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In a companion paper, we reported the self-consistent formation of quasar accretion disks with inflow rates $\sim 10\,{\rm M_{\odot}\,yr^{-1}}$ down to <300 Schwarzschild radii from cosmological radiation-magneto-thermochemical-hydrodynamical galaxy and star formation simulations. We see the formation of a well-defined, steady-state accretion disk which is stable against star formation at sub-pc scales. The disks are optically thick, with radiative cooling balancing accretion, but with properties that are distinct from those assumed in most previous accretion disk models. The pressure is strongly dominated by (primarily toroidal) magnetic fields, with a plasma $\beta \sim 10^{-4}$ even in the disk midplane. They are qualitatively distinct from magnetically elevated or arrested disks. The disks are strongly turbulent, with trans-Alfvenic and highly super-sonic turbulence, and balance this via a cooling time that is short compared to the disk dynamical time, and can sustain highly super-Eddington accretion rates. Their surface and 3D densities at $\sim 10^{3}-10^{5}$ gravitational radii are much lower than in a Shakura-Sunyaev disk, with important implications for their thermo-chemistry and stability. We show how the magnetic field strengths and geometries arise from rapid advection of flux with the inflow from much weaker galaxy-scale fields in these 'flux-frozen' disks, and how this stabilizes the disk and gives rise to efficient torques. Re-simulating without magnetic fields produces catastrophic fragmentation with a vastly smaller, lower-$\dot{M}$ Shakura-Sunyaev-like disk., Comment: 46 pages, 30 figures. Updated to match accepted version from The Open Journal of Astrophysics. Part of a series with arXiv:2309.13115 . Animations of the simulations can be viewed at http://www.tapir.caltech.edu/~phopkins/Site/animations/Movies_zoom.html

Recent numerical cosmological radiation-magnetohydrodynamic-thermochemical-star formation simulations have resolved the formation of quasar accretion disks with Eddington or super-Eddington accretion rates onto supermassive black holes (SMBHs) down to a few hundred gravitational radii. These 'flux-frozen' and hyper-magnetized disks appear to be qualitatively distinct from classical $\alpha$ disks and magnetically-arrested disks: the midplane pressure is dominated by toroidal magnetic fields with plasma $\beta \ll 1$ powered by advection of magnetic flux from the interstellar medium (ISM), and they are super-sonically and trans-Alfvenically turbulent with cooling times short compared to dynamical times yet remain gravitationally stable owing to magnetic support. In this paper, we present a simple analytic similarity model for such disks. For reasonable assumptions, the model is entirely specified by the boundary conditions (inflow rate at the BH radius of influence [BHROI]). We show that the scalings from this model are robust to various detailed assumptions, agree remarkably well with the simulations (given their simplicity), and demonstrate the self-consistency and gravitational stability of such disks even in the outer accretion disk (approaching the BHROI) at hyper-Eddington accretion rates., Comment: 12 pages, 1 figure. Replaced with accepted version. Companion paper to 'FORGE'D IN FIRE II' (arXiv:2310.04506, part of a series with arXiv:2309.13115 -- animations of the simulations referred to here can be viewed at http://www.tapir.caltech.edu/~phopkins/Site/animations/Movies_zoom.html)

The shape of the low-mass (faint) end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at z > 6 is an open question for understanding which galaxies primarily drove cosmic reionisation. Resolved photometry of Local Group low-mass galaxies allows us to reconstruct their star formation histories, stellar masses, and UV luminosities at early times, and this fossil record provides a powerful `near-far' technique for studying the reionisation-era SMF/UVLF, probing orders of magnitude lower in mass than direct HST/JWST observations. Using 882 low-mass (Mstar < 10^9 Msun) galaxies across 11 Milky Way- and Local Group-analogue environments from the FIRE-2 cosmological baryonic zoom-in simulations, we characterise their progenitors at z ~ 6 - 9, the mergers/disruption of those progenitors over time, and how well their present-day fossil record traces the high-redshift SMF. A present-day galaxy with Mstar ~ 10^5 Msun (10^9 Msun) had ~1 (~30) progenitors at z ~ 7, and its main progenitor comprised ~100% (~50%) of the total stellar mass of all its progenitors at z ~ 7. We show that although only ~ 15% of the early population of low-mass galaxies survives to present day, the fossil record of surviving Local Group galaxies accurately traces the low-mass slope of the SMF at z ~ 6 - 9. We find no obvious mass dependence to the mergers and accretion, and show that applying this reconstruction technique to just the low-mass galaxies at z = 0 and not the MW/M31 hosts correctly recovers the slope of the SMF down to Mstar ~ 10^4.5 Msun at z > 6. Thus, we validate the `near-far' approach as an unbiased tool for probing low-mass reionisation-era galaxies., Comment: 16 pages, 6 figures, 3 appendix figures, and 1 table. Submitted to MNRAS

It has recently become possible to zoom-in from cosmological to sub-pc scales in galaxy simulations to follow accretion onto supermassive black holes (SMBHs). However, at some point the approximations used on ISM scales (e.g. optically-thin cooling and stellar-population-integrated star formation [SF] and feedback [FB]) break down. We therefore present the first cosmological radiation-magnetohydrodynamic (RMHD) simulation which self-consistently combines the FIRE physics (relevant on galactic/ISM scales where SF/FB are ensemble-averaged) and STARFORGE physics (relevant on small scales where we track individual (proto)stellar formation and evolution), together with explicit RMHD (including non-ideal MHD and multi-band M1-RHD) which self-consistently treats both optically-thick and thin regimes. This allows us to span scales from ~100 Mpc down to <100 au (~300 Schwarzschild radii) around a SMBH at a time where it accretes as a bright quasar, in a single simulation. We show that accretion rates up to $\sim 10-100\,{\rm M_{\odot}\,yr^{-1}}$ can be sustained into the accretion disk at $\ll 10^{3}\,R_{\rm schw}$, with gravitational torques between stars and gas dominating on sub-kpc scales until star formation is shut down on sub-pc scales by a combination of optical depth to cooling and strong magnetic fields. There is an intermediate-scale, flux-frozen disk which is gravitoturbulent and stabilized by magnetic pressure sustaining strong turbulence and inflow with persistent spiral modes. In this paper we focus on how gas gets into the small-scale disk, and how star formation is efficiently suppressed., Comment: 38 pages, 18 figures. Replaced with accepted version

This paper reports the first measurement of the relationship between turbulent velocity and cloud size in the diffuse circumgalactic medium (CGM) in typical galaxy halos at redshift z~0.4-1. Through spectrally-resolved absorption profiles of a suite of ionic transitions paired with careful ionization analyses of individual components, cool clumps of size as small as l_cl~1 pc and density lower than nH = 0.001 cm^-3 are identified in galaxy halos. In addition, comparing the line widths between different elements for kinematically matched components provides robust empirical constraints on the thermal temperature T and the non-thermal motions bNT, independent of the ionization models. On average, bNT is found to increase with l_cl following bNT \propto l_cl^0.3 over three decades in spatial scale from l_cl~1 pc to l_cl~1 kpc. Attributing the observed bNT to turbulent motions internal to the clumps, the best-fit bNT-l_cl relation shows that the turbulence is consistent with Kolmogorov at <1 kpc with a roughly constant energy transfer rate per unit mass of epsilon~0.003 cm^2 s^-3 and a dissipation time scale of <~ 100 Myr. No significant difference is found between massive quiescent and star-forming halos in the sample on scales less than 1 kpc. While the inferred epsilon is comparable to what is found in CIV absorbers at high redshift, it is considerably smaller than observed in star-forming gas or in extended line-emitting nebulae around distant quasars. A brief discussion of possible sources to drive the observed turbulence in the cool CGM is presented., Comment: Accepted for publication in ApJ Letters; 10 pages, 4 figures

