arXiv subjects

Jorge Moreno

Publications and source records attributed to Jorge Moreno.

At least 19 recordsLinked to original sources

arXiv2026

Old Disks Die Hard: How Does AGN Feedback Suppress Disk Formation in Milky Way Mass Galaxies?

The connection between a galaxy's visual shape and its star formation rate is one of the oldest established trends in galaxy evolution, yet the origin of this trend remains unsettled. We analyze two Milky-Way mass halos simulated with FIRE-2 galaxy formation physics, each run without AGN and with up to three implementations of AGN feedback. We use them to study how and why AGN star formation suppression affects morphological evolution over cosmic time. Both runs without AGN feedback produce prominent, thin, star-forming spiral disks at $z=0$. Each of the AGN runs has less late-time star formation, and ends up with a higher spheroid fraction and a lower thin-disk mass fraction. In two instances, the AGN runs produce quenched lenticular galaxies with no cool gas at $z=0$. The main reason for these morphological differences is that AGN feedback becomes effective {\em just after} the onset of disk formation, at ``spin up," in every run. This time-differentiated impact suppresses the formation of disk stars, especially thin-disk stars, preferentially because thin-disk formation occurs late. The spheroidal components are assembled early in all runs, before AGN feedback is effective, and are therefore similar in mass and size across runs.

astro-ph.GA↗

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

Mergers Matter: Gravothermal Collapse in Dwarf Halos with Self-Interacting Dark Matter

Self-Interacting Dark Matter (SIDM) models with large cross sections at relative velocities below $\sim100\,{\rm km \, s}^{-1}$ can be tested with dwarf galaxy observations. We analyze six dark-matter-only zoom-in $\sim10^{10}\,{\rm M}_\odot$ halos with diverse assembly histories, adopting a cross section over mass of $\sigma/m = 70\,cm^2 \, g^{-1}$. We find that mergers inject orbital kinetic energy into the halo, altering the heat transport and the gravothermal evolution of the core. Three of the six halos -- those with the most quiescent merger histories -- show clear signs of core collapse in these simulations. Halos with sustained mergers do not collapse. Furthermore, merger-induced heat transport drives two non-collapsing halos to central densities well below the predictions of the gravothermal fluid model. These findings suggest a novel mechanism for producing dark-matter-deficient galaxies and expanding the diversity of rotation curves beyond what halo concentration alone predicts. Merger histories are thus essential for understanding central density distributions of dwarf galaxy halos in SIDM.

astro-ph.GA↗

arXiv2026

Resolving galaxy formation in the early Universe with BonFIRE and CampFIRE

The abundance and rapid growth of galaxies at cosmic dawn revealed by the James Webb Space Telescope challenges models of galaxy formation, motivating new simulations to uncover the processes driving early galaxy assembly. We present the first results from BonFIRE ($L\approx40$ cMpc, $m_{\rm baryon}\approx5\times10^4~\rm{M}_{\odot}$) and CampFIRE ($L\approx5$ cMpc, at both $m_{\rm baryon}\approx800~\rm{M}_{\odot}$ and $\approx6\times10^3~\rm{M}_{\odot}$), a suite of cosmological hydrodynamic simulations of early galaxy formation ($z\gtrsim6$) from the Feedback In Realistic Environments (FIRE) project, using the FIRE-3 model. We use a resampling procedure to combine the large statistics of BonFIRE with the higher resolution of CampFIRE and robustly predict galaxy properties over a wide dynamic range ($M_{\star}\sim10^4-10^{10}~\rm{M}_{\odot}$). Galaxy formation in this suite emerges through clustered, bursty star formation, with halo-scale star formation efficiencies reaching $10-30\%$ in high-mass halos. A subset of low-mass halos also have surprisingly high efficiencies of $\gtrsim1\%$ and host ultra-compact galaxies with narrow age spreads. We predict galaxy UV luminosity functions at $9\lesssim~z\lesssim25$ in broad agreement with observations at $M_{\rm UV}\gtrsim-19$, with a faint-end turnover at $M_{\rm UV}\approx-14$, but we slightly overpredict the abundance of brighter galaxies. We find that UV luminosity variability in early galaxies is strongly mass-dependent, with halo-to-halo scatter dominating at low masses and contributing comparably to rapid temporal burstiness at $M_{\rm halo}\gtrsim10^{10}~\rm{M}_{\odot}$. We also present first results from a simple Pop~III model with a top-heavy IMF, demonstrating broad agreement with independent Pop~III predictions and observational constraints.

