smoothness of the Stellar Halo: A Link to Accretion History

Authors: Lachlan Lancaster, Vasily Belokurov, N. Wyn Evans

Abstract: We investigate the utility of the 3-dimensional Two-Point Correlation Function (3D 2PCF) in quantifying substructure in the stellar halo of the Milky Way, particularly as a means of constraining the accretion history of our Galaxy. We use RR Lyrae variable stars from two different surveys as tracers of the structure in the Galactic stellar halo. We compare our measurements of the 3D 2PCF in these… ▽ More We investigate the utility of the 3-dimensional Two-Point Correlation Function (3D 2PCF) in quantifying substructure in the stellar halo of the Milky Way, particularly as a means of constraining the accretion history of our Galaxy. We use RR Lyrae variable stars from two different surveys as tracers of the structure in the Galactic stellar halo. We compare our measurements of the 3D 2PCF in these datasets to a suite of simulations of the formation of the stellar halo from Bullock and Johnston (2005). While there is some room for interpretation, we find that the amounts of structure to be broadly consistent with the simulations, while appearing smoother than average within the inner halo and at small scales. This suggests a preferred accretion history scenario in which the Milky Way's stellar halo acquired most of its mass about ~8 Gigayears ago, and has been largely quiescent since. Finally, we discuss the prospects of statistical tools such as the 2PCF in the Gaia era of galactic archaeology. △ Less

Submitted 24 April, 2018; originally announced April 2018.

Comments: 1o pages, 9 figures. Submitted to MNRAS. Comments are welcome

On the Schrodinger-Poisson--Vlasov-Poisson correspondence

Authors: Philip Mocz, Lachlan Lancaster, Anastasia Fialkov, Fernando Becerra, Pierre-Henri Chavanis

Abstract: The Schrödinger-Poisson equations describe the behavior of a superfluid Bose-Einstein condensate under self-gravity with a 3D wave function. As $\hbar/m\to 0$, $m$ being the boson mass, the equations have been postulated to approximate the collisionless Vlasov-Poisson equations also known as the collisionless Boltzmann-Poisson equations. The latter describe collisionless matter with a 6D classical… ▽ More The Schrödinger-Poisson equations describe the behavior of a superfluid Bose-Einstein condensate under self-gravity with a 3D wave function. As $\hbar/m\to 0$, $m$ being the boson mass, the equations have been postulated to approximate the collisionless Vlasov-Poisson equations also known as the collisionless Boltzmann-Poisson equations. The latter describe collisionless matter with a 6D classical distribution function. We investigate the nature of this correspondence with a suite of numerical test problems in 1D, 2D, and 3D along with analytic treatments when possible. We demonstrate that, while the density field of the superfluid always shows order unity oscillations as $\hbar/m\to 0$ due to interference and the uncertainty principle, the potential field converges to the classical answer as $(\hbar/m)^{2}$. Thus, any dynamics coupled to the superfluid potential is expected to recover the classical collisionless limit as $\hbar/m\to 0$. The quantum superfluid is able to capture rich phenomena such as multiple phase-sheets, shell-crossings, and warm distributions. Additionally, the quantum pressure tensor acts as a regularizer of caustics and singularities in classical solutions. This suggests the exciting prospect of using the Schrödinger-Poisson equations as a low-memory method for approximating the high-dimensional evolution of the Vlasov-Poisson equations. As a particular example we consider dark matter composed of ultra-light axions, which in the classical limit ($\hbar/m\to 0$) is expected to manifest itself as collisionless cold dark matter. △ Less

Submitted 10 April, 2018; v1 submitted 10 January, 2018; originally announced January 2018.

