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ABSTRACT Modern studies of galaxy formation rely heavily on numerical simulations, which in turn require tools to identify and track self-bound structures in stars and dark matter. In this paper, we present Bloodhound, a new halo tracking algorithm optimized to track and characterize substructure in cosmological simulations, a regime that is crucial for studies of the nature of dark matter but where standard methods often have difficulties. Using simulations of Milky Way-mass haloes, we demonstrate that Bloodhound extends subhalo tracking by $$3{\text{-}}4\, \mathrm{Gyr}$$ on average, and significantly longer for subhaloes with small pericentres, relative to the widely used Rockstar  $$+$$  consistent-trees halo tracking pipeline. We also show that Bloodhound provides continuous tracking, mitigating an issue for the standard technique where subhaloes can be lost and then found again – but assigned to a new merger tree – after several snapshots. This improved tracking leads to a substantially larger number of surviving subhaloes in the inner regions of dark matter haloes, which has several implications for studies of the Milky Way’s satellite galaxy system and its use for constraining properties of dark matter. For example, within the radius where current surveys are complete to ultra-faint galaxies ($$D_{\rm MW} \lesssim 50$$ kpc), Bloodhound finds more than twice as many subhaloes above the atomic cooling scale relative to the standard tracking method. Our results underscore the importance of robust subhalo tracking techniques in advancing our understanding of galaxy formation and cosmological models. 

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. 

Abstract We study the intrinsic and observable shapes of approximately 700 star-forming galaxies with stellar masses of 10⁸–10¹¹M_⊙from the FIREbox simulation atz= 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, 10^9.5, and 10^10.5M_⊙, respectively. Using the radiative transfer codeSKIRT, 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 10¹⁰M_⊙, we produce too few galaxies with observedq < 0.4 and none withq < 0.2, suggesting that FIREbox does not produce enough low-mass disk galaxies. At higher masses, 10¹⁰–10¹¹M_⊙, we find that the predictedqdistribution is sensitive to the dust-to-metal ratio; using our fiducial model, the distribution ofqvalues is formally consistent with observations, but there is tension with our ability to produce enough very thin systems withq < 0.2. Future observational and theoretical programs aimed at understanding disk and thin-disk fractions will provide crucial tests of galaxy formation models. 

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 investigate the central density structure of dark matter haloes in cold dark matter (CDM) and self-interacting dark matter (SIDM) models using simulations that are part of the Feedback In Realistic Environments (fire) project. For simulated haloes of dwarf galaxy scale ($$M_{\rm halo}(z=0)\approx 10^{10}\, \mathrm{ M}_\odot$$), we study the central structure in both dissipationless simulations and simulations with full fire-2 galaxy formation physics. As has been demonstrated extensively in recent years, both baryonic feedback and self-interactions can convert central cusps into cores, with the former process doing so in a manner that depends sensitively on stellar mass at fixed $$M_{\rm halo}$$. Whether the two processes (baryonic feedback and self-interactions) are distinguishable, however, remains an open question. Here we demonstrate that, compared to feedback-induced cores, SIDM-induced cores transition more quickly from the central region of constant density to the falling density at larger radial scales. This result holds true even when including identical galaxy formation modelling in SIDM simulations as is used in CDM simulations, since self-interactions dominate over galaxy formation physics in establishing the central structure of SIDM haloes in this mass regime. The change in density profile slope as a function of radius therefore holds the potential to discriminate between self-interactions and galaxy formation physics as the driver of core formation in dwarf galaxies. 

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. 

We use FIRE-2 zoom simulations of Milky Way size disk galaxies to derive easy-to-use relationships between the observed circular speed of the Galaxy at the Solar location,v_c, and dark matter properties of relevance for direct detection experiments: the dark matter density, the dark matter velocity dispersion, and the speed distribution of dark matter particles near the Solar location. We find that both the local dark matter density and 3D velocity dispersion follow tight power laws withv_c. Using this relation together with the observed circular speed of the Milky Way at the Solar radius, we infer the local dark matter density and velocity dispersion near the Sun to beρ= 0.42±0.06 GeV cm^-3and^σ_3D= 280⁺¹⁹_-18km s^-1. We also find that the distribution of dark matter particle speeds is well-described by a modified Maxwellian with two shape parameters, both of which correlate with the observedv_c. We use that modified Maxwellian to predict the speed distribution of dark matter near the Sun and find that it peaks at a most probable speed of 257 km s^-1and begins to truncate sharply above 470 km s^-1. This peak speed is somewhat higher than expected from the standard halo model, and the truncation occurs well below the formal escape speed to infinity, with fewer very-high-speed particles than assumed in the standard halo model. 

ABSTRACT The observationally inferred size versus stellar–mass relationship (SMR) for low-mass galaxies provides an important test for galaxy formation models. However, the relationship relies on assumptions that relate observed luminosity profiles to underlying stellar mass profiles. Here we use the Feedback in Realistic Environments simulations of low-mass galaxies to explore how the predicted SMR changes depending on whether one uses star-particle counts directly or mock observations. We reproduce the SMR found in The Exploration of Local Volume Satellites survey remarkably well only when we infer stellar masses and sizes using mock observations. However, when we use star particles to directly infer stellar masses and half-mass radii, we find that our galaxies are too large and obey an SMR with too little scatter compared to observations. This discrepancy between the ‘true’ galaxy size and mass and those derived in the mock observation approach is twofold. First, our simulated galaxies have higher and more varied mass-to-light ratios (MLR) at a fixed colour than those commonly adopted, which tends to underestimate their stellar masses compared to their true, simulated values. Second, our galaxies have radially increasing MLR gradients therefore using a single MLR tends to underpredict the mass in the outer regions. Similarly, the true half-mass radius is larger than the half-light radius because the light is more concentrated than the mass. If our simulations are accurate representations of the real Universe, then the relationship between galaxy size and stellar mass is even tighter for low-mass galaxies than is commonly inferred from observed relations. 

ABSTRACT We introduce an analytic surface density profile for dark matter haloes that accurately reproduces the structure of simulated haloes of mass Mvir = 107–1011 M⊙, making it useful for modelling line-of-sight (LOS) perturbers in strong gravitational lensing models. The two-parameter function has an analytic deflection potential and is more accurate than the projected Navarro, Frenk, and White profile commonly adopted at this mass scale for perturbers, especially at the small radii of most relevant for lensing perturbations. Using a characteristic radius, R−1, where the log slope of surface density is equal to −1, and an associated surface density, Σ−1, we can represent the expected lensing signal from LOS haloes statistically, for an ensemble of halo orientations, using a distribution of projected concentration parameters, $$\mathcal {C}_{\rm vir} := r_{\rm vir}/ R_{-1}$$. Though an individual halo can have a projected concentration that varies with orientation with respect to the observer, the range of projected concentrations correlates with the usual three-dimensional halo concentration in a way that enables ease of use.