  We report a definitive detection of chemically-enriched cool gas around
massive, quiescent galaxies at z~0.4-0.7. The result is based on a survey of
37621 luminous red galaxy (LRG)-QSO pairs in SDSS DR12 with projected distance
d<500 kpc. The LRGs are characterized by a predominantly old (age>~1Gyr)
stellar population with 13% displaying [OII] emission features and LINER-like
spectra. Both passive and [OII]-emitting LRGs share the same stellar mass
distribution with a mean of <log(M*/Msun)>~11.4 and a dispersion of 0.2 dex.
Both LRG populations exhibit associated strong MgII absorbers out to d<500 kpc.
The mean gas covering fraction at d<~120 kpc is <kappa>_MgII > 15% and declines
quickly to <kappa>_MgII ~ 5% at d<~500 kpc. No clear dependence on stellar mass
is detected for the observed MgII absorption properties. The observed velocity
dispersion of MgII absorbing gas relative to either passive or [OII]-emitting
LRGs is merely 60% of what is expected from virial motion in these massive
halos. While no apparent azimuthal dependence is seen for <kappa>_MgII around
passive LRGs at all radii, a modest enhancement in <kappa>_MgII is detected
along the major axis of [OII]-emitting LRGs at d<50 kpc. The suppressed
velocity dispersion of MgII absorbing gas around both passive and
[OII]-emitting LRGs, together with an elevated <kappa>_MgII along the major
axis of [OII]-emitting LRGs at d<50 kpc, provides important insights into the
origin of the observed chemically-enriched cool gas in LRG halos. We consider
different scenarios and conclude that the observed MgII absorbers around LRGs
are best-explained by a combination of cool clouds formed in thermally unstable
LRG halos and satellite accretion through filaments.

Astrophysics of Galaxies