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

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 lambda lambda 2796, 2803 absorption from the CGM around close projected foreground galaxies at transverse distances 10 kpc < R-perpendicular to < 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 < logM(*)/M-circle dot < 11.2 at redshifts of 0.35< zf/g < 0.8. We detectMgII absorption associated with these foreground galaxies with equivalent widths of 0.25 angstrom < W-2796 < 2.6 angstrom at > 2 sigma significance in 20 individual background sightlines passing within R-perpendicular to < 50 kpc and place 2s upper limits on W-2796 of. less than or similar to 0.5 angstrom in an additional 11 close sightlines. Within R-perpendicular to < 50 kpc, W2796 is anticorrelated with R., consistent with analyses of Mg II absorption detected along background QSO sightlines. Subsamples of these foreground hosts divided at logM(*)/M-circle dot = 9.9 exhibit statistically inconsistent W2796 distributions at 30 kpc < R-perpendicular to < 50 kpc, with the higher-M-* galaxies yielding a larger median W-2796 by 0.9 angstrom. Finally, we demonstrate that foreground galaxies with similar stellar masses exhibit the same median W-2796 at a given R. to within < 0.2 angstrom 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.