Galaxies Probing Galaxies in PRIMUS. I. Sample, Spectroscopy, and Characteristics of the z\sim 0.5 Mg II-absorbing Circumgalactic Medium

Abstract

The spectroscopy of background QSO sightlines passing close to foreground galaxies is a potent technique for studying the circumgalactic medium (CGM). However, QSOs are effectively point sources, limiting their potential to constrain the size of circumgalactic gaseous structures. Here we present the first large Keck/Low-resolution Imaging Spectrometer (LRIS) and Very Large Telescope (VLT)/Focal Reducer/Low-dispersion Spectrograph 2 (FORS2) spectroscopic survey of bright ({B}_{AB}< 22.3) background galaxies whose lines of sight probe Mg II λ λ 2796,2803 absorption from the CGM around close projected foreground galaxies at transverse distances 10 {kpc}< {R}_\perp < 150 {kpc}. Our sample of 72 projected pairs, drawn from the PRIsm MUlti-object Survey, includes 48 background galaxies that do not host bright active galactic nuclei, and both star-forming and quiescent foreground galaxies with stellar masses of 9.0< {log}{M}_* /{M}_⊙ < 11.2 at redshifts of 0.35< {z}_{{f}/{{g}}}< 0.8. We detect Mg II absorption associated with these foreground galaxies with equivalent widths of 0.25 \mathring{{A}} < {W}₂₇₉₆< 2.6 \mathring{{A}} at > 2σ significance in 20 individual background sightlines passing within {R}_\perp < 50 {kpc} and place 2σ upper limits on W ₂₇₉₆ of ≲ 0.5 \mathring{{A}} in an additional 11 close sightlines. Within {R}_\perp < 50 {kpc}, W ₂₇₉₆ is anticorrelated with R _⊥, consistent with analyses of Mg II absorption detected along background QSO sightlines. Subsamples of these foreground hosts divided at {log}{M}_* /{M}_⊙ =9.9 exhibit statistically inconsistent W ₂₇₉₆ distributions at 30 {kpc}< {R}_\perp < 50 {kpc}, with the higher-M _* galaxies yielding a larger median W ₂₇₉₆ by 0.9 \mathring{{A}} . Finally, we demonstrate that foreground galaxies with similar stellar masses exhibit the same median W ₂₇₉₆ at a given R _⊥ to within < 0.2 \mathring{{A}} toward both background galaxies and toward QSO sightlines drawn from the literature. Analysis of these data sets constraining the spatial coherence scale of circumgalactic Mg II absorption is presented in a companion paper.