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The cold (⁠|$\sim 10^{4}\, {\rm K}$|⁠) component of the circumgalactic medium (CGM) accounts for a significant fraction of all galactic baryons. However, using current galaxy-scale simulations to determine the origin and evolution of cold CGM gas poses a significant challenge, since it is computationally infeasible to directly simulate a galactic halo alongside the sub-pc scales that are crucial for understanding the interactions between cold CGM gas and the surrounding 'hot' medium. In this work, we introduce a new approach: the Cold Gas Subgrid Model (CGSM), which models unresolved cold gas as a second fluid in addition to the standard 'normal' gas fluid. The CGSM tracks the total mass density and bulk momentum of unresolved cold gas, deriving the properties of its unresolved cloudlets from the resolved gas phase. The interactions between the subgrid cold fluid and the resolved fluid are modelled by prescriptions from high-resolution simulations of 'cloud crushing' and thermal instability. Through a series of idealized tests, we demonstrate the CGSM's ability to overcome the resolution limitations of traditional hydrodynamics simulations, successfully capturing the correct cold gas mass, its spatial distribution, and the time-scales for cloud destruction and growth. We discuss the implications of using this model in cosmological simulations to more accurately represent the microphysics that govern the galactic baryon cycle. [ABSTRACT FROM AUTHOR]

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, ∼ L * galaxies could have substantial non-thermal pressure support in the form of cosmic rays. However, these predictions are sensitive to the specific model of cosmic ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a novel observational constraint for calculating the lower limit of the radially averaged, effective cosmic ray transport rate, |${\kappa _{\rm eff}^{\rm min}}$|⁠. Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disc, regardless of whether the cosmic ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between |${\kappa _{\rm eff}^{\rm min}}$| and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of Feedback in Realistic Environments 2 galaxy simulations with a variety of cosmic ray transport physics to demonstrate that our analytical model of |${\kappa _{\rm eff}^{\rm min}}$| is a robust lower limit of the true cosmic ray transport rate. We then apply our new model to calculate |${\kappa _{\rm eff}^{\rm min}}$| for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate that rises rapidly away from the interstellar medium to values |${\kappa _{\rm eff}^{\rm min}}\gtrsim 10^{30\!-\!31}\, {\rm cm}^2\, {\rm s}^{-1}$| (corresponding to anisotropic streaming velocities of |$v^{\rm stream}_{\rm eff} \gtrsim 1000\, {\rm km}\, {\rm s}^{-1}$|⁠) in the diffuse CGM, at impact parameters larger than 50–100 kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium. [ABSTRACT FROM AUTHOR]

We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In Milky Way-mass, low-redshift (z ≲ 1−2) haloes, which are expected to form 'hot haloes' with slowly cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure confined to overdense clumps, embedded in low-density, volume-filling hot gas with volume-weighted cooling time longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius (≈ R vir), and the shock is quasi-spherical. With CRs, we previously argued that haloes in this particular mass and redshift range build up CR-pressure-dominated gaseous haloes. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond R vir. Rapid cooling also maintains subvirial temperatures in the inflowing gas within ∼ R vir. [ABSTRACT FROM AUTHOR]

Observations of ultraviolet (UV) metal absorption lines have provided insight into the structure and composition of the circumgalactic medium (CGM) around galaxies. We compare these observations with the low-redshift (z ≤ 0.3) CGM around dwarf galaxies in high-resolution cosmological zoom-in runs in the FIRE-2 (Feedback In Realistic Environments) simulation suite. We select simulated galaxies that match the halo mass, stellar mass, and redshift of the observed samples. We produce absorption measurements using trident for UV transitions of C  iv , O  vi , Mg  ii , and Si  iii. The FIRE equivalent width (EW) distributions and covering fractions for the C  iv ion are broadly consistent with observations inside 0.5 R vir, but are underpredicted for O  vi , Mg  ii , and Si  iii. The absorption strengths of the ions in the CGM are moderately correlated with the masses and star formation activity of the galaxies. The correlation strengths increase with the ionization potential of the ions. The structure and composition of the gas from the simulations exhibit three zones around dwarf galaxies characterized by distinct ion column densities: the discy interstellar medium, the inner CGM (the wind-dominated regime), and the outer CGM (the IGM accretion-dominated regime). We find that the outer CGM in the simulations is nearly but not quite supported by thermal pressure, so it is not in hydrostatic equilibrium, resulting in halo-scale bulk inflow and outflow motions. The net gas inflow rates are comparable to the star formation rate of the galaxy, but the bulk inflow and outflow rates are greater by an order of magnitude, with velocities comparable to the virial velocity of the halo. These roughly virial velocities (⁠|${\sim } 100 \, \rm km\, s^{-1}$|⁠) produce large EWs in the simulations. This supports a picture for dwarf galaxies in which the dynamics of the CGM at large scales are coupled to the small-scale star formation activity near the centre of their haloes. [ABSTRACT FROM AUTHOR]