Comments: 17 pages, 9 figures, accepted (PRD)

Journal ref: Phys. Rev. D 97, 083519 (2018)

A tale of two modes: Neutrino free-streaming in the early universe

Authors: Lachlan Lancaster, Francis-Yan Cyr-Racine, Lloyd Knox, Zhen Pan

Abstract: We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant $ G_{\rm eff}$, and resulting in a ne… ▽ More We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant $ G_{\rm eff}$, and resulting in a neutrino opacity $\dotτ_ν\propto G_{\rm eff}^2 T_ν^5$. Using a conservative prior on the parameter $\log_{10}\left(G_{\rm eff} {\rm MeV}^2\right)$, we find a bimodal posterior distribution. The first of these modes is consistent with the standard $Λ$CDM cosmology and corresponds to neutrinos decoupling at redshift $z_{ν,{\rm dec}} > 1.3\times10^5$. The other mode of the posterior, dubbed the "interacting neutrino mode", corresponds to neutrino decoupling occurring within a narrow redshift window centered around $z_{ν,{\rm dec}}\sim8300$. This mode is characterized by a high value of the effective neutrino coupling constant, together with a lower value of the scalar spectral index and amplitude of fluctuations, and a higher value of the Hubble parameter. Using both a maximum likelihood analysis and the ratio of the two mode's Bayesian evidence, we find the interacting neutrino mode to be statistically disfavored compared to the standard $Λ$CDM cosmology. Interestingly, the addition of CMB polarization and direct Hubble constant measurements significantly raises the statistical significance of this secondary mode, indicating that new physics in the neutrino sector could help explain the difference between local measurements of $H_0$, and those inferred from CMB data. A robust consequence of our results is that neutrinos must be free streaming long before the epoch of matter-radiation equality. △ Less

Submitted 19 January, 2018; v1 submitted 21 April, 2017; originally announced April 2017.

Comments: 22 pages + references, 9 figures, 4 tables, references added

Journal ref: JCAP 1707 (2017) 033

Nonequilibrium current-carrying steady states in the anisotropic $XY$ spin chain

Authors: Jarrett L. Lancaster

Abstract: Out-of-equilibrium behavior is explored in the one-dimensional anisotropic $XY$ model. Initially preparing the system in the isotropic $XX$ model with a linearly varying magnetic field to create a domain-wall magnetization profile, dynamics is generated by rapidly changing the exchange interaction anisotropy and external magnetic field. Relaxation to a nonequilibrium steady state is studied analyt… ▽ More Out-of-equilibrium behavior is explored in the one-dimensional anisotropic $XY$ model. Initially preparing the system in the isotropic $XX$ model with a linearly varying magnetic field to create a domain-wall magnetization profile, dynamics is generated by rapidly changing the exchange interaction anisotropy and external magnetic field. Relaxation to a nonequilibrium steady state is studied analytically at the critical transverse Ising point, where correlation functions may be computed in closed form. For arbitrary values of anisotropy and external field, an effective generalized Gibbs' ensemble is shown to accurately describe observables in the long-time limit. Additionally, we find spatial oscillations in the exponentially decaying, transverse spin-spin correlation functions with wavelength set by the magnetization jump across the initial domain wall. This wavelength depends only weakly on anisotropy and magnetic field in contrast to the current, which is highly dependent on these parameters. △ Less

Submitted 19 May, 2016; v1 submitted 25 February, 2016; originally announced February 2016.

Comments: typos corrected, references added

Journal ref: Phys. Rev. E 93, 052136 (2016)

Comparison of canonical and microcanonical definitions of entropy

Authors: Michael Matty, Lachlan Lancaster, William Griffin, Robert H. Swendsen

Abstract: For more than 100 years, one of the central concepts in statistical mechanics has been the microcanonical ensemble, which provides a way of calculating the thermodynamic entropy for a specified energy. A controversy has recently emerged between two distinct definitions of the entropy based on the microcanonical ensemble: (1) The Boltzmann entropy, defined by the density of states at a specified en… ▽ More For more than 100 years, one of the central concepts in statistical mechanics has been the microcanonical ensemble, which provides a way of calculating the thermodynamic entropy for a specified energy. A controversy has recently emerged between two distinct definitions of the entropy based on the microcanonical ensemble: (1) The Boltzmann entropy, defined by the density of states at a specified energy, and (2) The Gibbs entropy, defined by the sum or integral of the density of states below a specified energy. A critical difference between the consequences of these definitions pertains to the concept of negative temperatures, which by the Gibbs definition, cannot exist. In this paper, we call into question the fundamental assumption that the microcanonical ensemble should be used to define the entropy. Our argument is based on a recently proposed canonical definition of the entropy as a function of energy. We investigate the predictions of the Boltzmann, Gibbs, and canonical definitions for a variety of classical and quantum models, including models which exhibit a first-order phase transition. Our results support the validity of the concept of negative temperature, but not for all models with a decreasing density of states. We find that only the canonical entropy consistently predicts the correct thermodynamic properties, while microcanonical definitions of entropy, including those of Boltzmann and Gibbs, are correct only for a limited set of simple models. △ Less

