Gas Accretion is Dominated by Warm Ionized Gas in Milky Way-Mass Galaxies at z ~ 0

TL;DR

This study uses high-resolution simulations to show that warm ionized gas dominates mass accretion in Milky Way-like galaxies at low redshift, primarily flowing along filaments and heated during infall.

Contribution

It provides detailed simulation-based insights into the kinematics and composition of gas inflows and outflows in galaxy halos, highlighting the dominance of warm ionized gas in accretion processes.

Findings

Warm-hot and hot ionized gases dominate mass accretion.

Most inflowing gas does not cool before reaching the galaxy.

Neutral clouds are a small fraction of total inflow.

Abstract

We perform high-resolution hydrodynamic simulations of a Milky Way-mass galaxy in a fully cosmological setting using the adaptive mesh refinement code, Enzo, and study the kinematics of gas in the simulated galactic halo. We find that the gas inflow occurs mostly along filamentary structures in the halo. The warm-hot (10^5 K < T < 10^6 K) and hot (T > 10^6 K) ionized gases are found to dominate the overall mass accretion in the system (with dM/dt = 3-5 M_solar/yr) over a large range of distances, extending from the virial radius to the vicinity of the disk. Most of the inflowing gas (by mass) does not cool, and the small fraction that manages to cool does so primarily close to the galaxy (R <~ 20 kpc), perhaps comprising the neutral gas that may be detectable as, e.g., high-velocity clouds. The neutral clouds are embedded within larger, accreting filamentary flows, and represent only a small fraction of the total mass inflow rate. The inflowing gas has relatively low metallicity (Z/Z_solar < 0.2). The outer layers of the filamentary inflows are heated due to compression as they approach the disk. In addition to the inflow, we find high-velocity, metal-enriched outflows of hot gas driven by supernova feedback. Our results are consistent with observations of halo gas at low z.