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The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and morphologies). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. We predict that ~30% of massive central galaxies (with at least 10 11 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 10 8 –10 9 solar masses in stars). This distinctive class of galaxies provides an additional layer in our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario. A cosmological simulation shows that low-mass galaxies can form with far less dark matter than expected, with results matching some observed characteristics. Roughly one-third of massive central galaxies may host at least one such dark-matter-deficient satellite.

We study dwarf satellite galaxy quenching using observations from the Geha et al. (2012) NSA/SDSS catalog together with CDM cosmological simulations to facilitate selection and interpretation. We show that fewer than 30% of dwarfs (M* ∼ 108.5–9.5 Msun ) identified as satellites within massive host halos (M host ∼ 1012.5–14 Msun) are quenched. We conclude that whatever the action triggering environmental quenching of dwarf satellites, the process must be highly inefficient. We investigate a series of simple, one-parameter quenching models in order to understand what is required to explain the low quenched fraction and conclude that either the quenching timescale is very long (> 9.5 Gyr, a “slow starvation” scenario) or that the environmental trigger is not well matched to accretion within the virial volume. We further present FIRE/Gizmo hydrodynamic zoom-in simulations of isolated dark matter halos, two each at the mass of classical dwarf galaxies (Mvir ∼ 1010 Msun) and ultra-faint galaxies (Mvir ∼ 10 9 Msun). The resulting central galaxies lie on an extrapolated abundance matching relation from M* ∼ 106 to 104 Msun without a break. Our dwarfs with M* ∼ 106 Msun each have 1-2 well-resolved satellites with M* = 3 – 200 × 103 Msun. Even our isolated ultra-faint galaxies have star-forming subhalos. We combine our results with the ELVIS simulations to show that targeting the ∼ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ∼35% compared to random pointings. The well-resolved ultra-faint galaxies in our simulations (M * ∼ 3 - 30 × 103 Msun) form within Mpeak ∼ 0.5 – 3 × 109 Msun halos. Each has a uniformly ancient stellar population (> 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that Mhalo ∼ 5 × 109 Msun is a probable dividing line between halos hosting reionization “fossils” and those hosting dwarfs that can continue to form stars in isolation after reionization. Finally, we perform a systematic Bayesian analysis of rotation vs. dispersion support (vrot/σ) in 40 dwarf galaxies throughout the Local Volume (LV) over a stellar mass range 103.5 M sun < M* < 108 Msun. We find that the stars in 80% of the LV dwarf galaxies studied — both satellites and isolated systems — are dispersion-supported. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion supported stars. We apply the same Bayesian analysis to four of the FIRE/Gizmo hydrodynamic zoom-in simulations of isolated dwarf galaxies (109 Msun < M vir < 1010 Msun) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar vrot/sigma ratios that are consistent with the observed population of dIrrs and dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-dominated systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.

NGC 6822 is the closest isolated dwarf irregular galaxy to the Milky Way. Its proximity and stellar mass (\\(10^8 M_\\), large for a dwarf galaxy) allow for a detailed study of its kinematic properties. The red giant branch (RGB) stars at the galaxy's center are particularly interesting because they are aligned on an axis perpendicular to the galaxy's more extended HI disk. We detected a velocity gradient among the RGB population using spectra from Keck DEIMOS. This rotation is aligned with the HI disk, but the sense of rotation is about the major axis of the central RGB population. We measured the rotation velocity (\\(v\\)) and velocity dispersion (\\(\\)) of the RGB population in five metallicity bins. We found an increase of rotation support (\\(v/\\)) with increasing metallicity, driven primarily by decreasing dispersion. We also deduced an increasing radial distance for lower metallicity stars at \\(-0.5\\)~kpc/dex by relating the observed stellar kinematics to position via NGC 6822's HI velocity curve. While the inverted metallicity gradient-like could be interpreted as evidence for an outside-in formation scenario, it may instead indicate that stellar feedback disturbed a centrally star forming galaxy over time.

In hydrodynamic simulations, prevailing subgrid chemical-evolution models often use a single, \"IMF-averaged\" supernova yield, ignoring variations in elemental abundance ratios (particularly [\\(\\alpha\\)/Fe]) in the ejecta of higher- and lower-mass supernova progenitors within a stellar population. To understand the impact of this simplification and understand the impact of more explicit models, we run FIRE simulations of a dwarf galaxy \\((M_\\star(\\)z = 0\\() \\sim 10^6 M_\\odot)\\) using nucleosynthetic yields from the NuGrid database that depend on the stellar progenitor mass and metallicity. While NuGrid exhibits lower aggregate \\(\\alpha\\)-element production than default-FIRE yields, we find that its explicit mass dependence substantially widens the intrinsic scatter in the simulated [Fe/H]-[\\(\\alpha\\)/Fe] -- a phenomenon potentially visible in recent observations of dwarf galaxies.

