The environment of QSOs (invited review)

The evidence for the association of QSOs with groups of galaxies and the available data on the global properties of such groups are reviewed. At least for redshifts less than 0.5, QSOs tend to occur in richer-than-average environments. There is some evidence that groups around steep-radio-spectrum QSOs are systematically richer than those around radio-quiet and flat-radiospectrum QSOs. The groups associated with the former may sometimes reach central galaxy number densities similar to those of moderately rich clusters, but are significantly more compact, and there is no evidence that any such groups have velocity dispersions above a few hundred km s^–1.A possible explanation for this environmental preference can be found in the recent evidence, from a variety of directions, that galaxy interactions play an important role in triggering nuclear activity. A significant fraction of low-redshift QSOs either have close companions at the same redshift, or show some indication of recent interaction. The results of a recent search for resolvable extended emission around a large sample of luminous low-redshift QSOs are described. There are now substantial reasons for believing that the presence of these extended emission-line regions, which are found around approximately 25% of all low-redshift OSOs, is often a signature of a recent interaction.Observed correlations between radio properties, optical spectra, and environment are mentioned briefly. The present evidence tends to support suggestions that there are two classes of QSOs, one being the extension of the Seyfert population to higher luminosities and having spiral host galaxies, the other being closely related to the broad-line radio galaxies. The host galaxies of this latter class appear often to be morphologically ellipticals, but may in fact be products of recent mergers.Paper presented at the Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.