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 $10text{ kpc}<d<100text{ kpc}$. 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 $3.7sigma$ 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.
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