James S. Bullock

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 disc, and then ‘cooled down’ to a final cold, thin stellar disc. 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 ($T \le 10^4$ K) converting into stars is low, star formation is bursty, and the baryonic mass ‘sloshes’ within the host potential with respect to the centre 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 disc phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disc 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 disc formation appears to be minimal: a stable centre-of-mass motion. Thin disc formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.

Aims. We aim to investigate the connection between star formation histories (SFHs) and the inner dark matter density profiles of simulated galaxies. In particular, we tested whether the burstiness and temporal distribution of star formation influence the formation of cored versus cuspy dark matter profiles. Methods. We homogeneously analysed simulated galaxies from the NIHAO and FIRE-2 projects. For each galaxy, we derived dark matter density profiles and measured the logarithmic slope in the inner region of the dark matter halo (1–2% of R _vir ). To characterise the star formation burstiness, we introduced a criterion based on comparing the star formation rate (SFR) averaged over two distinct timescales. We further quantified the temporal concentration of SFHs by computing the ratio of stellar mass formed after versus before the epoch of re-ionisation at redshift z ∼ 6.5, expressed as M _★, post / M _★, pre . Results. A homogeneous analysis reveals that inner slope versus stellar-to-halo mass ratio trends for NIHAO and FIRE-2 galaxies are in much better agreement than what has been reported in previous works. The burstiness and post-to-pre re-ionisation stellar mass ratio explain the scatter in the inner slope versus stellar-to-halo mass ratio relation, revealing that galaxies with a burstiness above average and a more extended SFH are more efficient at developing cored dark matter profiles. In contrast, galaxies with smoother SFHs and an earlier stellar mass assembly time tend to maintain cuspier dark matter profiles. We present an analytic expression that improves predictions for the inner slope using M _★, post / M _★, pre , which reduces the mean square error in both simulation suites relative to previous formulations based solely on the stellar-to-halo mass ratio.

While galaxy rotation curves (RCs) provide one of the most powerful methods for measuring dark matter profiles in the inner regions of rotation-supported galaxies, at the dwarf scale there are factors that can complicate this analysis. Given the expectation of a universal profile in dark-matter-only simulations, the diversity of the observed RCs has become an often-discussed issue in Lambda cold dark matter cosmology on galactic scales. We analyze a suite of Feedback in Realistic Environments simulations of 10 ¹⁰ –10 ¹² M _⊙ halos with standard cold dark matter and compare the true circular velocity to RC reconstructions. We find that, for galaxies with well-ordered gaseous disks, the measured RC may deviate from the true circular velocity by at most ∼10% within the radius of the disk. However, nonequilibrium behaviors, noncircular motions, and nonthermal and nonkinetic stresses may cause much larger discrepancies, of ∼50% or more. Most RC reconstructions underestimate the true circular velocity, while some reconstructions transiently overestimate it in the central few kiloparsecs, due to dynamical phenomena. We further demonstrate that the features that contribute to these failures are not always visibly obvious in H I observations. If such dwarf galaxies are included in galaxy catalogs, they may give rise to the appearance of “artificial” RC diversity that does not reflect the true variation in underlying dark matter profiles.

We study the intrinsic and observable shapes of approximately 700 star-forming galaxies with stellar masses of 10 ⁸ –10 ¹¹ M _⊙ 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 , 10 ^9.5 , and 10 ^10.5 M _⊙ , 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 _⊙ , 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 _⊙ , 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.

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 employ FIREbox , a large-volume cosmological simulation based on the Feedback In Realistic Environments physics 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 suppressed c _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.

We explore how a realistic surface brightness (SB) detection limit of μ _V ≈ 32.5 mag arcsec ⁻² for 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 ≈400–40,000 M _⊙ simulated with FIRE-2 physics and baryonic mass resolution of 30 M _⊙ . 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.