On the nature of QSO host galaxies

Abstract

We present the results of an analysis of deep images of 31 low-redshift (z < 0.3) QSOs and lower luminosity QSO/AGNs obtained with the prime focus CCD system on the CTIO 4 m telescope. We have used two image-modeling programs to deconvolve the QSO from its host galaxy and thereby derive the absolute magnitudes , isophotal radii, and (in 12 cases) the morphological type (disk or elliptical) of the hosts. The derived absolute magnitudes of the hosts have been statistically corrected for the presence of spatially resolved emission-line nebulosity. Combining our data with other corrected data in the literature on 58 additional host galaxies yields the following results. First, we find that the host galaxies of QSOs are not drawn randomly from the general population of galaxies but are instead inhabitants of the "exponential tail" of the Schechter luminosity function. No plausible selection effects to explain this result have been uncovered, especially since unresolved QSOs constitute only ~ 10% of the sample. We interpret this result as implying that highly luminous (massive?) galaxies are particularly adept at hosting a QSO, or that the QSO phenomenon may be accompanied by a global "burst" of star formation. These interpretations have important implications for cosmic evolution of QSOs and galaxies and for the search for "dead QSOs" in galaxies at the present epoch. Second, we find that the radio-loud QSO host galaxies are ~0.7-0.8 mag more luminous on average than radio-quiet QSO hosts. A trend for elliptical (disk) models to be preferred for the radio-loud (radio-quiet) QSO and QSO/QGN hosts was also found. These results strengthen the basis for associating radio-loud (radio-quiet) QSOs with radio (Seyfert) galaxies. Third, we find that about half the QSOs and QSO/AGNs we have imaged are hosted by morphologically peculiar galaxies. These results suggest that galaxy interactions may be an important trigger of QSO activity.