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Claude-André Faucher-Giguère

Publications and source records attributed to Claude-André Faucher-Giguère.

At least 19 recordsLinked to original sources
arXiv2026

From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?

APOGEE and Gaia data suggest the Milky Way's kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ``spun up'' into a thick stellar disk, and then ``cooled down'' to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas ($T \leq 10^4$ K) converting into stars is low, star formation is bursty, and the baryonic mass ``sloshes'' within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.

astro-ph.GA↗
arXiv2026

Star Formation Mitosis: The Effects of Particle Splitting on the Star Formation Outcome

Particle splitting can increase the dynamic range in Lagrangian astrophysical simulations by several orders of magnitude. However, such methods can introduce numerical noise that may affect the rate of convergence, particularly for collapse problems where fragmentation could be artificially suppressed or enhanced. Thus it is necessary to determine how these errors propagate to key quantities of interest, and to correct or mitigate them where possible. Using GIZMO's MFM-MHD method, we perform a set of $\approx 220$ simulations of a uniform-density turbulent gas sphere undergoing collapse, systematically varying the cloud mass and radius, feedback physics, initial resolution, the pace and cadence at which we split particles and the sink (accreting) or single-age stellar population (SSP; non-accreting) particle formation prescription. Crucially, we compare with control runs that start with uniformly-high resolution without splitting. While we find no evidence for artificial fragmentation caused by splitting particles, particle splitting runs have a smoother density field than their non-splitting counterparts. Due to this, particle splitting runs exhibit minor differences in the density and turbulence statistics. However, they form about the same total stellar mass as the control non-splitting runs, albeit with a slightly different mass spectrum. As a result, particle splitting runs have a different evolutionary history, compared to runs without particle splitting, especially when stellar feedback is present. Despite the changes seen in runs where we split particles, the numerical solution lies within the scatter inherently present in these chaotic systems, which we model by modifying the initial turbulent velocity field. Our results here argue that particle splitting generally ``works" for star formation applications, despite some systematic differences in the specifics.

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arXiv2026

LIMFAST. V. Probing Reionization with the Synergy Between 21 cm and Photometric Galaxy Surveys

Cross-correlating the 21\,cm signal from intergalactic neutral hydrogen with the galaxy distribution is a powerful probe of the Epoch of Reionization and the physical mechanisms driving it. While upcoming surveys with the SKA-Low radio telescope and the Roman Space Telescope offer ideal synergies for this measurement, foreground mitigation of 21\,cm data requires exquisite redshift precision of galaxies, effectively excluding the rich photometrically selected samples of Lyman-break galaxies (LBGs) with typical redshift uncertainties $σ_z \gtrsim 0.1$. We overcome this challenge using a squared statistic of the foreground-cleaned 21\,cm signal. Squaring the filtered 21\,cm signal couples surviving high $k_\parallel$ modes into low $k_\parallel$ fluctuations accessible to photometric LBG surveys. This higher-order cross-correlation statistic remains measurable even when direct cross-correlations are strongly suppressed. Applying Fisher matrix analysis to this LBG--21\,cm$^2$ cross-correlation signal simulated by the \texttt{LIMFAST} code over $6 \lesssim z \lesssim 10$, we investigate the prospects for constraining EoR physics combining SKA-Low 21\,cm measurements and Roman photometric galaxy samples. For an overlapping survey area covering 10\,deg$^2$, we find that the LBG--21\,cm$^2$ cross-correlation is detectable at high significance across much of the EoR through 1000\,hours of SKA-Low integration with 15\,MHz bandwidth and a Roman photometric galaxy sample from the High-Latitude Wide-Area Survey (HLWAS) reaching $m_\mathrm{AB,lim}=27$ and photo-$z$ uncertainties of $σ_z \sim 0.1$--0.3. The characteristic sign evolution of the large-scale cross-power spectrum---driven primarily, though not exclusively, by the evolving neutral fraction---provides strong constraints on the physical parameters governing the ionization and thermal history of the IGM.

astro-ph.CO↗
arXiv2026

Dual Signatures of Bursty Star Formation in High-Redshift UV Luminosity Functions