We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. We quantify the relative importance of cosmological disk settling and post-formation dynamical heating. We identify three eras: a Pre-Disk Era (typically >8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disk Era (~ 8 - 4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, sigma_form; and a Late-Disk Era (< 4 Gyr ago), when stars formed with low sigma_form. sigma_form increased with time during the Pre-Disk Era, peaking ~ 8 Gyr ago, then decreased throughout the Early-Disk Era as the disk settled and remained low throughout the Late-Disk Era. By contrast, the velocity dispersion measured today, sigma_now, increases monotonically with age because of stronger post-formation heating for Pre-Disk stars. Importantly, most of sigma_now was in place at formation, not added post-formation, for stars younger than ~ 10 Gyr. We compare the evolution of the three velocity components: at all times, sigma_R,form > sigma_phi,form > sigma_Z,form. Post-formation heating primarily increased sigma_R at ages < 4 Gyr but acted nearly isotropically for older stars. The kinematics of young stars in FIRE-2 broadly agree with the range observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time that the disk began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder disks. Including stellar cosmic-ray feedback does not significantly change disk rotational support at fixed stellar mass., Comment: 23 pages, accepted for publication in MNRAS

Spurred by rich, multi-wavelength observations and enabled by new simulations, ranging from cosmological to sub-pc scales, the last decade has seen major theoretical progress in our understanding of the circumgalactic medium. We review key physical processes in the CGM. Our conclusions include: (1) The properties of the CGM depend on a competition between gravity-driven infall and gas cooling. When cooling is slow relative to free fall, the gas is hot (roughly virial temperature) whereas the gas is cold (T~10^4 K) when cooling is rapid. (2) Gas inflows and outflows play crucial roles, as does the cosmological environment. Large-scale structure collimates cold streams and provides angular momentum. Satellite galaxies contribute to the CGM through winds and gas stripping. (3) In multiphase gas, the hot and cold phases continuously exchange mass, energy and momentum. The interaction between turbulent mixing and radiative cooling is critical. A broad spectrum of cold gas structures, going down to sub-pc scales, arises from fragmentation, coagulation, and condensation onto gas clouds. (4) Magnetic fields, thermal conduction and cosmic rays can substantially modify how the cold and hot phases interact, although microphysical uncertainties are presently large. Key open questions for future work include the mutual interplay between small-scale structure and large-scale dynamics, and how the CGM affects the evolution of galaxies., Comment: 69 pages, 13 figures. Accepted for publication in Annual Review of Astronomy and Astrophysics. Authors' draft. Edited version will appear in the next volume

As they grow, galaxies can transition from irregular/spheroidal with 'bursty' star formation histories (SFHs), to disky with smooth SFHs. But even in simulations, the direct physical cause of such transitions remains unclear. We therefore explore this in a large suite of numerical experiments re-running portions of cosmological simulations with widely varied physics, further validated with existing FIRE simulations. We show that gas supply, cooling/thermodynamics, star formation model, Toomre scale, galaxy dynamical times, and feedback properties do not have a direct causal effect on these transitions. Rather, both the formation of disks and cessation of bursty star formation are driven by the gravitational potential, but in different ways. Disk formation is promoted when the mass profile becomes sufficiently centrally-concentrated in shape (relative to circularization radii): we show that this provides a well-defined dynamical center, ceases to support the global 'breathing modes' which can persist indefinitely in less-concentrated profiles and efficiently destroy disks, promotes orbit mixing to form a coherent angular momentum, and stabilizes the disk. Smooth SF is promoted by the potential or escape velocity (not circular velocity) becoming sufficiently large at the radii of star formation that cool, mass-loaded (momentum-conserving) outflows are trapped/confined near the galaxy, as opposed to escaping after bursts. We discuss the detailed physics, how these conditions arise in cosmological contexts, their relation to other correlated phenomena (e.g. inner halo virialization, vertical disk 'settling'), and observations., Comment: Updated to match MNRAS accepted version. 45 pages, 32 figures

Lightcurves of many classical novae deviate from the canonical "fast rise - smooth decline" pattern and display complex variability behavior. We present the first TESS-space-photometry-based investigation of this phenomenon. We use TESS Sector 41 full-frame images to extract a lightcurve of the slow Galactic nova V606 Vul that erupted nine days prior to the start of the TESS observations. The lightcurve covers the first of two major peaks of V606 Vul that was reached 19 days after the start of the eruption. The nova reached its brightest visual magnitude V=9.9 in its second peak 64 days after the eruption onset, following the completion of Sector 41 observations. To increase the confidence level of the extracted lightcurve, we performed the analysis using four different codes implementing the aperture photometry (Lightkurve, VaST) and image subtraction (TESSreduce, tequila_shots) and find good agreement between them. We performed ground-based photometric and spectroscopic monitoring to complement the TESS data. The TESS lightcurve reveals two features: periodic variations (0.12771d, 0.01mag average peak-to-peak amplitude) that disappeared when the source was within 1mag of peak optical brightness and a series of isolated mini-flares (with peak-to-peak amplitudes of up to 0.5mag) appearing at seemingly random times. We interpret the periodic variations as the result of azimuthal asymmetry of the photosphere engulfing the nova-hosting binary that was distorted by and rotating with the binary. Whereas we use spectra to associate the two major peaks in the nova lightcurve with distinct episodes of mass ejection, the origin of mini-flares remains elusive., Comment: 31 pages, 10 figures, submitted to ApJ; comments welcome

The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and morphologies). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. We predict that approximately 30 percent of massive central galaxies (with at least 1e11 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 1e8 - 1e9 solar masses in stars). This distinctive class of galaxies provides an additional layer in our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario., Comment: 55 pages, 4 figures, 13 supplementary figures. 29 pages, 4 figures. Accepted for publication in Nature Astronomy. To appear on 14-February-2022. Published version: https://www.nature.com/articles/s41550-021-01598-4

Increasingly, uncertainties in predictions from galaxy formation simulations (at sub-Milky Way masses) are dominated by uncertainties in stellar evolution inputs. In this paper, we present the full set of updates from the FIRE-2 version of the Feedback In Realistic Environments (FIRE) project code, to the next version, FIRE-3. While the transition from FIRE-1 to FIRE-2 focused on improving numerical methods, here we update the stellar evolution tracks used to determine stellar feedback inputs, e.g. stellar mass-loss (O/B and AGB), spectra (luminosities and ionization rates), and supernova rates (core-collapse and Ia), as well as detailed mass-dependent yields. We also update the low-temperature cooling and chemistry, to enable improved accuracy at $T \lesssim 10^{4}\,$K and densities $n\gg 1\,{\rm cm^{-3}}$, and the meta-galactic ionizing background. All of these synthesize newer empirical constraints on these quantities and updated stellar evolution and yield models from a number of groups, addressing different aspects of stellar evolution. To make the updated models as accessible as possible, we provide fitting functions for all of the relevant updated tracks, yields, etc, in a form specifically designed so they can be directly 'plugged in' to existing galaxy formation simulations. We also summarize the default FIRE-3 implementations of 'optional' physics, including spectrally-resolved cosmic rays and supermassive black hole growth and feedback., Comment: 26 pages, 12 figures. Submitted to MNRAS. Comments welcome

