Revealing the physical properties of gas accreting to haloes in the EAGLE simulations

TL;DR

This study uses the EAGLE simulations to analyze the physical properties of gas accreting onto galaxy haloes, revealing differences between hot and cold modes, the impact of AGN feedback, and the chemical composition of inflowing gas.

Contribution

The paper provides a detailed characterization of gas accretion properties onto haloes, including temperature, metallicity, and the effects of feedback, offering new insights into galaxy evolution processes.

Findings

Cold-mode accretion has a lower covering fraction than hot-mode.

AGN feedback reduces gas inflow covering fraction by about 10%.

First-infall gas is significantly metal-depleted compared to pre-processed gas.

Abstract

The inflow of cosmological gas onto haloes, while challenging to directly observe and quantify, plays a fundamental role in the baryon cycle of galaxies. Using the EAGLE suite of hydrodynamical simulations, we present a thorough exploration of the physical properties of gas accreting onto haloes -- namely, its spatial characteristics, density, temperature, and metallicity. Classifying accretion as ``hot'' or `` cold'' based on a temperature cut of $10^{5.5}{\rm K}$, we find that the covering fraction ($f_{\rm cov}$) of cold-mode accreting gas is significantly lower than the hot-mode, with $z=0$ $f_{\rm cov}$ values of $\approx 50\%$ and $\approx 80\%$ respectively. Active Galactic Nuclei (AGN) feedback in EAGLE reduces inflow $f_{\rm cov}$ values by $\approx 10\%$, with outflows decreasing the solid angle available for accretion flows. Classifying inflow by particle history, we find that gas on first-infall onto a halo is metal-depleted by $\approx 2$~dex compared to pre-processed gas, which we find to mimic the circum-galactic medium (CGM) in terms of metal content. We also show that high (low) halo-scale gas accretion rates are associated with metal-poor (rich) CGM in haloes below $10^{12}M_{\odot}$, and that variation in halo-scale gas accretion rates may offer a physical explanation for the enhanced scatter in the star-forming main sequence at low ($\lesssim10^{9}M_{\odot}$) and high ($\gtrsim10^{10}M_{\odot}$) stellar masses. Our results highlight how gas inflow influences several halo- and galaxy-scale properties, and the need to combine kinematic and chemical data in order to confidently break the degeneracy between accreting and outgoing gas in CGM observations.