Mining Circumgalactic Baryons in the Low-Redshift Universe
  (Abridged) This paper presents an absorption-line study of the multiphase
circumgalactic medium (CGM) based on observations of Lya, CII, CIV, SiII,
SiIII, and SiIV absorption transitions in the vicinities of 195 galaxies at
redshift z<0.176. The galaxy sample is established based on a cross-comparison
between public galaxy and QSO survey data and is characterized by a median
redshift of <z>=0.041, a median projected distance of <d>=362 kpc to the
sightline of the background QSO, and a median stellar mass of log(M_star/M_sun)
= 9.7 \pm 1.1. Comparing the absorber features identified in the QSO apectra
with known galaxy properties has led to strong constraints for the CGM
absorption properties at z<~0.176. First, abundant hydrogen gas is observed out
to d~500 kpc, well beyond the dark matter halo radius Rh of individual
galaxies, with a mean covering fraction of ~60%. In contrast, no heavy elements
are detected at d>~0.7 Rh from either low-mass dwarfs or high-mass galaxies.
The lack of detected heavy elements in low- and high-ionization states suggests
that either there exists a chemical enrichment edge at d~0.7 Rh or gaseous
clumps giving rise to the observed absorption lines cannot survive at these
large distances. Considering all galaxies at d>Rh leads to a strict upper limit
for the covering fraction of heavy elements of ~3% (at a 95% confidence level)
over d=(1-9) Rh. At d<Rh, differential covering fraction between low- and
high-ionization gas is observed, suggesting that the CGM becomes progressively
more ionized from d<0.3 Rh to larger distances. Comparing CGM absorption
observations at low and high redshifts shows that at a fixed-fraction of Rh the
CGM exhibits stronger mean absorption at z=2.2 than at z~0. We discuss possible
pseudo-evolution of the CGM as a result of misrepresentation of halo radius.
