Galactic outflow rates in the EAGLE simulations

TL;DR

This paper analyzes galactic outflow rates in the EAGLE simulations, revealing how feedback mechanisms influence gas removal from galaxies and their halos, with implications for galaxy evolution models.

Contribution

It provides detailed measurements of outflow rates, velocities, and patterns in the EAGLE simulations, and compares these with other models to highlight differences in feedback implementations.

Findings

Gas removal scales with circular velocity as V_c^{-3/2} in low-mass galaxies.

AGN feedback causes increased outflows in halos above 10^{12} M_sun.

Outflows are bimodal and aligned with galactic minor axes.

Abstract

We present measurements of galactic outflow rates from the EAGLE suite of cosmological simulations. We find that gas is removed from the interstellar medium (ISM) of central galaxies with a dimensionless mass loading factor that scales approximately with circular velocity as $V_{\mathrm{c}}^{-3/2}$ in the low-mass regime where stellar feedback dominates. Feedback from active galactic nuclei (AGN) causes an upturn in the mass loading for halo masses $> 10^{12} \, \mathrm{M_\odot}$. We find that more gas outflows through the halo virial radius than is removed from the ISM of galaxies, particularly at low redshift, implying substantial mass loading within the circum-galactic medium (CGM). Outflow velocities span a wide range at a given halo mass/redshift, and on average increase positively with redshift and halo mass up to $M_{200} \sim 10^{12} \, \mathrm{M_\odot}$. Outflows exhibit a bimodal flow pattern on circum-galactic scales, aligned with the galactic minor axis. We present a number of like-for-like comparisons to outflow rates from other recent cosmological hydrodynamical simulations, and show that comparing the propagation of galactic winds as a function of radius reveals substantial discrepancies between different models. Relative to some other simulations, EAGLE favours a scenario for stellar feedback where agreement with the galaxy stellar mass function is achieved by removing smaller amounts of gas from the ISM, but with galactic winds that then propagate and entrain ambient gas out to larger radii.