Submitted 1 July, 2016; v1 submitted 9 November, 2015; originally announced November 2015.

Comments: 12 pages, 15 figures

Emergence of long-range force laws from classical particle exchange

Authors: Jarrett L. Lancaster, Colin McGuire, Aaron P. Titus

Abstract: The analogy of classical repulsive interactions emerging from the exchange of mediating particles is revisited with a quantitative approach. Simulations are presented for a particular toy model which are accessible to undergraduate students at any level in the physics curriculum. Analytic treatment of the various regimes shows rigorously how effective force laws can emerge from an underlying micro… ▽ More The analogy of classical repulsive interactions emerging from the exchange of mediating particles is revisited with a quantitative approach. Simulations are presented for a particular toy model which are accessible to undergraduate students at any level in the physics curriculum. Analytic treatment of the various regimes shows rigorously how effective force laws can emerge from an underlying microscopic model and should be accessible to advanced undergraduate physics majors. The analysis presented uses the concept of emergence as motivation for students to gain experience building and testing simplified models for complex physical processes. △ Less

Submitted 1 October, 2019; v1 submitted 9 September, 2015; originally announced September 2015.

Comments: 16 pages, 4 figures

Micron-Scale Mapping of Megagauss Magnetic Fields in Petawatt Laser-Solid Interactions

Authors: Gourab Chatterjee, Prashant Kumar Singh, A. P. L. Robinson, N. Booth, O. Culfa, R. J. Dance, L. A. Gizzi, R. J. Gray, J. S. Green, P. Koester, G. Ravindra Kumar, L. Labate, Amit D. Lad, K. L. Lancaster, J. Pasley, N. C. Woolsey, P. P. Rajeev

Abstract: We report spatially and temporally resolved measurements of magnetic fields generated by petawatt laser-solid interactions with high spatial resolution, using optical polarimetry. The polarimetric measurements map the megagauss magnetic field profiles generated by the fast electron currents at the target rear. The magnetic fields at the rear of a 50 $μ$m thick aluminum target exhibit distinct and… ▽ More We report spatially and temporally resolved measurements of magnetic fields generated by petawatt laser-solid interactions with high spatial resolution, using optical polarimetry. The polarimetric measurements map the megagauss magnetic field profiles generated by the fast electron currents at the target rear. The magnetic fields at the rear of a 50 $μ$m thick aluminum target exhibit distinct and unambiguous signatures of electron beam filamentation. These results are corroborated by hybrid simulations. △ Less

Submitted 30 July, 2013; originally announced July 2013.

Comments: 5 pages, 5 figures

Measuring fast electron spectra and laser absorption in relativistic laser-solid interactions using differential bremsstrahlung photon detectors

Authors: R. H. H. Scott, E. L. Clark, F. Perez, M. J. V Streeter, J. R. Davies, H. -P. Schlenvoigt, J. J. Santos, S. Hulin, K. L. Lancaster, S. D. Baton, S. J. Rose, P. A. Norreys

Abstract: A photon detector suitable for the measurement of bremsstrahlung spectra generated in relativistically-intense laser-solid interactions is described. The Monte Carlo techniques used to back-out the fast electron spectrum and laser energy absorbed into fast electrons are detailed. A relativistically-intense laser-solid experiment using frequency doubled laser light is used to demonstrate the effect… ▽ More A photon detector suitable for the measurement of bremsstrahlung spectra generated in relativistically-intense laser-solid interactions is described. The Monte Carlo techniques used to back-out the fast electron spectrum and laser energy absorbed into fast electrons are detailed. A relativistically-intense laser-solid experiment using frequency doubled laser light is used to demonstrate the effective operation of the detector. The experimental data was interpreted using the 3-spatial-dimension Monte Carlo code MCNPX (Pelowitz 2008), and the fast electron temperature found to be 125 keV. △ Less

Submitted 1 May, 2013; v1 submitted 30 April, 2013; originally announced April 2013.

