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We use FIRE simulations to study disc formation in z ∼ 0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar discs is the ability for accreting gas to develop an aligned angular momentum distribution via internal cancellation prior to joining the galaxy. Among galaxies with a high fraction (⁠|$\gt 70{{\ \rm per\ cent}}$|⁠) of their young stars in a thin disc (h / R ∼ 0.1), we find that: (i) hot, virial-temperature gas dominates the inflowing gas mass on halo scales (≳20 kpc), with radiative losses offset by compression heating; (ii) this hot accretion proceeds until angular momentum support slows inward motion, at which point the gas cools to |$\lesssim 10^4\, {\rm K}$|⁠ ; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disc, and while cooling transitions from a quasi-spherical spatial configuration to a more-flattened, disc-like configuration. We show that the existence of this 'rotating cooling flow' accretion mode is strongly correlated with the fraction of stars forming in a thin disc, using a sample of 17 z ∼ 0 galaxies spanning a halo mass range of 1010.5 M⊙ ≲ M h ≲ 1012 M⊙ and stellar mass range of 108 M⊙ ≲ M ⋆ ≲ 1011 M⊙. Notably, galaxies with a thick disc or irregular morphology do not undergo significant angular momentum alignment of gas prior to accretion and show no correspondence between halo gas cooling and flattening. Our results suggest that rotating cooling flows (or, more generally, rotating subsonic flows) that become coherent and angular momentum-supported prior to accretion on to the galaxy are likely a necessary condition for the formation of thin, star-forming disc galaxies in a ΛCDM universe. [ABSTRACT FROM AUTHOR]

We introduce and apply a new approach to probe the response of galactic stellar haloes to the interplay between cosmological merger histories and galaxy formation physics. We perform dark matter-only, zoomed simulations of two Milky Way-mass hosts and make targeted, controlled changes to their cosmological histories using the genetic modification technique. Populating each history's stellar halo with a semi-empirical, particle tagging approach then enables a controlled study, with all instances converging to the same large-scale structure, dynamical and stellar mass at z  = 0 as their reference. These related merger scenarios alone generate an extended spread in stellar halo mass fractions (1.5 dex) comparable to the observed population, with the largest scatter achieved by growing late (z ≤ 1) major mergers that spread out existing stars to create massive, in - situ dominated stellar haloes. Increasing a last major merger at z ∼ 2 brings more accreted stars into the inner regions, resulting in smaller scatter in the outskirts which are predominantly built by subsequent minor events. Exploiting the flexibility of our semi-empirical approach, we show that the diversity of stellar halo masses across scenarios is reduced by allowing shallower slopes in the stellar mass–halo mass relation for dwarf galaxies, while it remains conserved when central stars are born with hotter kinematics across cosmic time. The merger-dependent diversity of stellar haloes thus responds distinctly to assumptions in modelling the central and dwarf galaxies respectively, opening exciting prospects to constrain star formation and feedback at different galactic mass-scales with the coming generation of deep, photometric observatories. [ABSTRACT FROM AUTHOR]

Understanding the variability of galaxy star formation histories (SFHs) across a range of time-scales provides insight into the underlying physical processes that regulate star formation within galaxies. We compile the SFHs of galaxies at z  = 0 from an extensive set of models, ranging from cosmological hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE), zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models (Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the variability of these SFHs on different time-scales using the power spectral density (PSD) formalism. We find that the PSDs are well described by broken power laws, and variability on long time-scales (≳1 Gyr) accounts for most of the power in galaxy SFHs. Most hydrodynamical models show increased variability on shorter time-scales (≲300 Myr) with decreasing stellar mass. Quenching can induce ∼0.4−1 dex of additional power on time-scales >1 Gyr. The dark matter accretion histories of galaxies have remarkably self-similar PSDs and are coherent with the in situ star formation on time-scales >3 Gyr. There is considerable diversity among the different models in their (i) power due to star formation rate variability at a given time-scale, (ii) amount of correlation with adjacent time-scales (PSD slope), (iii) evolution of median PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs. The PSD framework is a useful space to study the SFHs of galaxies since model predictions vary widely. Observational constraints in this space will help constrain the relative strengths of the physical processes responsible for this variability. [ABSTRACT FROM AUTHOR]

We study the relationship between supermassive black hole (BH) feedback, BH luminosity, and the kinematics of stars and gas for galaxies in IllustrisTNG. We use galaxies with mock MaNGA observations to identify kinematic misalignment at z  = 0 (difference in rotation of stars and gas), for which we follow the evolutionary history of BH activity and gas properties over the last 8 Gyr. Misaligned low-mass galaxies (⁠|$\mathrm{\mathit{ M}_{stel} \lt 10^{10.2}\, \mathrm{M}_{\odot }}$|⁠) typically have boosted BH luminosity and BH growth, and have had more energy injected by BHs into the gas over the last 8 Gyr in comparison to low-mass aligned galaxies. These properties likely lead to outflows and gas removal, in agreement with active low mass galaxies in observations. Splitting on BH luminosity at z  = 0 produces consistent distributions of kinematic misalignment at z  = 0; however, splitting on the maximum BH luminosity over the last 8 Gyr produces statistically significant different distributions. While instantaneous correlation at z  = 0 is difficult due to misalignment persisting on longer time-scales, the relationship between BH activity and misalignment is clear. High-mass quenched galaxies (⁠|$\mathrm{\mathit{ M}_{stel} \gt 10^{10.2}\, \mathrm{M}_{\odot }}$|⁠) with misalignment typically have similar BH luminosities, show lower gas fractions, and have typically lower gas phase metallicity over the last 8 Gyr in comparison to the high mass aligned. [ABSTRACT FROM AUTHOR]