We describe a public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation, available at http://flathub.flatironinstitute.org/fire, from the Feedback In Realistic Environments (FIRE) project. The FIRE-2 simulations achieve parsec-scale resolution to explicitly model the multi-phase interstellar medium while implementing direct models for stellar evolution and feedback, including stellar winds, core-collapse and Ia supernovae, radiation pressure, photoionization, and photoelectric heating. We release complete snapshots from 3 suites of simulations. The first comprises 20 simulations that zoom in on 14 Milky Way-mass galaxies, 5 SMC/LMC-mass galaxies, and 4 lower-mass galaxies including 1 ultra-faint; we release snapshots at z = 0, 1, 2, 3, 4. The second comprises 4 more massive galaxies simulated to z = 1, with snapshots at z = 1, 2, 3, 4, 5, 6. Finally, a high-redshift suite comprises 22 simulations at z = 5 and 6. Each simulation also includes dozens of resolved lower-mass (satellite) galaxies in its zoom-in region. Snapshots include all stored properties for all dark matter, gas, and star particles, including 11 elemental abundances for stars and gas, and formation times (ages) of star particles. We also release accompanying (sub)halo catalogs, which include galaxy properties and member star particles. For the simulations to z = 0, including all Milky Way-mass galaxies, we release an "ex-situ" flag for star particles, pointers to track particles across snapshots, catalogs of stellar streams, and multipole basis expansions for the halo mass distributions. We describe several publicly available python packages for reading and analyzing these simulations., Comment: 15 pages, submitted to ApJS, data available at http://flathub.flatironinstitute.org/fire

Recent strides have been made developing dust evolution models for galaxy formation simulations but these approaches vary in their assumptions and degree of complexity. Here we introduce and compare two separate dust evolution models (labelled 'Elemental' and 'Species'), based on recent approaches, incorporated into the GIZMO code and coupled with FIRE-2 stellar feedback and ISM physics. Both models account for turbulent dust diffusion, stellar production of dust, dust growth via gas-dust accretion, and dust destruction from time-resolved supernovae, thermal sputtering in hot gas, and astration. The "Elemental" model tracks the evolution of generalized dust species and utilizes a simple, 'tunable' dust growth routine, while the "Species" model tracks the evolution of specific dust species with set chemical compositions and incorporates a physically motivated, two-phase dust growth routine. We test and compare these models in an idealized Milky Way-mass galaxy and find that while both produce reasonable galaxy-integrated dust-to-metals (D/Z) ratios and predict gas-dust accretion as the main dust growth mechanism, a chemically motivated model is needed to reproduce the observed scaling relation between individual element depletions and D/Z with column density and local gas density. We also find the inclusion of theoretical metallic iron and O-bearing dust species are needed in the case of specific dust species in order to match observations of O and Fe depletions, and the integration of a sub-resolution dense molecular gas/CO scheme is needed to both match observed C depletions and ensure carbonaceous dust is not overproduced in dense environments., Comment: 24 pages, 16 figures, submitted to MNRAS

We use FIRE simulations to study disk formation in z~0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar disks 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 of their young stars (>70%) in a thin disk (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 T~10^4 K or less; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disk, and while cooling transitions from a quasi-spherical spatial configuration to a more flattened, disk-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 disk among a sample of 17 z~0 galaxies spanning a halo mass range of 10^10.5 solar masses to 10^12 solar masses, or a stellar mass range 10^8 solar masses to 10^11 solar masses. Notably, galaxies with a thick disk 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 direct deposition onto the galaxy are likely a necessary condition for the formation of thin, star-forming disk galaxies in a LambdaCDM universe., Comment: 16 pages, 10 figures. Comments welcome. Accompanying interactive visualization at zhafen.github.io/rotating-cooling-flows

Low-mass galaxies are highly susceptible to environmental effects that can efficiently quench star formation. We explore the role of ram pressure in quenching low-mass galaxies ($M_{*}\sim10^{5-9}\,\rm{M}_{\odot}$) within 2 Mpc of Milky Way (MW) hosts using the FIRE-2 simulations. Ram pressure is highly variable across different environments, within individual MW haloes, and for individual low-mass galaxies over time. The impulsiveness of ram pressure -- the maximum ram pressure scaled to the integrated ram pressure prior to quenching -- correlates with whether a galaxy is quiescent or star-forming. The time-scale between maximum ram pressure and quenching is anticorrelated with impulsiveness, such that high impulsiveness corresponds to quenching time-scales $<1$ Gyr. Galaxies in low-mass groups ($M_\mathrm{*,host}\sim10^{7-9}\,\rm{M}_{\odot}$) outside of MW haloes experience typical ram pressure only slightly lower than ram pressure on MW satellites, helping to explain effective quenching via group pre-processing. Ram pressure on MW satellites rises sharply with decreasing distance to the host, and, at a fixed physical distance, more recent pericentre passages are typically associated with higher ram pressure because of greater gas density in the inner host halo at late times. Furthermore, the ram pressure and gas density in the inner regions of Local Group-like paired host haloes is higher at small angles off the host galaxy disc compared to isolated hosts. The quiescent fraction of satellites within these low-latitude regions is also elevated in the simulations and observations, signaling possible anisotropic quenching via ram pressure around MW-mass hosts., Comment: 13 pages, 10 figures, + appendices. Accepted for publication in MNRAS

The shape and orientation of dark matter (DM) halos are sensitive to the micro-physics of the DM particle, yet in many mass models, the symmetry axes of the Milky Way's DM halo are often assumed to be aligned with the symmetry axes of the stellar disk. This is well-motivated for the inner DM halo but not for the outer halo. We use zoomed cosmological-baryonic simulations from the Latte suite of FIRE-2 Milky Way-mass galaxies to explore the evolution of the DM halo's orientation with radius and time, with or without a major merger with a Large Magellanic Cloud (LMC) analog, and when varying the DM model. In three of the four CDM halos we examine, the orientation of the halo minor axis diverges from the stellar disk vector by more than 20 degrees beyond about 30 galactocentric kpc, reaching a maximum of 30--90 degrees depending on the individual halo's formation history. In identical simulations using a model of self-interacting DM with $\sigma = 1 \, \mathrm{cm}^2 \, \mathrm{g}^{-1}$, the halo remains aligned with the stellar disk out to $\sim$200--400 kpc. Interactions with massive satellites ($M \gtrsim 4 \times 10^{10} \, \rm{M_\odot}$ at pericenter; $M \gtrsim 3.3 \times 10^{10} \, \rm{M_\odot}$ at infall) affect the orientation of the halo significantly, aligning the halo's major axis with the satellite galaxy from the disk to the virial radius. The relative orientation of the halo and disk beyond 30 kpc is a potential diagnostic of SIDM if the effects of massive satellites can be accounted for., Comment: Submitted to ApJ, 19 pages, 12 figures, 3 tables

The circumgalactic medium (CGM) plays a pivotal role in regulating gas flows around galaxies and thus shapes their evolution. However, the details of how galaxies and their CGM co-evolve remain poorly understood. We present a new time-dependent two-zone model that self-consistently tracks not just mass and metal flows between galaxies and their CGM but also the evolution of the global thermal and turbulent kinetic energy of the CGM. Our model accounts for heating and turbulence driven by both supernova winds and cosmic accretion as well as radiative cooling, turbulence dissipation, and halo outflows due to CGM overpressurization. We demonstrate that, depending on parameters, the CGM can undergo a phase transition (``thermalization'') from a cool, turbulence-supported phase to a virial-temperature, thermally-supported phase. This CGM phase transition is largely determined by the ability of radiative cooling to balance heating from supernova winds and turbulence dissipation. We perform an initial calibration of our model to the FIRE-2 cosmological hydrodynamical simulations and show that it can approximately reproduce the baryon cycles of the simulated halos. In particular, we find that, for these parameters, the phase transition occurs at high-redshift in ultrafaint progenitors and at low redshift in classical $M_{\rm vir}\sim10^{11}M_{\odot}$ dwarfs, while Milky Way-mass halos undergo the transition at $z\approx0.5$. We see a similar transition in the simulations though it is more gradual, likely reflecting radial dependence and multi-phase gas not captured by our model. We discuss these and other limitations of the model and possible future extensions., Comment: Updated to reflect version accepted by ApJ; minor text edits to clarify limitations and caveats of model

Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionises and heats the gas, and (2) the depletion of metals onto dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M$_\odot$ per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Way-mass, using the FIRE galaxy formation models together with the CHIMES non-equilibrium chemistry and cooling module. In our fiducial model, we couple the chemistry to the stellar fluxes calculated from star particles using an approximate radiative transfer scheme, and we implement an empirical density-dependent prescription for metal depletion. For comparison, we also run simulations with a spatially uniform radiation field, and without metal depletion. Our fiducial model broadly reproduces observed trends in HI and H2 mass with stellar mass, and in line luminosity versus star formation rate for [CII] 158$\mu$m, [OI] 63$\mu$m, [OIII] 88$\mu$m, [NII] 122$\mu$m and H$\alpha$ 6563A. Our simulations with a uniform radiation field predict fainter luminosities, by up to an order of magnitude for [OIII] 88$\mu$m and H$\alpha$ 6563A, while ignoring metal depletion increases the luminosity of carbon and oxygen lines by a factor $\approx$2. However, the overall evolution of the galaxy is not strongly affected by local stellar fluxes or metal depletion, except in dwarf galaxies where the inclusion of local fluxes leads to weaker outflows and hence higher gas fractions., Comment: 29 pages, 24 figures (including appendices). Accepted for publication in MNRAS. Minor changes relative to v1

Increasingly, uncertainties in predictions from galaxy formation simulations (at sub-Milky Way masses) are dominated by uncertainties in stellar evolution inputs. In this paper, we present the full set of updates from the FIRE-2 version of the Feedback In Realistic Environments (FIRE) project code, to the next version, FIRE-3. While the transition from FIRE-1 to FIRE-2 focused on improving numerical methods, here we update the stellar evolution tracks used to determine stellar feedback inputs, e.g. stellar mass-loss (O/B and AGB), spectra (luminosities and ionization rates), and supernova rates (core-collapse and Ia), as well as detailed mass-dependent yields. We also update the low-temperature cooling and chemistry, to enable improved accuracy at $T \lesssim 10^{4}\,$K and densities $n\gg 1\,{\rm cm^{-3}}$, and the meta-galactic ionizing background. All of these synthesize newer empirical constraints on these quantities and updated stellar evolution and yield models from a number of groups, addressing different aspects of stellar evolution. To make the updated models as accessible as possible, we provide fitting functions for all of the relevant updated tracks, yields, etc, in a form specifically designed so they can be directly 'plugged in' to existing galaxy formation simulations. We also summarize the default FIRE-3 implementations of 'optional' physics, including spectrally-resolved cosmic rays and supermassive black hole growth and feedback., Comment: 26 pages, 12 figures. Submitted to MNRAS. Comments welcome

Recent strides have been made developing dust evolution models for galaxy formation simulations but these approaches vary in their assumptions and degree of complexity. Here we introduce and compare two separate dust evolution models (labelled 'Elemental' and 'Species'), based on recent approaches, incorporated into the GIZMO code and coupled with FIRE-2 stellar feedback and ISM physics. Both models account for turbulent dust diffusion, stellar production of dust, dust growth via gas-dust accretion, and dust destruction from time-resolved supernovae, thermal sputtering in hot gas, and astration. The "Elemental" model tracks the evolution of generalized dust species and utilizes a simple, 'tunable' dust growth routine, while the "Species" model tracks the evolution of specific dust species with set chemical compositions and incorporates a physically motivated, two-phase dust growth routine. We test and compare these models in an idealized Milky Way-mass galaxy and find that while both produce reasonable galaxy-integrated dust-to-metals (D/Z) ratios and predict gas-dust accretion as the main dust growth mechanism, a chemically motivated model is needed to reproduce the observed scaling relation between individual element depletions and D/Z with column density and local gas density. We also find the inclusion of theoretical metallic iron and O-bearing dust species are needed in the case of specific dust species in order to match observations of O and Fe depletions, and the integration of a sub-resolution dense molecular gas/CO scheme is needed to both match observed C depletions and ensure carbonaceous dust is not overproduced in dense environments., Comment: 28 pages, 18 figures, accepted for publication in MNRAS. Added appendix testing variations in ICs

We describe a public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation, available at http://flathub.flatironinstitute.org/fire, from the Feedback In Realistic Environments (FIRE) project. FIRE-2 simulations achieve parsec-scale resolution to explicitly model the multi-phase interstellar medium while implementing direct models for stellar evolution and feedback, including stellar winds, core-collapse and Ia supernovae, radiation pressure, photoionization, and photoelectric heating. We release complete snapshots from 3 suites of simulations. The first comprises 20 simulations that zoom in on 14 Milky Way-mass galaxies, 5 SMC/LMC-mass galaxies, and 4 lower-mass galaxies including 1 ultra-faint; we release 39 snapshots across z = 0 - 10. The second comprises 4 massive galaxies, with 19 snapshots across z = 1 - 10. Finally, a high-redshift suite comprises 22 simulations, with 11 snapshots across z = 5 - 10. Each simulation also includes dozens of resolved lower-mass (satellite) galaxies in its zoom-in region. Snapshots include all stored properties for all dark matter, gas, and star particles, including 11 elemental abundances for stars and gas, and formation times (ages) of star particles. We also release accompanying (sub)halo catalogs, which include galaxy properties and member star particles. For the simulations to z = 0, including all Milky Way-mass galaxies, we release the formation coordinates and an "ex-situ" flag for all star particles, pointers to track particles across snapshots, catalogs of stellar streams, and multipole basis expansions for the halo mass distributions. We describe publicly available python packages for reading and analyzing these simulations., Comment: 22 pages. Accepted for publication in ApJS, matches the published version, with updated references. Now includes up to 39 snapshots per simulation and formation coordinates for all star particles. Data available at http://flathub.flatironinstitute.org/fire

The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and morphologies). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. We predict that approximately 30 percent of massive central galaxies (with at least 1e11 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 1e8 - 1e9 solar masses in stars). This distinctive class of galaxies provides an additional layer in our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario., Comment: 55 pages, 4 figures, 13 supplementary figures. 29 pages, 4 figures. Accepted for publication in Nature Astronomy. To appear on 14-February-2022. Published version: https://www.nature.com/articles/s41550-021-01598-4

We use FIRE simulations to study disk formation in z~0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar disks 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 (>70%) of their young stars in a thin disk (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 T~10^4 K or less; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disk, and while cooling transitions from a quasi-spherical spatial configuration to a more flattened, disk-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 disk among a sample of 17 z~0 galaxies spanning a halo mass range of 10^10.5 solar masses to 10^12 solar masses, or a stellar mass range 10^8 solar masses to 10^11 solar masses. Notably, galaxies with a thick disk 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 onto the galaxy are likely a necessary condition for the formation of thin, star-forming disk galaxies in a LambdaCDM universe., Comment: 17 pages, 11 figures. Comments welcome. Accompanying interactive visualization at zhafen.github.io/rotating-cooling-flows