The UV luminosity function (UVLF) encodes key information about galaxy formation. The slowly evolving bright-end UVLFs at $z\gtrsim10$ have made UV variability from bursty star formation a promising explanation, but the impact of such variability need not be restricted to the bright end. Motivated by ultra-deep measurements of the $z\simeq7$ UVLF, we investigate whether UV variability can provide a unified interpretation of the low-mass star formation efficiency (SFE) at $z\simeq7$ and the abundance of UV-bright galaxies at $z\gtrsim12$. Using abundance matching, we show that a steep faint-end UVLF at $z\simeq7$ admits a range of interpretations. For modest, mass-independent UV variability, the inferred median SFE starts flattening below $M_{\rm h} \sim10^{10.5} M_{\odot}$. If UV variability instead grows with decreasing halo mass---a possibility motivated by simulations and observations---the inferred median SFE is steeper and lower by a factor of about three at $\sim10^9 M_{\odot}$, consistent with stronger feedback suppression in low-mass halos. Calibrated to the common target $z\simeq7$ UVLF, these two scenarios diverge strongly when extrapolated to $12\lesssim z\lesssim17$ under redshift-independent UV variability and SFE prescriptions. Strongly mass-dependent $M_\mathrm{UV}$ scatter links the steep $z\simeq7$ faint-end slope and high $z\gtrsim12$ bright-end abundance as dual signatures of burstiness in similarly low-mass halos. Stronger variability makes bright galaxies contribute a larger fraction of the UV luminosity density, whereas weaker variability favors faint sources, despite their similar predicted reionization histories. At fixed $M_\mathrm{UV}$, UV variability broadens the halo mass distribution and lowers the clustering bias, which provides a useful diagnostic of burstiness for JWST and Roman.

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arXiv2026

Widespread Extended [CII] Emission in High-Redshift Galaxies: Insights from the FIRE-2 Cosmological Zoom-in Simulations

Recent ALMA observations reveal diffuse [CII] emission ("[CII] halos") extending to $\sim 10\,$kpc in galaxies at $4 < z < 6$. These measurements provide new insights into high-redshift galactic ecosystems and processes that drive metal enrichment on inner circumgalactic scales. To better understand the nature of [CII] halos, we analyze a suite of high-redshift FIRE-2 simulations at $5 \leq z \leq 6$ in the stellar mass range of $10^{9}$--$10^{10.5}\,M_{\odot}$. By post-processing these simulations with three-dimensional dust radiative transfer and photoionization modeling, we generate synthetic images of [CII] and UV continuum emission, from which we extract one-dimensional surface brightness profiles. Our results reproduce both the galaxy-integrated and spatial distribution of [CII] and UV emission, capturing in particular the more extended profile of [CII] emission. Comparing the time evolution of [CII] halos with bursty star formation histories of the simulated galaxies, we find that [CII] emission becomes more spatially extended following the decline of star formation rate in recent starburst episodes. This implies a strong correlation between extended [CII] emission and bursty star formation, consistent with a key role for star formation-driven outflows in producing [CII] halos -- though the kinematics of [CII]-emitting gas suggest that inflows and turbulent motions are also significant contributors. We also find a modest contribution from satellite galaxies to extended [CII] emission. Our framework can be readily applied to predict the observability of [CII] halos at higher redshifts and extended to create spatially resolved synthetic observations of other important emission lines, such as [OIII] and H$α$.

astro-ph.GA↗
arXiv2026

Scaling Relations of Galactic Outflows Across Cosmic Time: New Insights from Cosmic Noon

