A Complete Census of Circumgalactic MgII at Redshift z<~ 0.5
  We present a survey of MgII absorbing gas in the vicinity of 380 random
galaxies, using 156 background quasi-stellar objects(QSOs) as absorption-line
probes. The sample comprises 211 isolated (73 quiescent and 138 star-forming
galaxies) and 43 non-isolated galaxies with sensitive constraints for both MgII
absorption and Ha emission. The projected distances span a range from d=9 to
497 kpc, redshifts of the galaxies range from z=0.10 to 0.48, and rest-frame
absolute B-band magnitudes range from $M_{\rm B}=-16.7$ to $-22.8$. Our
analysis shows that the rest-frame equivalent width of MgII, $W_r$(2796),
depends on halo radius($R_h$), $B$-band luminosity($L_{\rm B}$) and stellar
mass ($M_{\rm star}$) of the host galaxies, and declines steeply with
increasing $d$ for isolated, star-forming galaxies. $W_r$(2796) exhibits no
clear trend for either isolated, quiescent galaxies or non-isolated galaxies.
The covering fraction of MgII absorbing gas $\langle \kappa \rangle$ is high
with $\langle \kappa \rangle\gtrsim 60$% at $<40$ kpc for isolated galaxies and
declines rapidly to $\langle \kappa \rangle\approx 0$ at $d\gtrsim100$ kpc.
Within the gaseous radius, $\langle \kappa \rangle$ depends sensitively on both
$M_{\rm star}$ and the specific star formation rate inferred from Ha. Different
from massive quiescent halos, the observed velocity dispersion of MgII gas
around star-forming galaxies is consistent with expectations from virial
motion, which constrains individual clump mass to $m_{\rm cl} \gtrsim 10^5
\,\rm M_\odot$ and cool gas accretion rate of $\sim 0.7-2 \,M_\odot\,\rm
yr^{-1}$. We find no strong azimuthal dependence of MgII absorption for either
star-forming or quiescent galaxies. Our results highlight the need of a
homogeneous, absorption-blind sample for establishing a holistic description of
chemically-enriched gas in the circumgalactic space.
