A comparison of observed and simulated absorption from H I, C IV, and Si IV around z ≈ 2 star-forming galaxies suggests redshift–space distortions are due to inflows

Abstract

We study H I and metal-line absorption around z ≈ 2 star-forming galaxies by comparing an analysis of data from the Keck Baryonic Structure Survey to mock spectra generated from the Evolution and Assembly of Galaxies and their Environments (EAGLE) cosmological, hydrodynamical simulations. We extract sightlines from the simulations and compare the properties of the absorption by H I, C IV, and Si IV around simulated and observed galaxies using pixel optical depths. We mimic the resolution, pixel size, and signal-to-noise ratio of the observations, as well as the distributions of impact parameters and galaxy redshift errors. We find that the EAGLE reference model is in excellent agreement with the observations. In particular, the simulation reproduces the high metal-line optical depths found at small galactocentric distances, the optical depth enhancements out to impact parameters of 2 proper Mpc, and the prominent redshift–space distortions which we find are due to peculiar velocities rather than redshift errors. The agreement is best for halo masses ∼10^(12.0) M⊙, for which the observed and simulated stellar masses also agree most closely. We examine the median ion mass-weighted radial gas velocities around the galaxies, and find that most of the gas is infalling, with the infall velocity depending on halo rather than stellar mass. From this, we conclude that the observed redshift–space distortions are predominantly caused by infall rather than outflows.