The Geometry of Cold, Metal-Enriched Gas Around Galaxies at $z\sim1.2$

Abstract: We present the first results from a Hubble Space Telescope WFC3/IR program, which obtained direct imaging and grism observations of galaxies near quasar sightlines with a high frequency of uncorrelated foreground Mg II absorption. These highly efficient observations targeted 54 Mg II absorbers along the line of sight to nine quasars at $z_{qso}\sim2$. We find that 89% of the absorbers in the range $0.64< z < 1.6$ can be spectroscopically matched to at least one galaxy with an impact parameter less than 200 kpc and $|\Delta z|/(1+z)<0.006$. We have estimated the star formation rates and measured structural parameters for all detected galaxies with impact parameters in the range 7-200 kpc and star formation rates greater than 1.3 M$_{\odot}$ yr$^{-1}$. We find that galaxies associated with Mg II absorption have significantly higher mean star formation rates and marginally higher mean star formation rate surface densities compared to galaxies with no detected Mg II. Nearly half of the Mg II absorbers match to more than one galaxy, and the mean equivalent width of the Mg II absorption is found to be greater for groups, compared to isolated galaxies. Additionally, we observe a significant redshift evolution in the physical extent of Mg II-absorbing gas around galaxies and evidence of an enhancement of Mg II within 50 degrees of the minor axis, characteristic of outflows, which persists to 80 kpc around the galaxies, in agreement with recent predictions from simulations.