The standard picture of galaxy formation motivates the decomposition of the Milky Way into 3--4 stellar populations with distinct kinematic and elemental abundance distributions: the thin disk, thick disk, bulge, and stellar halo. To test this idea, we construct a Gaussian mixture model (GMM) for both simulated and observed stars in the Solar neighborhood, using measured velocities and iron abundances (i.e., an augmented Toomre diagram) as the distributions to be decomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog of the Solar neighborhood with those from a suite of synthetic Gaia-APOGEE crossmatches constructed from FIRE-2 cosmological simulations of Milky Way-mass galaxies. We find that in both the synthetic and real data, the best-fit GMM uses five independent components, some of whose properties resemble the standard populations predicted by galaxy formation theory. Two components can be identified unambiguously as the thin disk and another as the halo. However, instead of a single counterpart to the thick disk, there are three intermediate components with different age and alpha abundance distributions (although these data are not used to construct the model). We use decompositions of the synthetic data to show that the classified components indeed correspond to stars with different origins. By analogy with the simulated data, we show that our mixture model of the real Gaia-APOGEE crossmatch distinguishes the following 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 thick disk (27%), and (5) a stellar halo consistent with early, massive accretion (4%).

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 maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and is rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies, we analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray physics. We find that at z~0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominantly parallel to the disk with small but nonzero vertical velocity components, and joins the disk largely in the outskirts as opposed to "raining" down onto the disk. Once in the disk, gas trajectories are complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show clear but slow net radial infall with transport speeds of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass per year, comparable to the star formation rates of the galaxies and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses per year). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies.

Cosmic rays (CRs) are an important component in the interstellar medium (ISM), but their effect on the dynamics of the disk-halo interface (< 10 kpc from the disk) is still unclear. We study the influence of CRs on the gas above the disk with high-resolution FIRE-2 cosmological simulations of late-type Lstar galaxies at redshift around zero. We compare runs with and without CR feedback (with constant anisotropic diffusion around 3e29 cm^2/s and streaming). Our simulations capture the relevant disk halo interactions, including outflows, inflows, and galactic fountains. Extra-planar gas in all of the runs satisfies dynamical balance, where total pressure balances the weight of the overlying gas. While the kinetic pressure from non-uniform motion (>1-kpc scale) dominates in the midplane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (1e4K) gas is slowly accelerated by CRs; (2) the hot (> 5e5K) gas scale height is suppressed; (3) the warm-hot (2e4-5e5K) medium becomes the most volume-filling phase in the disk-halo interface. We develop a novel conceptual model of the near-disk gas dynamics in low-redshift Lstar galaxies: With CRs, the disk-halo interface is filled with CR-driven warm winds and hot super-bubbles that are propagating into the CGM with a small fraction falling back to the disk. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains., Comment: 20 pages, 15 figures. Submitted to MNRAS

We present the first simulations evolving resolved spectra of cosmic rays (CRs) from MeV-TeV energies (including electrons, positrons, (anti)protons, and heavier nuclei), in live kinetic-MHD galaxy simulations with star formation and feedback. We utilize new numerical methods including terms often neglected in historical models, comparing Milky Way analogues with phenomenological scattering coefficients $\nu$ to Solar-neighborhood (LISM) observations (spectra, B/C, $e^{+}/e^{-}$, $\bar{p}/p$, $^{10}$Be/$^{9}$Be, ionization, $\gamma$-rays). We show it is possible to reproduce observations with simple single-power-law injection and scattering coefficients (scaling with rigidity $R$), similar to previous (non-dynamical) calculations. We also find: (1) The circum-galactic medium in realistic galaxies necessarily imposes a $\sim10\,$kpc CR scattering halo, influencing the required $\nu(R)$. (2) Increasing the normalization of $\nu(R)$ re-normalizes CR secondary spectra but also changes primary spectral slopes, owing to source distribution and loss effects. (3) Diffusive/turbulent reacceleration is unimportant and generally sub-dominant to gyroresonant/streaming losses, which are sub-dominant to adiabatic/convective terms dominated by $\sim0.1-1\,$kpc turbulent/fountain motions. (4) CR spectra vary considerably across galaxies; certain features can arise from local structure rather than transport physics. (5) Systematic variation in CR ionization rates between LISM and molecular clouds (or Galactic position) arises naturally without invoking alternative sources. (6) Abundances of CNO nuclei require most CR acceleration occurs around when reverse shocks form in SNe, not in OB wind bubbles or later Sedov-Taylor stages of SNe remnants., Comment: 25 pages main text + 18 pages appendices, 19 figures. Revised to match accepted MNRAS version. Conclusions unchanged, but extensive appendices added describing CR drift velocities and loss/residence times, numerical methods and tests, different model variations, and validation of magnetic field properties

Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and is rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies, we analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray physics. We find that at z~0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominantly parallel to the disk with small but nonzero vertical velocity components, and joins the disk largely in the outskirts as opposed to "raining" down onto the disk. Once in the disk, gas trajectories are complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show clear but slow net radial infall with transport speeds of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass per year, comparable to the star formation rates of the galaxies and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses per year). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies., Comment: 22 pages, 15 figures, submitted to MNRAS

The standard picture of galaxy formation motivates the decomposition of the Milky Way into 3--4 stellar populations with distinct kinematic and elemental abundance distributions: the thin disk, thick disk, bulge, and stellar halo. To test this idea, we construct a Gaussian mixture model (GMM) for both simulated and observed stars in the Solar neighborhood, using measured velocities and iron abundances (i.e., an augmented Toomre diagram) as the distributions to be decomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog of the Solar neighborhood with those from a suite of synthetic Gaia-APOGEE crossmatches constructed from FIRE-2 cosmological simulations of Milky Way-mass galaxies. We find that in both the synthetic and real data, the best-fit GMM uses five independent components, some of whose properties resemble the standard populations predicted by galaxy formation theory. Two components can be identified unambiguously as the thin disk and another as the halo. However, instead of a single counterpart to the thick disk, there are three intermediate components with different age and alpha abundance distributions (although these data are not used to construct the model). We use decompositions of the synthetic data to show that the classified components indeed correspond to stars with different origins. By analogy with the simulated data, we show that our mixture model of the real Gaia-APOGEE crossmatch distinguishes the following 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 thick disk (27%), and (5) a stellar halo consistent with early, massive accretion (4%).

Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and is rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies, we analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray physics. We find that at z~0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominantly parallel to the disk with small but nonzero vertical velocity components, and joins the disk largely in the outskirts as opposed to "raining" down onto the disk. Once in the disk, gas trajectories are complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show clear but slow net radial infall with transport speeds of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass per year, comparable to the star formation rates of the galaxies and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses per year). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies.