Galactic outflows play a significant role in regulating galaxy evolution. Scaling relations between stellar mass ($M_\ast$), star formation (SFR), and outflow properties have been extensively studied at low redshifts ($z<1$) but less so beyond $z \sim 2$. We construct a joint sample of 387 galaxies from $z \sim$ 0-9, including 98 new Cosmic Noon galaxies from the Keck Baryonic Structure Survey and the Keck Lyman Continuum Spectroscopic Survey. Using high signal-to-noise emission lines (SNR $>$ 50 for H$α$ or [OIII] $\lambda5007$) from Keck/MOSFIRE spectra, we detect warm-ionized outflows by decomposing lines into narrow and broad components. With the joint sample, we explore the redshift evolution of outflow scaling relations. On average, outflows at Cosmic Noon have higher maximum velocities than those at low$-z$ by up to a factor of 3 for a fixed $M_\ast$, SFR, or SFR surface density. They also have higher mass outflow rates for a fixed $M_\ast$. Despite faster and stronger outflows, there is no evolution in the mass loading factor for a fixed $M_\ast$. We find evidence for galactic fountains, as the majority of outflowing gas is recycled at a radius of $\sim$0.03 R$_\textrm{vir}$. We also constrain the overall outflow occurrence rate in our galaxy sample to be at least 30$\%$ when taking galaxy orientation and outflow geometry into account. By analyzing the largest sample of warm-ionized outflows at Cosmic Noon to date and compiling large galaxy samples across all redshifts, we present a comprehensive analysis of galactic outflows throughout cosmic time.

astro-ph.GA↗
arXiv2026

Ardua: Unveiling the Baryon Cycle from Stars to the Cosmic Web

The circumgalactic medium (CGM) -- the multiphase gas reservoirs surrounding galaxies -- remains the least understood component of the baryon cycle governing galaxy growth, despite its central role in the Astro2020 Decadal Survey's priorities. Existing constraints come almost exclusively from pencil-beam absorption spectroscopy, leaving the spatial structure, kinematics, and phase interactions of CGM gas fundamentally unmapped. We present Ardua, a mission concept for NASA's ASTRA Initiative that combines wide-field far-ultraviolet spectroscopy with a Line Emission Mapper (LEM)-derived X-ray microcalorimeter instrument to obtain the first comprehensive emission maps spanning the full CGM temperature range, including cool neutral gas, ionized warm-hot phase gas, and the volume-filling hot corona. By observing more than 50 nearby galaxies comprehensively in the UV and X-ray, Ardua will test competing galaxy formation models, resolve multiphase gas flows and feedback-driven outflows, and extend baryon-cycle science to the intergalactic medium and the environments of exoplanet-hosting stars. Beyond its core CGM/IGM program, Ardua's wide-field, high-sensitivity instruments are designed to serve as a flexible community resource, supporting guest-investigator science across astrophysics. No planned or approved mission is designed to deliver this combined UV/X-ray survey capability.

astro-ph.IM↗
arXiv2026

A Hot DOG Forged in FIRE: Nuclear and Starburst Spectral Decomposition of a Luminous Infrared Galaxy Simulation with a Resolved Dust Torus

Ultraluminous infrared galaxies are powered by a combination of rapid star formation and active galactic nucleus (AGN) emission, but their relative importance is not always observationally clear. We study the galactic continuum spectrum of a cosmologically simulated $\sim 4 \times 10^{10} M_\odot$ stellar mass starburst galaxy at redshift $z\sim 4.4$ that refines down to resolve beyond the dust sublimation boundary of its super-Eddington-accreting $\sim 10^7 M_\odot$ supermassive black hole. We find that this system resembles the rare class of hot dust-obscured galaxy (Hot DOG), with a roughly flat (in $νF_ν$) IR emission spectrum that sharply drops off at wavelengths $\lesssim 5~μ\mathrm{m}$. Our system also matches with the observational properties of many Hot DOGs, including undergoing multiple galaxy mergers and being the most massive galaxy within a dense cosmological environment. The distinctive Hot DOG spectral shape in our system is caused by AGN-heated mid-IR warm dust, predominately starburst-heated far-IR cold dust, and a steep near- to mid-IR cutoff caused by strong absorption in the dense ISM of the galactic nucleus, rather than the dust torus itself. This system is lower luminosity ($L_\mathrm{IR} \sim 2 \times 10^{12} L_\odot$) than those detected by the WISE survey at similar redshifts, but will be a prime target for future far-IR surveys such as PRIMA. Our results show that Hot DOGs can naturally result as a transitional phase during rapid AGN accretion, but before significant AGN-driven outflows clear optically thin paths.

astro-ph.GA↗
arXiv2026

Dark Matter in Draco and Boötes I: Hints of a Core in an Ultra-Faint Dwarf from Simulation-Based Inference

