Revealing the physical properties of gas accreting to haloes in the EAGLE simulations
Ruby J. Wright, Claudia del P. Lagos, Chris Power, Camila A. Correa
Abstract
ABSTRACT The inflow of cosmological gas on to 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 on to haloes – namely, its spatial characteristics, density, temperature, and metallicity. Classifying accretion as ‘hot’ or ‘ cold’ based on a temperature cut-off 105.5 K, we find that the covering fraction (fcov) of cold-mode accreting gas is significantly lower than the hot-mode, with z = 0 fcov values of ${\approx}50{{\ \rm per\ cent}}$ and ${\approx}80{{\ \rm per\ cent}}$, respectively. Active galactic nucleus (AGN) feedback in eagle reduces inflow fcov values by ${\approx}10{{\ \rm per\ cent}}$, with outflows decreasing the solid angle available for accretion flows. Classifying inflow by particle history, we find that gas on first-infall on to a halo is metal depleted by ≈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 $10^{12}\, \mathrm{M}_{\odot }$, 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 (${\lesssim}10^{9}\, \mathrm{M}_{\odot }$) and high (${\gtrsim}10^{10}\, \mathrm{M}_{\odot }$) 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.