The environments of $z\leqslant0.3$ QSOs

We have carried out an investigation of the galaxy environments of low-redshift ($$z\leqslant0.3$$) QSOs by cross-correlating the positions on the sky of X-ray-selected QSOs/AGNs identified in the 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 (5σ) galaxy excess around $$z\leqslant0.3$$ QSOs. The amplitude of the low-redshift ($$z\leqslant0.3$$) 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 $$z\gt0.3$$ 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 < z < 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 thai QSOs are unbiased with respect to galaxies and make useful tracers of large-scale structure in the Universe.