The density profiles of dwarf spheroidal galaxies are among the most sensitive probes of dark matter physics, yet extracting them from noisy stellar kinematics remains a fundamental obstacle. We present GraphNPE, a simulation-based inference method for dynamical mass modeling that incorporates measurement uncertainties and spectroscopic selection functions in the forward model. Using mock data, we show that methods relying solely on line-of-sight velocity dispersion are biased toward cuspy density profiles, even in the absence of the mass-anisotropy degeneracy. By accessing higher-order velocity moments, particularly line-of-sight kurtosis, GraphNPE breaks key degeneracies and recovers density profiles with substantially less bias. We apply GraphNPE to Draco and Boötes I using MMT/Hectochelle and DESI for Draco, and the S5 survey for Boötes I. For each, we report density profiles and dark matter $J$- and $D$-factors. For Draco, GraphNPE yields consistent results across datasets, marginally preferring a cuspy inner profile ($ρ_{150} \sim 1.6-1.9 \times 10^8\,\mathrm{M}_\odot\,\mathrm{kpc}^{-3}$) in agreement with literature. On DESI, however, second-order Jeans modeling fits the dispersion but fails to reproduce the kurtosis, demonstrating higher-order moments are essential. For Boötes I, limited statistical power prevents definitive determination of the inner slope. GraphNPE recovers $ρ_{150} = 0.36^{+0.15}_{-0.11} \times 10^8\,\mathrm{M}_\odot\,\mathrm{kpc}^{-3}$, significantly lower than literature and consistent with a cored inner profile. This places Boötes I among the lowest density dwarfs at comparable stellar masses.

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arXiv2026

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

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

astro-ph.GA↗
arXiv2026

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

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

astro-ph.GA↗
arXiv2026

Cold vs. Hot Gas Accretion and Angular Momentum in FIRE Simulations: From Halo to Galaxy Scales

We present a systematic study of gas accretion and angular momentum in the circumgalactic medium (CGM) using high-resolution FIRE cosmological simulations. Our analysis includes halos spanning the critical $\sim 10^{12}\ \mathrm{M}_{\odot}$ scale where several transitions have been identified, including inner CGM virialization, the transition from bursty to steady star formation, and the emergence of thin disks. We find that the temperature of inflowing gas is correlated with the virialization of the inner CGM. CGM inflows are almost entirely cold ($T < 10^5$ K) in pre-virialized halos, while hot inflows ($T > 10^5$ K) dominate in virialized halos. When hot inflows dominate, cooling generally occurs simultaneously with circularization at galaxy radii. The dominance of hot inflows onto massive galaxies persists even at high redshift, where cold streams may coexist. Consistent with previous studies, cold inflows have higher specific angular momentum than dark matter and hot gas. However, in bursty, low-mass galaxies, cold inflows do not circularize prior to star formation, while in steady, massive galaxies, hot inflows circularize, cool, and form stars with disk-like kinematics. We additionally find that in bursty galaxies, accreted gas typically forms stars after residing in the galaxy for less than $\sim 5$ galaxy free-fall times, while in steady galaxies, gas can persist for up to $\sim 25$ free-fall times before forming stars. This highlights a key difference between star formation in bursty galaxies fed by cold accretion and steady equilibrium disks fed by hot accretion.

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arXiv2026

Hot accretion onto spiral galaxies: the origin of extended and warped HI discs

Gas accretion, hot ($\sim 10^6$ K) atmospheres, and a tilt between the rotation axes of the disc and the atmosphere are all common predictions of standard galaxy evolution theory for massive star-forming galaxies at low redshift. Using idealised hydrodynamic simulations, we demonstrate that the central regions of hot galaxy atmospheres continuously condense into cool ($\sim 10^4$ K) discs, while being replenished by an inflow from larger scales. The size and orientation of the condensed disc are determined by the angular momentum of the atmosphere, so the condensed disc is expected to often be tilted and more extended than the stellar disc. Continuous accretion from hot atmospheres can thus explain the ubiquity of extended and warped HI discs around local spirals, and also potentially provide the necessary fuel for star formation. This hot accretion scenario predicts the absence of significant HI from galaxy halos, consistent with recent 21 cm constraints on nearby spirals (the so-called `HI desert'). Moreover, our analysis indicates that observations of HI warps can be used to constrain the angular momentum, accretion rate, and gas metallicity of hot galaxy atmospheres, important parameters for disc galaxy evolution that are hard to determine by other means.