Exotic dense matter states pumped by relativistic laser plasma in the radiation dominant regime

Authors: J. Colgan, J. Abdallah, Jr., A. Ya. Faenov, S. A. Pikuz, E. Wagenaars, N. Booth, C. R. D. Brown, O. Culfa, R. J. Dance, R. G. Evans, R. J. Gray, D. J. Hoarty, T. Kaempfer, K. L. Lancaster, P. McKenna, A. L. Rossall, I. Yu. Skobelev, K. S. Schulze, I. Uschmann, A. G. Zhidkov, N. C. Woolsey

Abstract: The properties of high energy density plasma are under increasing scrutiny in recent years due to their importance to our understanding of stellar interiors, the cores of giant planets$^{1}$, and the properties of hot plasma in inertial confinement fusion devices$^2$. When matter is heated by X-rays, electrons in the inner shells are ionized before the valence electrons. Ionization from the inside… ▽ More The properties of high energy density plasma are under increasing scrutiny in recent years due to their importance to our understanding of stellar interiors, the cores of giant planets$^{1}$, and the properties of hot plasma in inertial confinement fusion devices$^2$. When matter is heated by X-rays, electrons in the inner shells are ionized before the valence electrons. Ionization from the inside out creates atoms or ions with empty internal electron shells, which are known as hollow atoms (or ions)$^{3,4,5}$. Recent advances in free-electron laser (FEL) technology$^{6,7,8,9}$ have made possible the creation of condensed matter consisting predominantly of hollow atoms. In this Letter, we demonstrate that such exotic states of matter, which are very far from equilibrium, can also be formed by more conventional optical laser technology when the laser intensity approaches the radiation dominant regime$^{10}$. Such photon-dominated systems are relevant to studies of photoionized plasmas found in active galactic nuclei and X-ray binaries$^{11}$. Our results promote laser-produced plasma as a unique ultra-bright x-ray source for future studies of matter in extreme conditions as well as for radiography of biological systems and for material science studies$^{12,13,14,15}$. △ Less

Submitted 27 June, 2012; originally announced June 2012.

Controlling fast electron beam divergence using two laser pulses

Authors: R. H. H. Scott, C. Beaucourt, H. -P. Schlenvoigt, K. Markey, K. L. Lancaster, C. P. Ridgers, C. M. Brenner, J. Pasley, R. J. Gray, I. O. Musgrave, A. P. L Robinson, K. Li, M. M. Notley, J. R. Davies, S. D. Baton, J. J. Santos, J. -L. Feugeas, Ph. Nicolaï, G. Malka, V. T. Tikhonchuk, P. McKenna, D. Neely, S. J. Rose, P. A. Norreys

Abstract: This paper describes the first experimental demonstration of the guiding of a relativistic electron beam in a solid target using two co-linear, relativistically intense, picosecond laser pulses. The first pulse creates a magnetic field which guides the higher current fast electron beam generated by the second pulse. The effects of intensity ratio, delay, total energy and intrinsic pre-pulse are ex… ▽ More This paper describes the first experimental demonstration of the guiding of a relativistic electron beam in a solid target using two co-linear, relativistically intense, picosecond laser pulses. The first pulse creates a magnetic field which guides the higher current fast electron beam generated by the second pulse. The effects of intensity ratio, delay, total energy and intrinsic pre-pulse are examined. Thermal and Kα imaging showed reduced emission size, increased peak emission and increased total emission at delays of 4 - 6 ps, an intensity ratio of 10 : 1 (second:first) and a total energy of 186 J. In comparison to a single, high contrast shot, the inferred fast electron divergence is reduced by 2.7 times, while the fast electron current density is increased by a factor of 1.8. The enhancements are reproduced with modelling and are shown to be due to the self-generation of magnetic fields. Such a scheme could be of considerable benefit to fast ignition inertial fusion. △ Less

Submitted 15 May, 2012; v1 submitted 9 December, 2010; originally announced December 2010.

