Galaxies probing galaxies: Cool halo gas from A z = 0.47 post-starburst galaxy

We study the cool gas around a galaxy at z = 0.4729 using Keck/LRIS spectroscopy of a bright (B = 21.7) background galaxy at z = 0.6942 at a transverse distance of 16.5h ^-1 ₇₀ kpc. The background galaxy spectrum reveals strong Fe II, Mg II, Mg I, and Ca II absorption at the redshift of the foreground galaxy, with an Mg II λ2796 rest equivalent width of 3.93 0.08, indicative of a velocity width exceeding 400 km s ^-1. Because the background galaxy is large (>4h ^-1 ₇₀ kpc), the high covering fraction of the absorbing gas suggests that it arises in a spatially extended complex of cool clouds with large velocity dispersion. Spectroscopy of the massive (log M _*/M _⊙ = 11.15 0.08) host galaxy reveals that it experienced a burst of star formation about 1 Gyr ago and that it harbors a weak active galactic nucleus. We discuss the possible origins of the cool gas in its halo, including multiphase cooling of hot halo gas, cold inflow, tidal interactions, and galactic winds. We conclude that the absorbing gas was most likely ejected or tidally stripped from the interstellar medium of the host galaxy or its progenitors during the past starburst event. Adopting the latter interpretation, these results place one of only a few constraints on the radial extent of cool gas driven or stripped from a galaxy in the distant universe. Future studies with integral field unit spectroscopy of spatially extended background galaxies will provide multiple sight lines through foreground absorbers and permit analysis of the morphology and kinematics of the gas surrounding galaxies with a diverse set of properties and environments.