astro-ph.GA↗
arXiv2026

Accretion-Driven Turbulence in the Circumgalactic Medium

Simulations suggest that turbulence is ubiquitous in the circumgalactic medium (CGM), though the source and properties of CGM turbulence is uncertain. Using analytic considerations and hydrodynamic simulations we study how CGM turbulence is driven by gas accretion, thus providing a baseline for additional turbulence driving processes such as galaxy feedback. We demonstrate that in halos with mass $\sim 10^{10}-10^{12} M_{\odot}$ at $0 < z < 2$, accretion amplifies mild turbulent velocities near the virial radius of $σ_t(R_{\rm vir}) \sim 10 \, {\rm km \, s^{-1}}$ to virial velocities at inner CGM radii, $σ_t(0.1 R_{\rm vir}) \approx v_{\rm vir} \sim 100 \, {\rm km \, s^{-1}}$. Rapid cooling at these inner radii further implies that thermal pressure support is small, and the gas is dominated by the cool and warm ($\sim 10^4-10^5 \, {\rm K}$) phases. Inner CGM energetics in these halos is thus dominated by turbulence, with gas density distributions and velocity structure functions similar to those seen in simulations of isothermal supersonic turbulence, rather than those seen in subsonically turbulent stratified media such as the ICM. The accretion rate in these systems is regulated by the turbulence dissipation rate, in contrast with being regulated by the cooling rate as in more massive halos. We argue that galaxy feedback is unlikely to qualitatively change our conclusions unless it significantly depletes the CGM or continuously injects high specific energy material ($\gg v^2_{\rm vir}$). Such `turbulence-dominated' CGM can be identified in observations via the predicted wide lognormal ionization distributions and large velocity dispersions in UV absorption spectra.

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arXiv2026

Little Red Dots on FIRE: The Ability of Bursty Galaxies to Host an Abundant Population of High-Redshift AGN

The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift ($z\gtrsim4$) known as little red dots (LRDs), which are likely hosted in relatively low-mass galaxies. However, previous theoretical models have highlighted the difficulty in continuously feeding massive black holes in the central regions of bursty, high-redshift galaxies because of repeated gas evacuation by stellar feedback. We analyze galaxies in high-redshift FIRE-2 simulations to understand whether they are capable of hosting the observed abundant population of high-redshift AGN. We use a gravitational torque-driven accretion (GTDA) model and a simple free-fall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for $z=5-7$. The GTDA model and the free-fall model with black holes accreting $\lesssim 1$ percent of their central gas supply ($<100 \rm \ pc$) per free-fall time predict AGN abundances that are more than sufficient to explain the most recent LRD observations. The fiducial models, in fact, overpredict the number of low-luminosity AGN as compared with observations. We explore possible resolutions of this tension. A plausible, though likely not unique, scenario for alleviating the AGN overpredictions and which also provides a good match to the host-galaxy UV luminosity distribution suggests that LRDs are super Eddington-accreting, Eddington luminosity-limited, $M_{\rm BH}\gtrsim 2\times10^5 \ \rm M_\odot$ black holes residing in $M_\star\gtrsim 2\times10^7 \ \rm M_\odot$ galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.