Galaxies covering several orders of magnitude in stellar mass and a variety of Hubble types have been shown to follow the \"Radial Acceleration Relation\" (RAR), a relationship between \\(g_{\\rm obs}\\), the observed circular acceleration of the galaxy, and \\(g_{\\rm bar}\\), the acceleration due to the total baryonic mass of the galaxy. For accelerations above \\(10^{10}~{\\rm m \\, s}^{-2}\\), \\(g_{\\rm obs}\\) traces \\(g_{\\rm bar}\\), asymptoting to the 1:1 line. Below this scale, there is a break in the relation such that \\(\\rm g_{\\rm obs} \\sim g_{\\rm bar}^{1/2}\\). We show that the RAR slope, scatter and the acceleration scale are all natural consequences of the well-known baryonic Tully-Fisher relation (BTFR). We further demonstrate that galaxies with a variety of baryonic and dark matter (DM) profiles and a wide range of dark halo and galaxy properties (well beyond those expected in CDM) lie on the RAR if we simply require that their rotation curves satisfy the BTFR. We explore conditions needed to break this degeneracy: sub-kpc resolved rotation curves inside of \"cored\" DM-dominated profiles and/or outside \\(\\gg 100\\,\\)kpc could lie on BTFR but deviate in the RAR, providing new constraints on DM.

Dwarf galaxies located in extremely under-dense cosmic voids are excellent test-beds for disentangling the effects of large-scale environment on galaxy formation and evolution. We present integral field spectroscopy for low-mass galaxies (\\(M_=10^7-10^9~M_\\)) located inside (N=21) and outside (N=9) cosmic voids using the Keck Cosmic Web Imager (KCWI). Using measurements of stellar line-of-sight rotational velocity \\(v_rot\\) and velocity dispersion \\(_\\), we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values of \\(v_rot/_2\\) for our sample of isolated dwarf galaxies, and we find no trend between \\(v_rot/_\\) and distance from a massive galaxy \\(d_L^\\) out to \\(d_L^10\\) Mpc. These suggest that dwarf galaxies can become dispersion-supported \"puffy\" systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend between \\(v_rot/_\\) and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.

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.

Type Ia supernovae play a critical role in stellar feedback and elemental enrichment in galaxies. Recent transient surveys like the All-Sky Automated Survey for Supernova (ASAS-SN) and the Dark Energy Survey (DES) find that the specific Ia rate at z ~ 0 may be ~ 15-50 times higher in lower-mass galaxies than at Milky Way-mass. Independently, Milky Way observations show that the close-binary fraction of solar-type stars is higher at lower metallicity. Motivated by these observations, we use the FIRE-2 cosmological zoom-in simulations to explore the impact of varying Ia rate models, including metallicity dependence, on galaxies across a range of stellar masses: 10^7 Msun - 10^{11} Msun. First, we benchmark our simulated star-formation histories (SFHs) against observations. We show that assumed SFHs and stellar mass functions play a major role in determining the degree of tension between observations and metallicity-independent Ia rate models, and potentially cause ASAS-SN and DES observations to be much more consistent with each other than might naively appear. Models in which the Ia rate increases with decreasing metallicity (as ~ Z^{-0.5} to Z^{-1}) provide significantly better agreement with observations. Encouragingly, these increases in Ia rate (> 10 times in low-mass galaxies) do not significantly impact galaxy stellar masses and morphologies: effective radii, axis ratios, and v/sigma remain largely unaffected except for our most extreme rate models. We explore implications for both [Fe/H] and [alpha/Fe] enrichment: metallicity-dependent Ia rate models can improve agreement with observed stellar mass-metallicity relations in low-mass galaxies. Our results demonstrate that a wide range of metallicity-dependent Ia models are viable for galaxy formation and motivate future work in this area.

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 uniform projected apertures 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 MW+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 meaningful differences. SAGA and ELVES exhibit gently declining quenched fractions with projected distance, reflecting strong environmental quenching at small radii. TNG50 most closely matches this behavior, FIREbox, remains consistent with with a nearly flat trend within uncertainties, and the FIRE-2 zoom-ins show suppressed inner quenched fractions driven almost entirely by their paired MW-M31 hosts, which lack high-mass satellites and show strong radial segregation between star-forming and quenched systems. This environmental imprint suggests that host environment and assembly history can influence satellite quenching outcomes and may contribute to diversity across simulations. Overall, while the simulations consistently recover the stellar-mass dependence of quenching their radial trends vary, highlighting the influence of host-halo conditions and motivating deeper exploration of how host environments shape satellite quenching.

We explore how a realistic surface brightness detection limit of \\(_V 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 \\( 400 - 40,000~ M_\\) simulated with FIRE-2 physics and baryonic mass resolution of \\(30~M_\\). 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 \\( [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.