Quasars Have Fewer Close Companions than Normal Galaxies
  We investigate the distribution of companion galaxies around quasars using
{\em Hubble Space Telescope} ({\em HST}) Advanced Camera for Surveys Wide Field
Camera (ACS/WFC) archival images. Our master sample contains 532 quasars which
have been observed by {\em HST} ACS/WFC, spanning a wide range of luminosity
$(-31<M_i(z=2)<-23)$ and redshift ($0.3<z<3$). We search for companions around
the quasars with projected distance of $10\text{ kpc}<d<100\text{ kpc}$. PSF
subtraction is performed to enhance the completeness for close companions. The
completeness is estimated to be high $(>90\%)$ even for the faintest companions
of interest. The number of physical companions is estimated by subtracting a
background density from the number density of projected companions. We divide
all the companions into three groups (faint, intermediate and bright) according
to their fluxes. A control sample of galaxies is constructed to have similar
redshift distribution and stellar mass range as the quasar sample using the
data from {\em HST} deep fields. We find that quasars and control sample
galaxies have similar numbers of faint and bright companions, while quasars
show a $3.7\sigma$ deficit of intermediate companions compared to galaxies. The
numbers of companions in all three groups do not show strong evolution with
redshift, and the number of intermediate companions around quasars decreases
with quasar luminosity. Assuming that merger-triggered quasars have entered the
final coalescence stage during which individual companions are no longer
detectable at large separations, our result is consistent with a picture in
which a significant fraction of quasars is triggered by mergers.