Cosmic rays (CRs) are an important component in the interstellar medium (ISM), but their effect on the dynamics of the disk-halo interface (< 10 kpc from the disk) is still unclear. We study the influence of CRs on the gas above the disk with high-resolution FIRE-2 cosmological simulations of late-type Lstar galaxies at redshift around zero. We compare runs with and without CR feedback (with constant anisotropic diffusion around 3e29 cm^2/s and streaming). Our simulations capture the relevant disk halo interactions, including outflows, inflows, and galactic fountains. Extra-planar gas in all of the runs satisfies dynamical balance, where total pressure balances the weight of the overlying gas. While the kinetic pressure from non-uniform motion (>1-kpc scale) dominates in the midplane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (1e4K) gas is slowly accelerated by CRs; (2) the hot (> 5e5K) gas scale height is suppressed; (3) the warm-hot (2e4-5e5K) medium becomes the most volume-filling phase in the disk-halo interface. We develop a novel conceptual model of the near-disk gas dynamics in low-redshift Lstar galaxies: With CRs, the disk-halo interface is filled with CR-driven warm winds and hot super-bubbles that are propagating into the CGM with a small fraction falling back to the disk. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains., Comment: 21 pages, 15 figures. Accepted for publication by MNRAS

We present the first simulations evolving resolved spectra of cosmic rays (CRs) from MeV-TeV energies (including electrons, positrons, (anti)protons, and heavier nuclei), in live kinetic-MHD galaxy simulations with star formation and feedback. We utilize new numerical methods including terms often neglected in historical models, comparing Milky Way analogues with phenomenological scattering coefficients $\nu$ to Solar-neighborhood (LISM) observations (spectra, B/C, $e^{+}/e^{-}$, $\bar{p}/p$, $^{10}$Be/$^{9}$Be, ionization, $\gamma$-rays). We show it is possible to reproduce observations with simple single-power-law injection and scattering coefficients (scaling with rigidity $R$), similar to previous (non-dynamical) calculations. We also find: (1) The circum-galactic medium in realistic galaxies necessarily imposes a $\sim10\,$kpc CR scattering halo, influencing the required $\nu(R)$. (2) Increasing the normalization of $\nu(R)$ re-normalizes CR secondary spectra but also changes primary spectral slopes, owing to source distribution and loss effects. (3) Diffusive/turbulent reacceleration is unimportant and generally sub-dominant to gyroresonant/streaming losses, which are sub-dominant to adiabatic/convective terms dominated by $\sim0.1-1\,$kpc turbulent/fountain motions. (4) CR spectra vary considerably across galaxies; certain features can arise from local structure rather than transport physics. (5) Systematic variation in CR ionization rates between LISM and molecular clouds (or Galactic position) arises naturally without invoking alternative sources. (6) Abundances of CNO nuclei require most CR acceleration occurs around when reverse shocks form in SNe, not in OB wind bubbles or later Sedov-Taylor stages of SNe remnants., Comment: 25 pages main text + 18 pages appendices, 19 figures. Revised to match accepted MNRAS version. Conclusions unchanged, but extensive appendices added describing CR drift velocities and loss/residence times, numerical methods and tests, different model variations, and validation of magnetic field properties

Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and is rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies, we analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray physics. We find that at z~0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominantly parallel to the disk with small but nonzero vertical velocity components, and joins the disk largely in the outskirts as opposed to "raining" down onto the disk. Once in the disk, gas trajectories are complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show clear but slow net radial infall with transport speeds of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass per year, comparable to the star formation rates of the galaxies and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses per year). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies., Comment: 22 pages, 15 figures, published by MNRAS

The standard picture of galaxy formation motivates the decomposition of the Milky Way into 3--4 stellar populations with distinct kinematic and elemental abundance distributions: the thin disk, thick disk, bulge, and stellar halo. To test this idea, we construct a Gaussian mixture model (GMM) for both simulated and observed stars in the Solar neighborhood, using measured velocities and iron abundances (i.e., an augmented Toomre diagram) as the distributions to be decomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog of the Solar neighborhood with those from a suite of synthetic Gaia-APOGEE crossmatches constructed from FIRE-2 cosmological simulations of Milky Way-mass galaxies. We find that in both the synthetic and real data, the best-fit GMM uses five independent components, some of whose properties resemble the standard populations predicted by galaxy formation theory. Two components can be identified unambiguously as the thin disk and another as the halo. However, instead of a single counterpart to the thick disk, there are three intermediate components with different age and alpha abundance distributions (although these data are not used to construct the model). We use decompositions of the synthetic data to show that the classified components indeed correspond to stars with different origins. By analogy with the simulated data, we show that our mixture model of the real Gaia-APOGEE crossmatch distinguishes the following 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 thick disk (27%), and (5) a stellar halo consistent with early, massive accretion (4%)., Comment: 20 pages, 14 figures, 1 table

Previous studies of fueling black holes (BHs) in galactic nuclei have argued (on scales ~0.01-1000pc) accretion is dynamical with inflow rates $\dot{M}\sim\eta\,M_{\rm gas}/t_{\rm dyn}$ in terms of gas mass $M_{\rm gas}$, dynamical time $t_{\rm dyn}$, and some $\eta$. But these models generally neglected expulsion of gas by stellar feedback, or considered extremely high densities where expulsion is inefficient. Studies of star formation, however, have shown on sub-kpc scales the expulsion efficiency $f_{\rm wind}=M_{\rm ejected}/M_{\rm total}$ scales with the gravitational acceleration as $(1-f_{\rm wind})/f_{\rm wind}\sim\bar{a}_{\rm grav}/\langle\dot{p}/m_{\ast}\rangle\sim \Sigma_{\rm eff}/\Sigma_{\rm crit}$ where $\bar{a}_{\rm grav}\equiv G\,M_{\rm tot}(

The relation between infrared excess (IRX) and UV spectral slope (beta(UV)) is an empirical probe of dust properties of galaxies. The shape, scatter, and redshift evolution of this relation are not well understood, however, leading to uncertainties in estimating the dust content and star formation rates (SFRs) of galaxies at high redshift. In this study, we explore the nature and properties of the IRX-beta(UV) relation with a sample of z = 2-6 galaxies (M-* approximate to 10(9)-10(12) M-circle dot) extracted from high-resolution cosmological simulations (MassiveFIRE) of the Feedback in Realistic Environments (FIRE) project. The galaxies in our sample show an IRX-beta(UV) relation that is in good agreement with the observed relation in nearby galaxies. IRX is tightly coupled to the UV optical depth, and is mainly determined by the dust-to-star geometry instead of total dust mass, while beta(UV) is set both by stellar properties, UV optical depth, and the dust extinction law. Overall, much of the scatter in the IRX-beta(UV) relation of our sample is found to be driven by variations of the intrinsic UV spectral slope. We further assess how the IRX-beta(UV) relation depends on viewing direction, dust-to-metal ratio, birth-cloud structures, and the dust extinction law and we present a simple model that encapsulates most of the found dependencies. Consequently, we argue that the reported 'deficit' of the infrared/sub-millimetre bright objects at z greater than or similar to 5 does not necessarily imply a non-standard dust extinction law at those epochs.

The relation between infrared excess (IRX) and UV spectral slope (beta(UV)) is an empirical probe of dust properties of galaxies. The shape, scatter, and redshift evolution of this relation are not well understood, however, leading to uncertainties in estimating the dust content and star formation rates (SFRs) of galaxies at high redshift. In this study, we explore the nature and properties of the IRX-beta(UV) relation with a sample of z = 2-6 galaxies (M-* approximate to 10(9)-10(12) M-circle dot) extracted from high-resolution cosmological simulations (MassiveFIRE) of the Feedback in Realistic Environments (FIRE) project. The galaxies in our sample show an IRX-beta(UV) relation that is in good agreement with the observed relation in nearby galaxies. IRX is tightly coupled to the UV optical depth, and is mainly determined by the dust-to-star geometry instead of total dust mass, while beta(UV) is set both by stellar properties, UV optical depth, and the dust extinction law. Overall, much of the scatter in the IRX-beta(UV) relation of our sample is found to be driven by variations of the intrinsic UV spectral slope. We further assess how the IRX-beta(UV) relation depends on viewing direction, dust-to-metal ratio, birth-cloud structures, and the dust extinction law and we present a simple model that encapsulates most of the found dependencies. Consequently, we argue that the reported 'deficit' of the infrared/sub-millimetre bright objects at z greater than or similar to 5 does not necessarily imply a non-standard dust extinction law at those epochs.

