The Red Dead Redemption Survey of Circumgalactic Gas About Massive
  Galaxies. I. Mass and Metallicity of the Cool Phase
  We present a search for HI in the circumgalactic medium (CGM) of 21 massive
($\langle \log M_\star \rangle \sim 11.4$), luminous red galaxies (LRGs) at
$z\sim0.5$. Using UV spectroscopy of QSO sightlines projected within 500 kpc
($\sim R_{vir}$) of these galaxies, we detect HI absorption in 11/21
sightlines, including two partial Lyman limit systems and two Lyman limit
systems. The covering factor of $\log N(HI) \ge 16.0$ gas within the virial
radius of these LRGs is $f_c(\rho \le R_{vir}) = 0.27^{+0.11}_{-0.10}$, while
for optically-thick gas ($\log N(HI) \ge 17.2$) it is $f_c(\rho \le R_{vir}) =
0.15^{+0.10}_{-0.07}$. Combining this sample of massive galaxies with previous
galaxy-selected CGM studies, we find no strong dependence of the HI covering
factor on galaxy mass, although star-forming galaxies show marginally higher
covering factors. There is no evidence for a critical mass above which dense,
cold ($T \sim 10^4$ K) gas is suppressed in the CGM of galaxies (spanning
stellar masses $9.5 \lesssim \log M_\star \lesssim 11.8$). The metallicity
distribution in LRGs is indistinguishable from those found about lower-mass
star-forming galaxies, and we find low-metallicity gas with $[{\rm X/H}]
\approx -1.8$ (1.5% solar) and below about massive galaxies. About half the
cases show super-solar [FeII/MgII] abundances as seen previously in cool gas
near massive galaxies. While the high-metallicity cold gas seen in LRGs could
plausibly result from condensation from a corona, the low-metallicity gas is
inconsistent with this interpretation.