astro-ph.GA↗

arXiv2026

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

APOGEE and Gaia data have revealed that the Milky Way's structure appears to have evolved through three distinct kinematic phases. First, at early cosmic times, the Milky Way was a disordered protogalaxy, which subsequently "spun up" to a second kinematic phase marked by star formation occurring in a rotating, thick stellar disk. The thick disk phase later transitioned to a third (and final) phase with star formation occurring in a cold, thin stellar disk. In this paper, we use a suite of FIRE-2 simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in our cosmological zoom-in simulations, and study their physical origin. In all of our galaxies, the early disordered phase occurs when the rate of cool gas ($T \leq 10^4$ K) converting into stars is low, the star formation rate is bursty, and the baryonic mass "sloshes" within the host potential with respect to the center of mass motion. The gas in the galaxy begins to spin coherently after the sloshing phase ends, followed by the spin-up of young stars. The central potential of the galaxy is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with a period when the rate of cool gas converting into stars is highest, even though the star formation rate remains bursty in this phase. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes. The thin disk phase is associated with a time of steady star formation and intermediate 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 the formation of a thick disk appears to be fairly minimal: a stable center of mass motion. The formation of a thin disk 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

Carrier Revival in Long Trapped-Ion Chains

For a single trapped ion, the excitation spectrum of a narrow optical transition consists of a Doppler- and recoil-free carrier accompanied by motional sidebands, which are equally spaced by the trap secular frequency and lie under a Doppler-broadened envelope that is shifted by the photon recoil. Outside the Lamb-Dicke regime, the large photon recoil distributes the line strength across many sidebands and suppresses excitation of the carrier. With multiple ions, the motional spectrum becomes dense, and the carrier is further weakened. Here, we predict a counterintuitive revival effect: increasing the number of ions in a linear chain can restore strong carrier excitation even under trapping conditions far from the single-ion Lamb-Dicke regime. Using a quantum-mechanical model of the excitation dynamics in linear ion chains, we find that sufficiently long chains concentrate the spectrum into the carrier. This effect enables efficient excitation of light ions at short wavelengths. It may also benefit multi-ion optical clocks and mixed-species quantum-logic spectroscopy.

physics.atom-ph↗

arXiv2026

Radial redistribution of stellar orbits in FIRE simulations of Milky-Way-mass galaxies

A central question in galactic dynamics and galactic archeology is: how much do the orbits of stars redistribute (migrate) after birth? We use the FIRE-2 cosmological zoom-in simulations of 11 Milky Way-mass galaxies to quantify the change in the orbital specific angular momentum, j_phi, orbital radius, R_orbit, and azimuthal velocity, v_phi, of stars from birth to today. We examine the dependences on stellar age, present-day R_orbit, and birth R_orbit, characterizing both the median (net) change, Delta R_orbit, and its scatter, sigma(Delta R_orbit). We comprehensively compare five ways of measuring orbital radius; we find generally consistent trends, but only when measuring radius today and radial redistribution self-consistently. Stars selected by their birth R_orbit typically decreased in R_orbit, j_phi, and v_phi since birth. The trend for stars at a given R_orbit today depends on age: those younger than ~5 Gyr generally decreased in R_orbit, j_phi, and v_phi since birth, while those older generally increased in R_orbit, j_phi, and v_phi since birth. sigma(Delta R_orbit), a standard metric of radial redistribution, increases with stellar age only up to ~ 3 Gyr; it saturates at sigma(Delta R_orbit) ~2 kpc for older stars. This saturation contradicts a common expectation of a monotonic increase with age. Our results broadly agree with recent observational inferences of Delta R_orbit and sigma(Delta R_orbit) in the Milky Way. Across our FIRE-2 sample, the timing of disk formation does not correlate with sigma(Delta R_orbit), but it correlates with (net) Delta R_orbit.

