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We compare satellite quenched fractions across three cosmological simulation suites (FIREbox, the FIRE-2 zoom-ins, and IllustrisTNG50) and observational datasets from Satellites Around Galactic Analogs (SAGA), ELVES, and the combined satellite population of the Milky Way (MW) and M31. To enable consistent comparisons, we select MW-mass hosts withM_halo = 10^11.9–10^12.2M_⊙and satellites with stellar masses of 10⁷–10¹⁰M_⊙, 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 MW 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. 

Abstract We explore how a realistic surface brightness (SB) detection limit ofμ_V≈ 32.5 mag arcsec⁻²for stars at the edges of ultrafaint galaxies affects our ability to infer their underlying properties. We use a sample of 19 galaxies with stellar masses ≈400–40,000M_⊙simulated with FIRE-2 physics and baryonic mass resolution of 30M_⊙. The SB 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 SB 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 J. Wolf et al. dynamical mass estimator is less accurate when the SB 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 ultrafaint 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. 

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 part of the solution to the so-called “missing satellites problem.” In this paper we employFIREbox, a large-volume cosmological simulation based on theFeedback In Realistic Environmentsphysics model, to characterize the mechanisms governing galaxy ignition in the postreionization 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 helps 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_NFW), while the remaining ∼17% exhibit enhanced cold dense gas content and suppressedc_NFWvalues. Lastly, our simulation suggests that galaxy ignition can occur as late asz= 2, potentially allowing us to observationally catch this process “in the act” in the foreseeable future. 

ABSTRACT We study the morphology of hundreds of simulated central galaxies in the stellar mass range $$M_\star =$$ 107.5–1011  $$\rm M_\odot$$ from the FIREbox cosmological volume. We demonstrate that FIREbox is able to predict a wide variety of morphologies, spanning from disc-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 disc-dominated while the presence of stellar discs mostly vanishes for dwarfs with $$M_\star < 10^9 ~$$\rm M_\odot$$. This defines a ‘morphology transition’ regime for galaxies with $$10^9 < M_\star /\rm {M_\odot }< 10^{10}$$ in which discs become increasingly common, but below which discs are rare. We show that burstiness in the star formation history and the deepening of the gravitational potential strongly correlate in our simulations with this transition regime, with discs forming in objects with lower levels of burstiness in the last $$\sim 6$$ Gyr and haloes with mass $$\sim 10^{11} ~ \rm {{\rm M}_{\odot }}$$ and above. While observations support a transition towards thicker discs in the regime of dwarfs, our results are in partial disagreement with observations of at least some largely rotationally supported gas discs in dwarfs with $$M_\star < 10^9$$\rm M_\odot$$. This study highlights dwarf morphology as a fundamental benchmark for testing future galaxy formation models. 

Abstract We utilize the cosmological volume simulation FIREbox to investigate how a galaxy’s environment influences its size and dark matter content. Our study focuses on approximately 1200 galaxies (886 central and 332 satellite halos) in the low-mass regime, with stellar masses between 10⁶and 10⁹M_⊙. We analyze the size–mass relation (r₅₀–M_⋆), the inner dark matter mass–stellar mass ( $M_{\mathrm{DM}}^{50}$ –M_⋆) relation, and the halo mass–stellar mass (M_halo–M_⋆) relation. At fixed stellar mass, we find that galaxies experiencing stronger tidal influences, indicated by higher Perturbation Indices (PI > 1) are generally larger and have lower halo masses relative to their counterparts with lower Perturbation Indices (PI < 1). Applying a Random Forest regression model, we show that both the environment (PI) and halo mass (M_halo) are significant predictors of a galaxy’s relative size and dark matter content. Notably, becauseM_halois also strongly affected by the environment, our findings indicate that environmental conditions not only influence galactic sizes and relative inner dark matter content directly, but also indirectly, through their impact on halo mass. Our results highlight a critical interplay between environmental factors and halo mass in shaping galaxy properties, affirming the environment as a fundamental driver in galaxy formation and evolution. 

