The environments of $z\leqslant0.3$ QSOs

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