THE LOCAL ENVIRONMENTS OF LOW-REDSHIFT QUASARS AND POWERFUL RADIO GALAXIES

We have studied the local environments (r < 100h-1 kpc) of 31 low-redshift (z < 0.3) QSOs and 35 powerful radio galaxies (PRGs - defined to have P178 > 1025 W Hz-1) using V and R CCD frames processed through FOCAS to generate catalogs of companion galaxies. We find that the PRGs and the radio-loud QSOs typically inhabit regions with local galaxy densities similar to normal giant ellipticals and 3-4 times less dense than the regions inhabited by lower power radio-loud elliptical galaxies. The radio-loud QSOs and PRGs have about twice the average number of bright (L > 0.2 L*), nearby (r < 100 kpc) companion galaxies (at the 95% confidence level) compared to the radio-quiet QSOs, with the respective averages being about two companions and one companion. The luminosity function of the companions of the radio-loud objects (PRGs and QSOs) is unusually flat at the high end (> 0.2 L*) compared to a standard Schechter function (a 99% confidence level difference), while the luminosity function of the companions to the radio-quiet QSOs is consistent with a Schechter function. The brightest companion galaxy is on-average about 2 mag less bright than the QSO host galaxy or PRG (reflecting the large optical luminosities of the QSO hosts and PRGs). Finally, we find no difference between the local environments of radio-loud QSOs and PRGs in terms of galaxy density or the shape of the companion galaxy luminosity function. These results have some interesting implications for theories of the origin of nuclear activity and the relationships between the various classes of active galaxies and QSOs. Radio-quiet and radio-loud QSOs must be intrinsically different polulations without any obvious evolutionary connection. This agrees with previously reported differences between the properties of the host galaxies. In contrast, the radio-loud QSOs and PRGs are likely to be closely related objects. They could be related in an evolutionary sense or might differ only in their geometric orientation with respect to the line of sight.