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arXiv2026

Enhanced Multiphase Circumgalactic Medium and Gas Cycling in Galaxy Mergers

We investigate the impact of galaxy mergers on the circumgalactic medium (CGM) using the FIREbox cosmological hydrodynamic simulation. By comparing matched samples of merging and isolated galaxies with stellar masses $M_\star \sim 10^{10}$--$10^{11} M_{\odot}$ at $z=0$ and mass ratio of merging galaxies larger than $1:10$, we find that mergers significantly alter CGM properties. Merging systems exhibit enhanced radiative cooling, leading to shorter cooling times than free-fall times across large CGM volumes. This results in amplified multiphase structure and increased cool/cold gas content ($T \sim 10^4K$) compared to isolated galaxies. Both inflow and outflow mass fluxes are elevated by at least $\sim$1 dex in mergers across all temperature phases, with cool gas primarily generated in-situ via radiative cooling rather than from pre-existing streams. Gas cycling analysis reveals that mergers fundamentally accelerate CGM processing, amplifying the effective transfer rate from cold/cool cosmic inflow to galaxy inflow by factors of $\sim 30$, through rapid cycling of inflowing gas through intermediate CGM phases, efficiently fueling the ISM and star formation. The enhanced cool gas content in mergers produces elevated column densities for low- and intermediate-temperature ion species in the inner CGM, while high-temperature ones remain largely unaffected.

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arXiv2026

Under Pressure: UV Emission Line Ratios as Barometers of AGN Feedback Mechanisms

Feedback from active galactic nuclei (AGN) is widely acknowledged to regulate the growth of massive galaxies, though its driving mechanisms are debated. Prevailing theories suggest that AGN-driven outflows are driven either by radiation pressure acting directly on the dusty interstellar medium (ISM) or by hot winds entraining cooler ISM gas, but the relative contribution of each mechanism remains uncertain. By combining optical emission line measurements with highly ionized UV emission lines, it is possible to constrain whether the pressure source applied to ionized clouds is primarily radiation or primarily hydrodynamic, and thus constrain the dominant driver. This study presents the first multi-object analysis of far-ultraviolet (FUV) spectra from galactic-scale AGN-driven outflows in obscured quasars, based on Cosmic Origins Spectrograph observations of five low-redshift targets. By comparing narrow-line region UV emission line ratios to theoretical models that vary the importance of the two pressure sources, we find three out of five targets fall within the radiation pressure-dominated regime. A fourth target exhibits intermediate emission-line ratios that suggest radiation pressure and pressure from a hot wind are both dynamically important. Finally, the lowest-luminosity object in our sample may have a dynamically important hot wind component, but non-detections prevent a clear conclusion in this case. These results suggest radiation pressure dominates circum-nuclear narrow-line region cloud dynamics, but pressure from a hot wind also plays a role in some cases. This is consistent with AGN feedback scenarios mediated by radiation pressure or a short-lived hot wind phase that dissipates after initially accelerating outflows.

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arXiv2026

The Shape of FIREbox Galaxies and a Potential Tension with Low-mass Disks

We study the intrinsic and observable shapes of approximately 700 star-forming galaxies with stellar masses of $10^8 - 10^{11}$ M$_\odot$ from the FIREbox simulation at $z=0$. We calculate intrinsic axis ratios using inertia tensors weighted by three morphology types: "All Stars," "Young Stars," and "Luminosity-weighted Stars." Young Stars shows mass-dependent 3D configurations, with spheroidal, elongated, and disky shapes dominant at stellar masses of $10^{8.5}$ M$_\odot$, $10^{9.5}$ M$_\odot$, and $10^{10.5}$ M$_\odot$, respectively. Using the radiative transfer code SKIRT, we construct mock images for each galaxy and show that projected short-to-long axis ratios, $q$, inferred from 2D Sérsic fits are most closely related to Luminosity-weighted Stars tensor shapes and least resemble the All Stars' shapes. This suggests observed 2D shape distributions should not be compared to predictions based on 3D stellar mass shapes. Next, we construct a sample of mock images projected in random orientations and compare them to observed axis ratio distributions from the GAMA survey. At stellar masses below $10^{10}$ M$_\odot$, we produce too few galaxies with observed $q<0.4$ and none with $q<0.2$, suggesting that FIREbox does not produce enough low-mass disk galaxies. At higher masses, $10^{10} - 10^{11}$ M$_\odot$, we find that the predicted q distribution is sensitive to the dust-to-metal ratio; using our fiducial model, the distribution of $q$ values is formally consistent with observations, but there is tension with our ability to produce enough very thin systems with $q<0.2$. Future observational and theoretical programs aimed at understanding disk and thin-disk fractions will provide crucial tests of galaxy formation models.

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