Abstract Dwarf galaxies located in extremely underdense cosmic voids are excellent test beds for disentangling the effects of large-scale environments on galaxy formation and evolution. We present the first results of the Dwarfs in Void Environments Survey, which has obtained 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. Using measurements of stellar line-of-sight rotational velocity v rot and velocity dispersion σ ⋆ , 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 / σ ⋆ ≲ 2 for our sample of isolated dwarf galaxies, and we find no trend between v rot / σ ⋆ and the 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 / σ ⋆ 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. 

Abstract Type Ia supernovae are critical for feedback and elemental enrichment in galaxies. Recent surveys like the All-Sky Automated Survey for Supernova (ASAS-SN) and the Dark Energy Survey (DES) find that the specific supernova Ia rate at z ∼ 0 may be ≲ 20 − 50 × higher in lower-mass galaxies than at Milky Way-mass. Independently, observations show that the close-binary fraction of solar-type Milky Way stars is higher at lower metallicity. Motivated by these observations, we use the FIRE-2 cosmological zoom-in simulations to explore the impact of metallicity-dependent rate models on galaxies of $$M_* \sim 10^7\, \rm {M}_{\odot }-10^{11}\, \rm {M}_{\odot }$$. First, we benchmark our simulated star-formation histories (SFHs) against observations, and show that assumed stellar mass functions play a major role in determining the degree of tension between observations and metallicity-independent rate models, potentially causing ASAS-SN and DES observations to agree more than might appear. Models in which the supernova Ia rate increases with decreasing metallicity ($$\propto Z^{-0.5 \; \rm {to} \; -1}$$) provide significantly better agreement with observations. Encouragingly, these rate increases (≳ 10 × in low-mass galaxies) do not significantly impact galaxy masses and morphologies, which remain largely unaffected except for our most extreme models. We explore implications for both [Fe/H] and [$$\alpha /\rm {Fe}$$] enrichment; metallicity-dependent rate models can improve agreement with the observed stellar mass-metallicity relations in low-mass galaxies. Our results demonstrate that a range of metallicity-dependent rate models are viable for galaxy formation and motivate future work. 

ABSTRACT We explore the effect of including progenitor mass- and metallicity-dependent yields, supernova rates and energetics on variations in elemental abundance ratios (particularly [α/Fe]) in dwarf galaxies. To understand how the scatter and overall trends in [α/Fe] are affected by including variable metal yields from a discretely sampled initial mass function, we run FIRE simulations of a dwarf galaxy (M⋆(z = 0$$) \sim 10^6\rm \, M_{\odot })$$ using nucleosynthetic yields from the NuGrid data base that depend on the stellar progenitor mass and metallicity. While NuGrid exhibits lower aggregate α-element production than default FIRE yields, we find that its explicit mass dependence, even when including turbulent metal diffusion, substantially widens the intrinsic scatter in the simulated [Fe/H]-[α/Fe] – a phenomenon visible in some observations of dwarf galaxies. 

Abstract As they grow, galaxies can transition from irregular/spheroidal with “bursty” star formation histories (SFHs), to disky with smooth SFHs. But even in simulations, the direct physical cause of such transitions remains unclear. We therefore explore this in a large suite of numerical experiments re-running portions of cosmological simulations with widely varied physics, further validated with existing FIRE simulations. We show that gas supply, cooling/thermodynamics, star formation model, Toomre scale, galaxy dynamical times, and feedback properties do not have a direct causal effect on these transitions. Rather, both the formation of disks and cessation of bursty star formation are driven by the gravitational potential, but in different ways. Disk formation is promoted when the mass profile becomes sufficiently centrally-concentrated in shape (relative to circularization radii): we show that this provides a well-defined dynamical center, ceases to support the global “breathing modes” which can persist indefinitely in less-concentrated profiles and efficiently destroy disks, promotes orbit mixing to form a coherent angular momentum, and stabilizes the disk. Smooth SF is promoted by the potential or escape velocity Vesc (not circular velocity Vc) becoming sufficiently large at the radii of star formation that cool, mass-loaded (momentum-conserving) outflows are trapped/confined near the galaxy, as opposed to escaping after bursts. We discuss the detailed physics, how these conditions arise in cosmological contexts, their relation to other correlated phenomena (e.g. inner halo virialization, vertical disk “settling”), and observations.