Galaxies Probing Galaxies in PRIMUS - I. Sample, Spectroscopy, and
  Characteristics of the z~0.5 MgII-Absorbing Circumgalactic Medium
  Spectroscopy of background QSO sightlines passing close to foreground
galaxies is a potent technique for studying the circumgalactic medium (CGM).
QSOs are effectively point sources, however, limiting their potential to
constrain the size of circumgalactic gaseous structures. Here we present the
first large Keck/LRIS and VLT/FORS2 spectroscopic survey of bright (B_AB <
22.3) background galaxies whose lines of sight probe MgII 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 (PRIMUS), includes 48 background galaxies which do
not host bright AGN, and both star-forming and quiescent foreground galaxies
with stellar masses 9.0 < log M_*/M_sun < 11.2 at redshifts 0.35 < z_f/g < 0.8.
We detect MgII absorption associated with these foreground galaxies with
equivalent widths 0.25 Ang < W_2796 < 2.6 Ang at >2sigma significance in 20
individual background sightlines passing within R_perp < 50 kpc, and place
2sigma upper limits on W_2796 of <0.5 Ang in an additional 11 close sightlines.
Within R_perp < 50 kpc, W_2796 is anticorrelated with R_perp, consistent with
analyses of MgII absorption detected along background QSO sightlines.
Subsamples of these foreground hosts divided at log M_*/M_sun = 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 Ang.
Finally, we demonstrate that foreground galaxies with similar stellar masses
exhibit the same median W_2796 at a given R_perp to within <0.2 Ang toward both
background galaxies and toward QSO sightlines drawn from the literature.
Analysis of these datasets constraining the spatial coherence scale of
circumgalactic MgII absorption is presented in a companion paper.
