On the origin of excess cool gas in quasar host halos
  Previous observations of quasar host halos at z=2 have uncovered large
quantities of cool gas that exceed what is found around inactive galaxies of
both lower and higher masses. To better understand the source of this excess
cool gas, we compiled an exhaustive sample of 195 quasars at z=1 with
constraints on chemically enriched, cool gas traced by MgII absorption in
background quasar spectra from the Sloan Digital Sky Survey. This quasar sample
spans a broad range of luminosities from Lbol=10^44.4 to 10^46.8 erg/s and
allows an investigation of whether halo gas properties are connected with
quasar properties. We find a strong correlation between luminosity and cool gas
covering fraction. In particular, low-luminosity quasars exhibit a mean gas
covering fraction comparable to inactive galaxies of similar masses, but more
luminous quasars exhibit excess cool gas approaching what is reported
previously at z=2. Moreover, 30-40% of the MgII absorption occurs at radial
velocities of |v|>300 km/s from the quasar, inconsistent with gas bound to a
typical quasar host halo. The large velocity offsets and observed luminosity
dependence of the cool gas near quasars can be explained if the gas arises
from: (1) neighboring halos correlated through large-scale structure at Mpc
scales, (2) feedback from luminous quasars, or (3) debris from the mergers
thought to trigger luminous quasars. The first of these scenarios is in tension
with the lack of correlation between quasar luminosity and clustering while the
latter two make distinct predictions that can be tested with additional
observations.
