Hydrogen and metal line absorption around low-redshift galaxies in cosmological hydrodynamic simulations

Abstract

We study the physical conditions of the circumgalactic medium (CGM) around z=0.25 galaxies as traced by HI and metal line absorption, using cosmological hydrodynamic simulations that include galactic outflows. Using lines of sight targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo masses from 1011-1013M, we study the physical conditions and their variation with impact parameter b and line-of-sight velocity Δv in the CGM as traced by HI, MgII, Si IV, CIV, OVI and Ne VIII absorbers. All ions show a strong excess of absorption near galaxies compared to random lines of sight. The excess continues beyond 1 Mpc, reflecting the correlation of metal absorption with large-scale structure. Absorption is particularly enhanced within about Δv ≪ 300 km s-1 and roughly 300 kpc of galaxies (with distances somewhat larger for the highest ion), approximately delineating the CGM; this range contains the majority of global metal absorption. Low ions like Mg II and Si IV predominantly arise in denser gas closer to galaxies and drop more rapidly with b, while high ions OVI and Ne VIII trace more diffusely distributed gas with a comparatively flat radial profile; C IV is intermediate. All ions predominantly trace T ~ 104-4.5 K photoionized gas at all b, but when hot CGM gas is present (mostly in larger haloes), we see strong collisionally ionized OVI and Ne VIII at b ≤100 kpc. Larger halo masses generally produce more absorption, though overall the trends are not as strong as that with impact parameter. These findings arise using our favoured outflow scalings as expected for momentum-driven winds; with no winds, the CGM gas remains mostly unenriched, while our outflow model with a constant velocity and mass loading factor produce hotter, more widely dispersed metals. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.