Observation of post-soliton expansion following laser propagation through an underdense plasma

Authors: G. Sarri, 1 D. K. Singh, 2 J. R. Davies, 2 K. L. Lancaster, 3 E. L. Clark, 4 S. Hassan, 4 J. Jiang, 2 N. Kageiwa, N. Lopes, A. Rehman, C. Russo, R. H. H. Scott, T. Tanimoto, Z. Najmudin, K. A. Tanaka, M. Tatarakis, M. Borghesi, P. A. Norreys

Abstract: The expansion of electromagnetic post-solitons emerging from the interaction of a 30 ps, $3\times 10^{18}$ W cm$^{-2}$ laser pulse with an underdense deuterium plasma has been observed up to 100 ps after the pulse propagation, when large numbers of post-solitons were seen to remain in the plasma. The temporal evolution of the post-solitons has been accurately characterized with a high spatial and… ▽ More The expansion of electromagnetic post-solitons emerging from the interaction of a 30 ps, $3\times 10^{18}$ W cm$^{-2}$ laser pulse with an underdense deuterium plasma has been observed up to 100 ps after the pulse propagation, when large numbers of post-solitons were seen to remain in the plasma. The temporal evolution of the post-solitons has been accurately characterized with a high spatial and temporal resolution. The observed expansion is compared to analytical models and three dimensional particle-in-cell results providing indication of the polarisation dependence of the post-soliton dynamics. △ Less

Submitted 1 July, 2010; originally announced July 2010.

Modeling Excitable Systems: Reentrant Tachycardia

Authors: Jarrett L. Lancaster, Esther M. Leise, Edward H. Hellen

Abstract: Excitable membranes are an important type of nonlinear dynamical system and their study can be used to provide a connection between physical and biological circuits. We discuss two models of excitable membranes important in cardiac and neural tissues. One model is based on the Fitzhugh-Nagumo equations and the other is based on a three-transistor excitable circuit. We construct a circuit that si… ▽ More Excitable membranes are an important type of nonlinear dynamical system and their study can be used to provide a connection between physical and biological circuits. We discuss two models of excitable membranes important in cardiac and neural tissues. One model is based on the Fitzhugh-Nagumo equations and the other is based on a three-transistor excitable circuit. We construct a circuit that simulates reentrant tachycardia and its treatment by surgical ablation. This project is appropriate for advanced undergraduates as a laboratory capstone project, or as a senior thesis or honors project, and can also be a collaborative project, with one student responsible for the computational predictions and another for the circuit construction and measurements. △ Less

Submitted 15 January, 2010; v1 submitted 2 July, 2008; originally announced July 2008.

Comments: 9 pages, twocolumn, revised and published in American Journal of Physics

Journal ref: Am. J. Phys. 78, 56-63 (2010)

Longitudinal Ion Acceleration from High-Intensity Laser Interactions with Underdense Plasma

Authors: L. Willingale, S. P. D. Mangles, P. M Nilson, R. J. Clarke, A. E. Dangor, M. C. Kaluza, S. Karsch, K. L. Lancaster, W. B. Mori, J. Schreiber, A. G. R. Thomas, M. S. Wei, K. Krushelnick, Z. Najmudin

Abstract: Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed. The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations have been… ▽ More Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed. The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations have been used to investigate the acceleration mechanism. The time varying magnetic field associated with the fast electron current provides a contribution to the accelerating electric field as well as providing a collimating field for the ions. A strong correlation between the plasma density and the ion acceleration was found. A short plasma scale-length at the vacuum interface was observed to be beneficial for the maximum ion energies, but the collimation appears to be improved with longer scale-lengths due to enhanced magnetic fields in the ramp acceleration region. △ Less

Submitted 17 December, 2007; originally announced December 2007.

Comments: 18 pages, 6 figures