We examine the thermodynamic state and cooling of the low-$z$ Circum-Galactic 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 nonlinear density perturbations sourced by the accretion of gas from the Inter-Galactic 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 timescales $t_{\rm cool}/t_{\rm ff}$ in the hot component of the CGM spans a wide range $\sim 1-100$ at a given radius, but exhibits approximately constant median values $\sim 5-20$ at all radii $0.1 R_{\rm vir} < r < R_{\rm vir}$. These are similar to the $\approx 10-20$ 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 $t_{\rm cool}/t_{\rm ff}$ 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 $t_{\rm cool}/t_{\rm ff}$ value of a gas parcel in the hot component of the CGM does not predict its probability of subsequently accreting onto the central galaxy. This suggests that the value of $t_{\rm cool}/t_{\rm ff}$ is a poor predictor of thermal stability in gaseous halos in which large-amplitude density perturbations are prevalent., Comment: Updated to match published version. Note: a bug was found in the code used to create Figure 8 - corrected version strengthens conclusions of paper, which remain unchanged

Galaxy evolution is thought to be driven in large part by the flow of gas between galaxies and the circumgalactic medium (CGM), a halo of metal-enriched gas extending out to $\gtrsim100$ kpc from each galaxy. Studying the spatial structure of the CGM holds promise for understanding these gas flow mechanisms; however, the common method using background quasar sightlines provides minimal spatial information. Recent works have shown the utility of extended background sources such as giant gravitationally lensed arcs. Using background lensed arcs from the CSWA 38 lens system, we continuously probed, at a resolution element of about 15 kpc$^2$, the spatial and kinematic distribution of MgII absorption in a star-forming galaxy at $z=0.77$ (stellar mass $\approx 10^{9.7}$ M$_\odot$, star formation rate $\approx 10$ M$_\odot$ yr$^{-1}$) at impact parameters $D \simeq 5-30$ kpc. Our results present an anisotropic, optically thick medium whose absorption strength decreases with increasing impact parameter, in agreement with the statistics towards quasars and other gravitational arcs. Furthermore, we find generally low line-of-sight velocities in comparison to the relatively high velocity dispersion in the MgII gas (with typical $\sigma\approx 50$ km s$^{-1}$). While the galaxy itself exhibits a clear outflow (with MgII velocities up to $\sim 500$ km s$^{-1}$) in the down-the-barrel spectrum, the outflow component is sub-dominant and only weakly detected at larger impact parameters probed by the background arcs. Our results provide evidence of mainly dispersion-supported, metal-enriched gas recycling through the CGM., Comment: 27 pages, 10 figures, 5 tables

Cosmic rays (CRs) with ~GeV energies can contribute significantly to the energy and pressure budget in the interstellar, circumgalactic, and intergalactic medium (ISM, CGM, IGM). Recent cosmological simulations have begun to explore these effects, but almost all studies have been restricted to simplified models with constant CR diffusivity and/or streaming speeds. Physical models of CR propagation/scattering via extrinsic turbulence and self-excited waves predict transport coefficients which are complicated functions of local plasma properties. In a companion paper, we consider a wide range of observational constraints to identify proposed physically-motivated cosmic-ray propagation scalings which satisfy both detailed Milky Way (MW) and extra-galactic $\gamma$-ray constraints. Here, we compare the effects of these models relative to simpler 'diffusion+streaming' models on galaxy and CGM properties at dwarf through MW mass scales. The physical models predict large local variations in CR diffusivity, with median diffusivity increasing with galacto-centric radii and decreasing with galaxy mass and redshift. These effects lead to a more rapid dropoff of CR energy density in the CGM (compared to simpler models), in turn producing weaker effects of CRs on galaxy star formation rates (SFRs), CGM absorption profiles and galactic outflows. The predictions of the more physical CR models tend to lie 'in between' models which ignore CRs entirely and models which treat CRs with constant diffusivity., Comment: 6 pages, 2 figures. Updated to match published MNRAS version

We present optical light curves from the Transiting Exoplanet Survey Satellite (TESS) for the archetypical dwarf active galactic nucleus (AGN) in the nearby galaxy NGC 4395 hosting a $\sim 10^5\,M_\odot$ supermassive black hole (SMBH). Significant variability is detected on timescales from weeks to hours before reaching the background noise level. The $\sim$month-long, 30 minute-cadence, high-precision TESS light curve can be well fit by a simple damped random walk (DRW) model, with the damping timescale $\tau_{\rm DRW}$ constrained to be $2.3_{-0.7}^{+1.8}$~days ($1\sigma$). NGC 4395 lies almost exactly on the extrapolation of the $\tau_{\rm DRW}-M_{\rm BH}$ relation measured for AGNs with BH masses that are more than three orders of magnitude larger. The optical variability periodogram can be well fit by a broken power law with the high-frequency slope ($-1.88\pm0.15$) and the characteristic timescale ($\tau_{\rm br}\equiv 1/(2\pi f_{\rm br})=1.4_{-0.5}^{+1.9}\,$days) consistent with the DRW model within 1$\sigma$. This work demonstrates the power of TESS light curves in identifying low-mass accreting SMBHs with optical variability, and a potential global $\tau_{\rm DRW}-M_{\rm BH}$ relation that can be used to estimate SMBH masses with optical variability measurements., Comment: 12 pages, 5 figures, 1 table. Accepted for publication in ApJ

We study the effects of cosmic rays (CRs) on outflows from star-forming galaxies in the circum and inter-galactic medium (CGM/IGM), in high-resolution, fully-cosmological FIRE-2 simulations (accounting for mechanical and radiative stellar feedback, magnetic fields, anisotropic conduction/viscosity/CR diffusion and streaming, and CR losses). We showed previously that massive ($M_{\rm halo}\gtrsim 10^{11}\,M_{\odot}$), low-redshift ($z\lesssim 1-2$) halos can have CR pressure dominate over thermal CGM pressure and balance gravity, giving rise to a cooler CGM with an equilibrium density profile. This dramatically alters outflows. Absent CRs, high gas thermal pressure in massive halos "traps" galactic outflows near the disk, so they recycle. With CRs injected in supernovae as modeled here, the low-pressure halo allows "escape" and CR pressure gradients continuously accelerate this material well into the IGM in "fast" outflows, while lower-density gas at large radii is accelerated in-situ into "slow" outflows that extend to $>$Mpc scales. CGM/IGM outflow morphologies are radically altered: they become mostly volume-filling (with inflow in a thin mid-plane layer) and coherently biconical from the disk to $>$Mpc. The CR-driven outflows are primarily cool ($T\sim10^{5}\,$K) and low-velocity. All of these effects weaken and eventually vanish at lower halo masses ($\lesssim 10^{11}\,M_{\odot}$) or higher redshifts ($z\gtrsim 1-2$), reflecting the ratio of CR to thermal+gravitational pressure in the outer halo. We present a simple analytic model which explains all of the above phenomena., Comment: 24 pages, 20 figures, revised to match published MNRAS version. Comments welcome. Animations in CGM/IGM section at http://www.tapir.caltech.edu/~phopkins/Site/animations

We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which in our models sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H$_2$ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T < 300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H$_{2}$ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model which controls the amount of radiation that penetrates giant molecular clouds., Comment: Accepted to MNRAS. 16 pages, 10 figures