astro-ph.GA↗

arXiv2026

Beyond Cloud-9: The case for discovering more HI-rich failed halos

HI-rich starless halos, should they exist, hold great promise for elucidating dark matter halo structure. Yet realizing this potential demands reliable theoretical predictions for their properties and abundances. Indeed, the recent identification of Cloud-9 as a strong HI-rich starless halo candidate in the nearby universe makes such predictions timely. This Letter examines HI-rich ($M_\mathrm{HI} \geq 10^6\,M_\odot$) starless (isolated/central) halos at $z=0$ across three cosmological simulations: FIREbox, Recal-EAGLE and NIVARIA-LG. All three successfully produce such objects, with $M_\mathrm{HI}$ extending up to $\sim\!1$--$2$ dex above Cloud-9, but with number densities that vary by a factor of $\sim30$. These populations span different regions of the $M_\mathrm{HI}$--$M_\mathrm{gas}$--$M_{200}$ space: NIVARIA-LG produces objects with higher $M_\mathrm{HI}$ and $M_\mathrm{gas}$ values ($\gtrsim 10^7$ and $\gtrsim 10^8\,M_\odot$), while FIREbox predicts they lie within extremely narrow ranges of $M_\mathrm{gas} \sim(1.1$--$1.6)\times10^8\,M_\odot$ and $M_{200} \sim(7.8$--$8.6)\times10^9\,M_\odot$. Recal-EAGLE and NIVARIA-LG exhibit a strong $M_\mathrm{gas}-M_{200}$ correlation, with similar slopes but different normalizations. The simulations predict numerical Cloud-9 analogs -- though similarities in the shapes of their HI column-density profiles may be driven by FAST's modest beam; halving it already reveals differences. Collectively, these inter-simulation discrepancies make a compelling case for discoveries beyond Cloud-9: a statistical sample of well-resolved HI-rich starless halos is needed to discriminate amongst competing predictions.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2026

Constraining Nuclear Molecular Gas Content with High-resolution CO Imaging of GOALS Galaxies

We present measurements of the cool molecular gas mass around the nuclei of two gas-rich mergers, III Zw 035 and IRAS F01364-1042, whose enclosed masses (M$_\mathrm{enc}$) within the central 40-80 pc would be overmassive if attributed entirely to the supermassive black hole mass (SMBH) and compared to SMBH-galaxy scaling relations. Our gas mass measurements are derived from Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 long-baseline observations of CO(J=2-1) and 230 GHz continuum emission at 14-20 pc resolution, which probes below the resolving limit of the previous black hole mass measurements. Subtracting molecular gas mass from these enclosed masses is not enough to reconcile with BH-galaxy relationships, but independently measuring M$_\mathrm{enc}$ using the cold CO(2-1) gas does shift the black holes down to their expected values. Still, these ALMA data reveal respective molecular gas masses of $\sim$3$\times$10$^7$ to $\sim$6$\times$10$^8$ M$_\odot$ within 70 pc of these black holes, which could challenge some black hole accretion models that assume nuclear gas like this has no angular momentum.

astro-ph.GA↗

arXiv2025

The quenched fraction of satellites around simulated Milky Way-mass galaxies

We compare satellite quenched fractions across three cosmological simulation suites (FIREbox, the FIRE-2 zoom-ins, and IllustrisTNG50) and observational datasets from SAGA, ELVES, and the combined satellite population of the Milky Way and M31. To enable consistent comparisons, we select Milky Way-mass hosts with $M_{\rm halo} = 10^{11.9}$ - $10^{12.2} \, M_{\odot}$ and satellites with stellar masses of $10^7$ - $10^{10} \, M_{\odot}$, applying nearly uniform radial selections and a common quenching definition. All three simulations reproduce the strong observed trend that lower-mass satellites are more likely to be quenched, closely matching the stellar mass dependence seen in SAGA, ELVES, and the Milky Way and M31 system. This agreement indicates that the mass dependence of satellite quenching is a robust outcome of contemporary galaxy formation models. Radial trends, however, show greater diversity. SAGA and ELVES exhibit gently declining quenched fractions with projected distance, consistent with stronger quenching at small radii. TNG50 most closely matches this behavior, while FIREbox remains broadly consistent with a weak radial trend within uncertainties. The FIRE-2 zoom-ins show suppressed quenched fractions at small projected distances, driven primarily by their paired MW-M31 analogs. We show that this discrepancy is not explained by host environment alone, but instead reflects atypical satellite populations in the paired systems, where star-forming and quenched satellites occupy distinct spatial distributions. Overall, our results demonstrate that stellar mass-quenched fraction trends are robust across simulations and observations, while radial trends are more sensitive to the detailed properties and distributions of satellite populations

