The environments of $z\leqslant0.3$ QSOs

Abstract

We have carried out an investigation of the galaxy environments of low redshift ($z<0.3$) QSOs by cross-correlating the positions on the sky of X-ray-selected QSOs/AGN identified in the {\it Einstein} Medium Sensitivity Survey (EMSS) with those of $B_J<20.5$ galaxies in the APM galaxy catalogues. At $<5\,$arcmin, we find a significant ($5\sigma$) galaxy excess around $z<0.3$ QSOs. The amplitude of the low redshift ($z<0.3$) QSO-galaxy angular cross-correlation function is identical to that of the APM galaxy angular correlation function, implying that these (predominantly radio-quiet) QSOs inhabit environments similar to those of normal galaxies. No significant galaxy excess was found around a `control' sample of $z>0.3$ QSOs. Coupled with previous observations, these results imply that the environment of radio-quiet QSOs undergoes little evolution over a wide range in redshift ($0 less than z less than 1.5$). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range. The similarity between QSO-galaxy clustering and galaxy-galaxy clustering also suggests that QSOs are unbiased with respect to galaxies and make useful tracers of large-scale structure in the Universe.