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 a 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 FIRE-2 cosmological simulations, including 6 isolated MW/M31-mass galaxies and 6 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 a MW/M31-mass galaxy emerged typically at z ~ 3-4 (11.6-12.2 Gyr ago), while stars in the bulge region (inner 2 kpc) at z = 0 formed primarily in a single main progenitor at z < 5 (< 12.6 Gyr ago). Compared with isolated hosts, the main progenitors of LG-like paired hosts emerged significantly earlier (\Delta z ~ 2, \Delta t ~ 1.6 Gyr), with ~ 4x higher stellar mass at all z > 4 (> 12.2 Gyr ago). This highlights the importance of environment in MW/M31-mass galaxy formation, especially at early times. Overall, about 100 galaxies with M_star > 10^5 M_sun formed a typical MW/M31-mass system. Thus, surviving satellites represent a highly incomplete census (by ~ 5x) of the progenitor population., Comment: 20 pages, 8 figures, accepted for publication in MNRAS

The microphysics of ~GeV cosmic ray (CR) transport on galactic scales remain deeply uncertain, with almost all studies adopting simple prescriptions (e.g. constant-diffusivity). We explore different physically-motivated, anisotropic, dynamical CR transport scalings in high-resolution cosmological FIRE simulations of dwarf and ~$L_{\ast}$ galaxies where scattering rates vary with local plasma properties motivated by extrinsic turbulence (ET) or self-confinement (SC) scenarios, with varying assumptions about e.g. turbulent power spectra on un-resolved scales, Alfven-wave damping, etc. We self-consistently predict observables including $\gamma$-rays ($L_{\gamma}$), grammage, residence times, and CR energy densities to constrain the models. We demonstrate many non-linear dynamical effects (not captured in simpler models) tend to enhance confinement. For example, in multi-phase media, even allowing arbitrary fast transport in neutral gas does not substantially reduce CR residence times (or $L_{\gamma}$), as transport is rate-limited by the ionized WIM and 'inner CGM' gaseous halo ($10^{4}-10^{6}$ K gas within 10-30 kpc), and $L_{\gamma}$ can be dominated by trapping in small 'patches.' Most physical ET models contribute negligible scattering of ~1-10 GeV CRs, but it is crucial to account for anisotropy and damping (especially of fast modes) or else scattering rates would violate observations. We show that the most widely-assumed scalings for SC models produce excessive confinement by factors >100 in the WIM and inner CGM, where turbulent and Landau damping dominate. This suggests either a breakdown of quasi-linear theory used to derive the CR transport parameters in SC, or that other novel damping mechanisms dominate in intermediate-density ionized gas., Comment: 29 pages, 12 figures. Replaced with published MNRAS version

Project AMIGA (Absorption Maps In the Gas of Andromeda) is a large ultraviolet Hubble Space Telescope program, which has assembled a sample of 43 QSOs that pierce the circumgalactic medium (CGM) of Andromeda (M31) from R=25 to 569 kpc (25 of them probing gas from 25 kpc to about the virial radius-Rvir = 300 kpc-of M31). Our large sample provides an unparalleled look at the physical conditions and distribution of metals in the CGM of a single galaxy using ions that probe a wide range of gas phases (Si II, Si III, Si IV, C II, C IV, and O VI, the latter being from the Far Ultraviolet Spectroscopic Explorer). We find that Si III and O VI have near unity covering factor maintained all the way out to 1.2Rvir and 1.9Rvir, respectively. We show that Si III is the dominant ion over Si II and Si IV at any R. While we do not find that the properties of the CGM of M31 depend strongly on the azimuth, we show that they change remarkably around 0.3-0.5Rvir, conveying that the inner regions of the CGM of M31 are more dynamic and have more complicated multi-phase gas-structures than at R>0.5Rvir. We estimate the metal mass of the CGM within Rvir as probed by Si II, Si III, and Si IV is 2x10^7 Msun and by O VI is >8x10^7 Msun, while the baryon mass of the 10^4-10^5.5 K gas is ~4x10^10 (Z/0.3 Zsun)^(-1) Msun within Rvir. We show that different zoom-in cosmological simulations of L* galaxies better reproduce the column density profile of O VI with R than Si III or the other studied ions. We find that observations of the M31 CGM and zoom-in simulations of L* galaxies have both lower ions showing higher column density dispersion and dependence on R than higher ions, indicating that the higher ionization structures are larger and/or more broadly distributed., Comment: Submitted to the Astrophysical Journal. Comments welcome

Observations of emission lines in Active Galactic Nuclei (AGN) often find fast (~1000 km s^-1) outflows extending to kiloparsec scales, seen in ionised, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient ISM or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [OIV] 25 ${\mu}$m / [NeII] 12 ${\mu}$m , [NeV] 14 ${\mu}$m / [NeII] 12 ${\mu}$m and [NIII] 57 ${\mu}$m / [NII] 122 ${\mu}$m constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurised compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that >50 per cent of the mass outflow rate, momentum flux and kinetic energy flux of the outflow are traced by lines such as [NII] 122 ${\mu}$m and [NeIII] 15 ${\mu}$m (produced in the 10^4 K phase) and [CII] 158 ${\mu}$m (produced in the transition from 10^4 K to 100 K)., Comment: 19 pages, 16 figures (including appendix). Accepted for publication in MNRAS; minor changes relative to previous version

The galaxy size-stellar mass and central surface density-stellar mass relationships are observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects. Consequently, forward-modeling light-based sizes from simulations is desirable. In this work, we use the {\skirt} dust radiative transfer code to generate synthetic observations of massive galaxies ($M_{*}\sim10^{11}\,\rm{M_{\odot}}$ at $z=2$, hosted by haloes of mass $M_{\rm{halo}}\sim10^{12.5}\,\rm{M_{\odot}}$) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments (FIRE) project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius ($R_e$), as well as the stellar mass surface density within this radius and within $1\,\rm{kpc}$ ($\Sigma_e$ and $\Sigma_1$, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The predicted sizes and surface densities are within a factor of two of the intrinsic values. We then compare our predictions to the observed size-mass relationship and the $\Sigma_1-M_\star$ and $\Sigma_e-M_\star$ relationships. At $z\gtrsim2$, the simulated massive galaxies are in general agreement with observational scaling relations. At $z\lesssim2$, they evolve to become too compact but still star-forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at $z\lesssim2$., Comment: 12 pages, 7 figures, accepted to mnras, final accepted version

We examine the thermodynamic state and cooling of the low-$z$ Circum-Galactic 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 nonlinear density perturbations sourced by the accretion of gas from the Inter-Galactic 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 timescales $t_{\rm cool}/t_{\rm ff}$ in the hot component of the CGM spans a wide range $\sim 1-100$ at a given radius, but exhibits approximately constant median values $\sim 5-20$ at all radii $0.1 R_{\rm vir} < r < R_{\rm vir}$. These are similar to the $\approx 10-20$ 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 $t_{\rm cool}/t_{\rm ff}$ 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 $t_{\rm cool}/t_{\rm ff}$ value of a gas parcel in the hot component of the CGM does not predict its probability of subsequently accreting onto the central galaxy. This suggests that the value of $t_{\rm cool}/t_{\rm ff}$ is a poor predictor of thermal stability in gaseous halos in which large-amplitude density perturbations are prevalent., Comment: Updated to match published version. Note: a bug was found in the code used to create Figure 8 - corrected version strengthens conclusions of paper, which remain unchanged