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ABSTRACT We examine the thermodynamic state and cooling of the low-z circumgalactic medium (CGM) in five FIRE-2 galaxy formation simulations of Milky Way-mass galaxies. We find that the CGM in these simulations is generally multiphase and dynamic, with a wide spectrum of largely non-linear density perturbations sourced by the accretion of gas from the intergalactic medium (IGM) and outflows from both the central and satellite galaxies. We investigate the origin of the multiphase structure of the CGM with a particle-tracking analysis and find that most of the low-entropy gas has cooled from the hot halo as a result of thermal instability triggered by these perturbations. The ratio of cooling to free-fall time-scales tcool/tff in the hot component of the CGM spans a wide range of ∼1−100 at a given radius but exhibits approximately constant median values of ∼5−20 at all radii 0.1Rvir < r < Rvir. These are similar to the ≈10−20 value typically adopted as the thermal instability threshold in ‘precipitation’ models of the ICM. Consequently, a one-dimensional model based on the assumption of a constant tcool/tff and hydrostatic equilibrium approximately reproduces the number density and entropy profiles of each simulation but only if it assumes the metallicity profile and temperature boundary condition taken directly from the simulation. We explicitly show that the tcool/tff value of a gas parcel in the hot component of the CGM does not predict its probability of subsequently accreting on to the central galaxy. This suggests that the value of tcool/tff is a poor predictor of thermal stability in gaseous haloes in which large-amplitude density perturbations are prevalent.

Observations indicate that a continuous supply of gas is needed to maintainobserved star formation rates in large, disky galaxies. To fuel star formation,gas must reach the inner regions of such galaxies. Despite its crucialimportance for galaxy evolution, how and where gas joins galaxies is poorlyconstrained observationally and is rarely explored in fully cosmologicalsimulations. To investigate gas accretion in the vicinity of galaxies, weanalyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way massgalaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic rayphysics. We find that at z~0, gas approaches the disk with angular momentumsimilar to the gaseous disk edge and low radial velocities, piling-up near theedge and settling into full rotational support. Accreting gas movespredominantly parallel to the disk with small but nonzero vertical velocitycomponents, and joins the disk largely in the outskirts as opposed to "raining"down onto the disk. Once in the disk, gas trajectories are complex, beingdominated by spiral arm induced oscillations and feedback. However, time andazimuthal averages show clear but slow net radial infall with transport speedsof 1-3 km/s and net mass fluxes through the disk on the order of one solar massper year, comparable to the star formation rates of the galaxies and decreasingtowards galactic center as gas is sunk into star formation. These rates areslightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar massesper year). We find overall consistency of our results with observationalconstraints and discuss prospects of future observations of gas flows in andaround galaxies.

Observations indicate that a continuous supply of gas is needed to maintainobserved star formation rates in large, disky galaxies. To fuel star formation,gas must reach the inner regions of such galaxies. Despite its crucialimportance for galaxy evolution, how and where gas joins galaxies is poorlyconstrained observationally and is rarely explored in fully cosmologicalsimulations. To investigate gas accretion in the vicinity of galaxies, weanalyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way massgalaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic rayphysics. We find that at z~0, gas approaches the disk with angular momentumsimilar to the gaseous disk edge and low radial velocities, piling-up near theedge and settling into full rotational support. Accreting gas movespredominantly parallel to the disk with small but nonzero vertical velocitycomponents, and joins the disk largely in the outskirts as opposed to "raining"down onto the disk. Once in the disk, gas trajectories are complex, beingdominated by spiral arm induced oscillations and feedback. However, time andazimuthal averages show clear but slow net radial infall with transport speedsof 1-3 km/s and net mass fluxes through the disk on the order of one solar massper year, comparable to the star formation rates of the galaxies and decreasingtowards galactic center as gas is sunk into star formation. These rates areslightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar massesper year). We find overall consistency of our results with observationalconstraints and discuss prospects of future observations of gas flows in andaround galaxies.

The standard picture of galaxy formation motivates the decomposition of theMilky Way into 3--4 stellar populations with distinct kinematic and elementalabundance distributions: the thin disk, thick disk, bulge, and stellar halo. Totest this idea, we construct a Gaussian mixture model (GMM) for both simulatedand observed stars in the Solar neighborhood, using measured velocities andiron abundances (i.e., an augmented Toomre diagram) as the distributions to bedecomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog ofthe Solar neighborhood with those from a suite of synthetic Gaia-APOGEEcrossmatches constructed from FIRE-2 cosmological simulations of Milky Way-massgalaxies. We find that in both the synthetic and real data, the best-fit GMMuses five independent components, some of whose properties resemble thestandard populations predicted by galaxy formation theory. Two components canbe identified unambiguously as the thin disk and another as the halo. However,instead of a single counterpart to the thick disk, there are three intermediatecomponents with different age and alpha abundance distributions (although thesedata are not used to construct the model). We use decompositions of thesynthetic data to show that the classified components indeed correspond tostars with different origins. By analogy with the simulated data, we show thatour mixture model of the real Gaia-APOGEE crossmatch distinguishes thefollowing components: (1) a classic thin disk of young stars on circular orbits(46%), (2) thin disk stars heated by interactions with satellites (22%), (3, 4)two components representing the velocity asymmetry of the alpha-enhanced thickdisk (27%), and (5) a stellar halo consistent with early, massive accretion(4%).

Surveys of the Milky Way (MW) and M31 enable detailed studies of stellar populations across ages and metallicities, with the goal of reconstructing formation histories across cosmic time. These surveys motivate key questions for galactic archaeology in a cosmological context: When did the main progenitor of an MW/M31-mass galaxy form, and what were the galactic building blocks that formed it? We investigate the formation times and progenitor galaxies of MW/M31-mass galaxies using the Feedback In Realistic Environments-2 cosmological simulations, including six isolated MW/M31-mass galaxies and six galaxies in Local Group (LG)-like pairs at z = 0. We examine main progenitor 'formation' based on two metrics: (1) transition from primarily ex-situ to in-situ stellar mass growth and (2) mass dominance compared to other progenitors. We find that the main progenitor of an MW/M31-mass galaxy emerged typically at z ∼3-4 ($11.6\!\!-\!\!12.2\, \rm {Gyr}$ ago), while stars in the bulge region (inner 2 kpc) at z = 0 formed primarily in a single main progenitor at z ≲ 5 (${\lesssim} \!12.6\, \rm {Gyr}$ ago). Compared with isolated hosts, the main progenitors of LG-like paired hosts emerged significantly earlier (Δz ∼2, $\Delta t\!\sim \!1.6\, \rm {Gyr}$), with ∼4× higher stellar mass at all z ≳ 4 (${\gtrsim} \!12.2\, \rm {Gyr}$ ago). This highlights the importance of environment in MW/M31-mass galaxy formation, especially at early times. On average, about 100 galaxies with $\rm {\it{ M}}_\rm {star}\!\gtrsim \!10^5\, \rm {M}_\odot$ went into building a typical MW/M31-mass system. Thus, surviving satellites represent a highly incomplete census (by ∼5×) of the progenitor population.