astro-ph.GA↗

arXiv2025

Igniting galaxy formation in the post-reionization universe

It is widely believed that the ultraviolet background produced during the epoch of reionization conspires against the formation of low-mass galaxies. Indeed, this mechanism is often invoked as a solution to the so-called `missing satellites problem.' In this paper we employ FIREbox, a large-volume cosmological simulation based on the Feedback In Realistic Environments (FIRE-2) physics model, to characterize the mechanisms governing galaxy ignition in the post-reionization era. By carefully matching recently-ignited halos (with stellar ages below $100$ Myr at the time of selection) to halos that failed to form any stars, we conclude that the presence of cold-dense gas and halo concentration help incite the process of galaxy formation. Concretely, we find that $100\%$ of recently-ignited halos experience cold-dense gas enhancements relative to their matched failed counterparts. Likewise, approximately $83\%$ display enhancements in both cold-dense gas and Navarro-Frenk-White concentration ($c_{\rm NFW}$), while the remaining $\sim17\%$ exhibit enhanced cold-dense gas content and suppressed $c_{\rm NFW}$ values. Lastly, our simulation suggests that galaxy ignition can occur as late as $z=2$, potentially allowing us to observationally catch this process `in the act' in the foreseeable future.

astro-ph.GA↗

arXiv2025

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

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

astro-ph.GA↗

arXiv2025

The Nature of Post-Starburst Galaxies: Real Deal or Masquerading Impostors?

Post-starburst galaxies (PSBs) are a population of galaxies with spectral and photometric features indicative of rapid quenching following a recent starburst. The origin and nature of PSBs are currently debated. For example, a number of observed PSBs unexpectedly host substantial molecular gas despite their low inferred star-formation activity. Furthermore, the relative roles of galaxy interactions and quenching mechanisms in PSBs remain unclear. We study PSBs at $z=0.7$ and $z=1$ in the FIREbox cosmological simulation, selecting them primarily via their rest-frame optical photometric properties. The fraction of PSBs in FIREbox broadly agrees with observations, although some candidates are clear impostors with star-formation rates comparable to star-forming galaxies of similar mass. Impostors are rich in molecular gas and have a larger near-to-mid infrared flux ratios compared to quenched PSBs in the sample. The role of galaxy interactions of PSBs in FIREbox depends on their stellar mass. At low stellar masses ($\lesssim 10^{10}~\mathrm{M}_\odot$), PSBs have interaction fractions comparable to those of non-PSBs in the simulation, consistent with a scenario in which stellar feedback and gas consumption drive temporary quenching of star formation. At higher stellar masses ($\gtrsim 10^{10}~\mathrm{M}_\odot$), PSBs are preferentially interacting systems compared to non-PSBs, with major mergers providing the dominant contribution. We conclude that stellar feedback and galaxy interactions in FIREbox can produce galaxies with observational properties akin to those of observed PSBs, many of which are actively forming stars. Additional quenching channels, such as massive black hole feedback, are likely required to explain a long-lived, quenched population of PSBs.

astro-ph.GA↗

arXiv2025

Second public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation

We describe the second data release (DR2) of the FIRE-2 cosmological zoom-in simulations of galaxy formation, from the Feedback In Realistic Environments (FIRE) project, available at http://flathub.flatironinstitute.org/fire. DR2 includes all snapshots for most simulations, starting at z ~ 99, with all snapshot time spacings <~ 25 Myr. The Core suite -- comprising 14 Milky Way-mass galaxies, 5 SMC/LMC-mass galaxies, and 4 lower-mass galaxies -- includes 601 snapshots to z = 0. For the Core suite, we also release resimulations with physics variations: (1) dark-matter-only versions; (2) a modified ultraviolet background with later reionization at z = 7.8; (3) magnetohydrodynamics, anisotropic conduction, and viscosity in gas; and (4) a model for cosmic-ray injection, transport, and feedback (assuming a constant diffusion coefficient). The Massive Halo suite now includes 8 massive galaxies with 278 snapshots to z = 1. The High Redshift suite includes 34 simulations: in addition to the 22 simulations run to z = 5, we now include 12 additional simulations run to z = 7 and z = 9. We also release 4 dark-matter-only cosmological boxes used to generate zoom-in initial conditions for many FIRE simulations. Most simulations include catalogs of (sub)halos and galaxies at all available snapshots, and most Core simulations to z = 0 include full halo merger trees.

