We apply the empirical galaxy--halo connection model UniverseMachine to dark matter-only zoom-in simulations of isolated Milky Way (MW)--mass halos along with their parent cosmological simulations. This application extends textsc{UniverseMachine} pre
dictions into the ultra-faint dwarf galaxy regime ($ 10^{2},mathrm{M_{odot}} leqslant M_{ast} leqslant 10^{5},mathrm{M_{odot}}$) and yields a well-resolved stellar mass--halo mass (SMHM) relation over the peak halo mass range $10^8,mathrm{M_{odot}}$ to $10^{15},mathrm{M_{odot}}$. The extensive dynamic range provided by the zoom-in simulations allows us to assess specific aspects of dwarf galaxy evolution predicted by textsc{UniverseMachine}. In particular, although UniverseMachine is not constrained for dwarf galaxies with $M_* lesssim 10^{8},mathrm{M_{odot}}$, our predicted SMHM relation is consistent with that inferred for MW satellite galaxies at $z=0$ using abundance matching. However, UniverseMachine predicts that nearly all galaxies are actively star forming below $M_{ast}sim 10^{7},mathrm{M_{odot}}$ and that these systems typically form more than half of their stars at $zlesssim 4$, which is discrepant with the star formation histories of Local Group dwarf galaxies that favor early quenching. This indicates that the current UniverseMachine model does not fully capture galaxy quenching physics at the low-mass end. We highlight specific improvements necessary to incorporate environmental and reionization-driven quenching for dwarf galaxies, and provide a new tool to connect dark matter accretion to star formation over the full dynamic range that hosts galaxies.
A key feature of a large population of low-mass, late-type disk galaxies are star-forming disks with exponential light distributions. They are typically also associated with thin and flat morphologies, blue colors, and dynamically cold stars moving a
long circular orbits within co-planar thin gas disks. However, the latter features do not necessarily always imply the former, in fact, a variety of different kinematic configurations do exist. In this work, we use the cosmological hydrodynamical IllustrisTNG Simulation to study the nature and origin of dynamically hot, sometimes even counter-rotating, star-forming disk galaxies in the lower stellar mass range (between $5times 10^9,mathrm{M_{odot}}$ and $2times 10^{10},mathrm{M_{odot}}$). We find that being dynamically hot arises in most cases as an induced transient state, for example due to galaxy interactions and merger activities, rather than as an age-dependent evolutionary phase of star-forming disk galaxies. The dynamically hot but still actively star-forming disks show a common feature of hosting kinematically misaligned gas and stellar disks, and centrally concentrated on-going star formation. The former is often accompanied by disturbed gas morphologies, while the latter is reflected in low gas and stellar spins in comparison to their dynamically cold, normal disk counterparts. Interestingly, observed galaxies from MaNGA with kinematic misalignment between gas and stars show remarkably similar general properties as the IllustrisTNG galaxies, and therefore are plausible real-world counterparts. In turn, this allows us to make predictions for the stellar orbits and gas properties of these misaligned galaxies.
We study the evolutionary trend of the total density profile of early-type galaxies (ETGs) in IllustrisTNG. To this end, we trace ETGs from $z=0$ to $z=4$ and measure the power-law slope $gamma^{prime}$ of the total density profile for their main pro
genitors. We find that their $gamma^{prime}$ steepen on average during $zsim4-2$, then becoming shallower until $z=1$, after which they remain almost constant, aside from a residual trend of becoming shallower towards $z=0$. We also compare to a statistical sample of ETGs at different redshifts, selected based on their luminosity profiles and stellar masses. Due to different selection effects, the average slopes of the statistical samples follow a modified evolutionary trend. They monotonically decrease since $z=3$, and after $zapprox 1$, they remain nearly invariant with a mild increase towards $z=0$. These evolutionary trends are mass-dependent for both samples, with low-mass galaxies having in general steeper slopes than their more massive counterparts. Galaxies that transitioned to ETGs more recently have steeper mean slopes as they tend to be smaller and more compact at any given redshift. By analyzing the impact of mergers and AGN feedback on the progenitors evolution, we conjecture a multi-phase path leading to isothermality in ETGs: dissipation associated with rapid wet mergers tends to steepen $gamma^{prime}$ from $z=4$ to $z=2$, whereas subsequent AGN feedback (especially in the kinetic mode) makes $gamma^{prime}$ shallower again from $z=2$ to $z=1$. Afterwards, passive evolution from $z=1$ to $z=0$, mainly through gas-poor mergers, mildly decreases $gamma^{prime}$ and maintains the overall mass distribution close to isothermal.
We explore the isothermal total density profiles of early-type galaxies (ETGs) in the IllustrisTNG simulation. For the selected 559 ETGs at $z = 0$ with stellar mass $10^{10.7}mathrm{M}_{odot} leqslant M_{ast} leqslant 10^{11.9}mathrm{M}_{odot}$, the
total power-law slope has a mean of $langlegamma^{prime}rangle = 2.011 pm 0.007$ and a scatter of $sigma_{gamma^{prime}} = 0.171$ over the radial range 0.4 to 4 times the stellar half mass radius. Several correlations between $gamma^{prime}$ and galactic properties including stellar mass, effective radius, stellar surface density, central velocity dispersion, central dark matter fraction and in-situ-formed stellar mass ratio are compared to observations and other simulations, revealing that IllustrisTNG reproduces many correlation trends, and in particular, $gamma^{prime}$ is almost constant with redshift below $z = 2$. Through analyzing IllustrisTNG model variations we show that black hole kinetic winds are crucial to lowering $gamma^{prime}$ and matching observed galaxy correlations. The effects of stellar winds on $gamma^{prime}$ are subdominant compared to AGN feedback, and differ due to the presence of AGN feedback from previous works. The density profiles of the ETG dark matter halos are well-described by steeper-than-NFW profiles, and they are steeper in the full physics (FP) run than their counterparts in the dark matter only (DMO) run. Their inner density slopes anti-correlates (remain constant) with the halo mass in the FP (DMO) run, and anti-correlates with the halo concentration parameter $c_{200}$ in both types of runs. The dark matter halos of low-mass ETGs are contracted whereas high-mass ETGs are expanded, suggesting that variations in the total density profile occur through the different halo responses to baryons.