We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈 z 〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈 z fg〉 = 2.23 and 〈 z bg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess |$\rm{H\,{\small I}}$| |$\rm Ly\,\alpha$| optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ D tran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of |$\rm{H\,{\small I}}$| gas as a function of projected distance D tran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of |$\rm{H\,{\small I}}$|⁠. The best-fitting model suggests that galaxy-scale outflows with initial velocity v out ≃ 600 km s |$^{-1}\,$| dominate the kinematics of circumgalactic |$\rm{H\,{\small I}}$| out to D tran ≃ 50 kpc, while |$\rm{H\,{\small I}}$| at D tran ≳ 100 kpc is dominated by infall with characteristic v in ≲ circular velocity. Over the impact parameter range 80 ≲ D tran/pkpc ≲ 200, the |$\rm{H\,{\small I}}$| line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame |$\rm Ly\,\alpha$| equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond D tran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion. [ABSTRACT FROM AUTHOR]

We present and study a large suite of high-resolution cosmological zoom-in simulations, using the FIRE-2 treatment of mechanical and radiative feedback from massive stars, together with explicit treatment of magnetic fields, anisotropic conduction and viscosity (accounting for saturation and limitation by plasma instabilities at high β), and cosmic rays (CRs) injected in supernovae shocks (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultrafaint dwarf (⁠|$M_{\ast }\sim 10^{4}\, \mathrm{M}_{\odot }$|⁠ , |$M_{\rm halo}\sim 10^{9}\, \mathrm{M}_{\odot }$|⁠) through Milky Way/Local Group (MW/LG) masses, systematically vary uncertain CR parameters (e.g. the diffusion coefficient κ and streaming velocity), and study a broad ensemble of galaxy properties [masses, star formation (SF) histories, mass profiles, phase structure, morphologies, etc.]. We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved (⁠|$\gtrsim 1\,$|  pc) scales have only small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs (⁠|$M_{\ast } \ll 10^{10}\, \mathrm{M}_{\odot }$|⁠ , |$M_{\rm halo} \lesssim 10^{11}\, \mathrm{M}_{\odot }$|⁠), or at high redshifts (z ≳ 1–2), for any physically reasonable parameters. However, at higher masses (⁠|$M_{\rm halo} \gtrsim 10^{11}\, \mathrm{M}_{\odot }$|⁠) and z ≲ 1–2, CRs can suppress SF and stellar masses by factors ∼2–4, given reasonable injection efficiencies and relatively high effective diffusion coefficients |$\kappa \gtrsim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$|⁠. At lower κ, CRs take too long to escape dense star-forming gas and lose their energy to collisional hadronic losses, producing negligible effects on galaxies and violating empirical constraints from spallation and γ-ray emission. At much higher κ CRs escape too efficiently to have appreciable effects even in the CGM. But around |$\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$|⁠ , CRs escape the galaxy and build up a CR-pressure-dominated halo which maintains approximate virial equilibrium and supports relatively dense, cool (T ≪ 106 K) gas that would otherwise rain on to the galaxy. CR 'heating' (from collisional and streaming losses) is never dominant. [ABSTRACT FROM AUTHOR]

We present the extended GALEX Arecibo SDSS Survey (xGASS), a gas fraction-limited census of the atomic hydrogen (H I) gas content of 1179 galaxies selected only by stellar mass (M* = 109-1011.5 M⊙) and redshift (0.01

Cosmological hydrodynamical simulations of galaxy evolution are increasingly able to produce realistic galaxies, but the largest hurdle remaining is in constructing subgrid models that accurately describe the behaviour of stellar feedback. As an alternate way to test and calibrate such models, we propose to focus on the circumgalactic medium (CGM). To do so, we generate a suite of adaptive mesh refinement simulations for a Milky-Way-massed galaxy run to z = 0, systematically varying the feedback implementation. We then post-process the simulation data to compute the absorbing column density for a wide range of common atomic absorbers throughout the galactic halo, including H i, Mg ii, Si ii, Si iii, Si iv, C iv, N v, O vi and O vii. The radial profiles of these atomic column densities are compared against several quasar absorption line studies to determine if one feedback prescription is favoured. We find that although our models match some of the observations (specifically those ions with lower ionization strengths), it is particularly difficult to match O vi observations. There is some indication that the models with increased feedback intensity are better matches. We demonstrate that sufficient metals exist in these haloes to reproduce the observed column density distribution in principle, but the simulated CGM lacks significant multiphase substructure and is generally too hot. Furthermore, we demonstrate the failings of inflow-only models (without energetic feedback) at populating the CGM with adequate metals to match observations even in the presence of multiphase structure. Additionally, we briefly investigate the evolution of the CGM from z = 3 to present. Overall, we find that quasar absorption line observations of the gas around galaxies provide a new and important constraint on feedback models. [ABSTRACT FROM AUTHOR]

Errata, addenda, galaxies: evolution, galaxies: high-redshift, galaxies: ISM, galaxies: intergalatic medium This is an erratum to the paper 'The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z~ 2' (2020, MNRAS, 499, 1721-1746). Erratum: The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ~ 2. [Extracted from the article]