The impact of feedback and the hot halo on the rates of gas accretion onto galaxies

TL;DR

This study uses cosmological simulations and a semi-analytic model to analyze how feedback processes and hot halo properties influence gas accretion rates onto galaxies across different halo masses and redshifts.

Contribution

It introduces a semi-analytic model that explains the non-monotonic gas accretion trends by hot halo cooling and feedback effects, highlighting the role of AGN and stellar feedback.

Findings

Gas accretion increases with halo mass up to 10^{11.7} M_sun

Cooling from hot halos causes the observed flattening in accretion rates

AGN feedback efficiency impacts high-mass halo accretion rates

Abstract

We investigate the physics that drives the gas accretion rates onto galaxies at the centers of dark matter haloes using the EAGLE suite of hydrodynamical cosmological simulations. We find that at redshifts $z{\le}2$ the accretion rate onto the galaxy increases with halo mass in the halo mass range $10^{10}-10^{11.7}M_{\odot}$, flattens between the halo masses $10^{11.7}-10^{12.7}M_{\odot}$, and increases again for higher-mass haloes. However, the galaxy gas accretion does not flatten at intermediate halo masses when AGN feedback is switched off. To better understand these trends, we develop a physically motivated semi-analytic model of galaxy gas accretion. We show that the flattening is produced by the rate of gas cooling from the hot halo. The ratio of the cooling radius and the virial radius does not decrease continuously with increasing halo mass as generally thought. While it decreases up to ${\sim}10^{13}M_{\odot}$ haloes, it increases for higher halo masses, causing an upturn in the galaxy gas accretion rate. This may indicate that in high-mass haloes AGN feedback is not sufficiently efficient. When there is no AGN feedback, the density of the hot halo is higher, the ratio of the cooling and virial radii does not decrease as much and the cooling rate is higher. Changes in the efficiency of stellar feedback can also increase or decrease the accretion rates onto galaxies. The trends can plausibly be explained by the re-accretion of gas ejected by progenitor galaxies and by the suppression of black hole growth, and hence AGN feedback, by stellar feedback.