astro-ph.GA↗

arXiv2025

Disks no more: the morphology of low-mass simulated galaxies in FIREbox

We study the morphology of hundreds of simulated central galaxies in the stellar mass range $M_\star=10^{7.5} \rm - 10^{11}~$\msun\, from the FIREbox cosmological volume. We demonstrate that FIREbox is able to predict a wide variety of morphologies, spanning from disk-dominated objects to spheroidal galaxies supported by stellar velocity dispersion. However, the simulations predict a strong relation between morphology (degree of rotational support) and stellar mass: galaxies comparable to the Milky Way are often disk-dominated while the presence of stellar disks mostly vanishes for dwarfs with $M_\star <10^9 ~$\msun. This defines a ``morphology transition'' regime for galaxies with $10^9 <M_\star/\rm{M_\odot}< 10^{10}$ in which disks become increasingly common, but below which disks are rare. We show that burstiness in the star formation history and the deepening of the gravitational potential strongly correlate in our simulations with this transition regime, with disks forming in objects with lower levels of burstiness in the last $\sim 6$ Gyr and halos with mass $\sim 10^{11} ~ \rm{M_{\odot}}$ and above. While observations support a transition towards thicker disks in the regime of dwarfs, our results are in partial disagreement with observations of at least some largely rotationally supported gas disks in dwarfs with $M_\star < 10^9$\msun. This study highlights dwarf morphology as a fundamental benchmark for testing future galaxy formation models.

astro-ph.GA↗

arXiv2025

How invisible stellar halos bias our understanding of ultra-faint galaxies

We explore how a realistic surface brightness detection limit of $\mu_V \approx 32.5$ mag arcsec$^{-2}$ for stars at the edges of ultra-faint galaxies affects our ability to infer their underlying properties. We use a sample of 19 galaxies with stellar masses $\approx 400 - 40,000~{\rm M}_\odot$ simulated with FIRE-2 physics and baryonic mass resolution of $30~M_{\odot}$. The surface brightness cut leads to smaller sizes, lower stellar masses, and lower stellar velocity dispersions than the values inferred without the cut. However, by imposing this realistic limit, our inferred galaxy properties lie closer to observed populations in the mass-size plane, better match observed velocity dispersions as a function of stellar mass, and better reproduce derived circular velocities as a function of half-light radius. For the most massive galaxies in our sample, the surface brightness cut leads to higher mean $\rm [Fe/H]$ values, but the increase is not enough to match the observed MZR. Finally, we demonstrate that the common Wolf et al. (2010) mass estimator is less accurate when the surface brightness cut is applied. For our lowest-mass galaxies, in particular, excluding the low-surface brightness outskirts causes us to overestimate their central dark-matter densities and virial masses. This suggests that attempts to use mass estimates of ultra-faint galaxies to constrain dark-matter physics or to place constraints on the low-mass threshold of galaxy formation must take into account surface brightness limits or risk significant biases.

astro-ph.GA↗

arXiv2025

Stellar Velocity Dispersion versus Age: Consistency across Observations and Simulations, with the Milky Way as an Outlier

Within disk galaxies, the velocity dispersion, $\sigma$, of stars increases with age, $\tau$, as measured in the Milky Way (MW) and nearby galaxies. This relation provides a key window into galactic formation history, tracing both the kinematics of stars at birth and the dynamical heating of stars after birth. We compile and compare observational measurements of the MW, M31, M33, and 16 galaxies from the PHANGS survey. The MW exhibits significantly colder stellar kinematics, with 2-3 times lower $\sigma(\tau)/v_{\phi,0}$ at a given age, than all but one other observed galaxy. Therefore, the MW is a kinematic outlier. To assess how measurement effects influence $\sigma(\tau)$, we analyze the FIRE-2 cosmological simulations, quantifying the impact of uncertainties in stellar age, aperture size, galactocentric radius, and galaxy inclination. Aperture size and galactocentric radius affect $\sigma(\tau)$ by up to a factor of $\approx2$ for stars younger than 100 Myr, with milder effects on older stars. Age uncertainties up to 40\% change the \textit{value} of $\sigma(\tau)$ at a given age by $\lesssim20\%$ but can reshape the relation with age and erase merger signatures. We compare $\sigma(\tau)/v_{\phi,0}$ in FIRE-2 simulations with observations. FIRE-2 agrees well with M31 and M33 at all measured ages, and with PHANGS for stars older than $\approx500$ Myr. The average $\sigma(\tau)/v_{\phi,0}$ in FIRE-2 is about two times higher than the MW at most ages, but the youngest stars show better agreement. The velocity ratios ($\sigma_{\phi}/\sigma_{R}$, $\sigma_{Z}/\sigma_{\phi}$, $\sigma_{Z}/\sigma_{R}$) in FIRE-2 broadly agree with the MW. We conclude that $\sigma(\tau)$ in FIRE-2, and most cosmological zoom-in simulations, reasonably matches observed nearby galaxies, but matching the MW is rare, because it is a kinematic outlier.

astro-ph.GA↗