Luminous Quasars in Luminous Early-Type Host Galaxies

Some recent observations of nearby quasars with Hubble Space Telescope ( HST ) have apparently failed to detect host galaxies. We review the HST observations as well as near-infrared ground-based observations of the same objects. We find that the quasar hosts can be detected in the HST data if they are smoothed sufficiently to reveal low surface brightness. The smooth hosts are very difficult to detect with HST but are more easily visible in the deeper, ground-based IR images. The V - H colors obtained by combining the HST and IR data are compatible with normal galaxy colors at the redshifts of the quasars. This behavior can be explained if the hosts are massive early-type galaxies. All together, HST images have been reported for 15 luminous quasars, ~13 of which have smooth early-type hosts. This kind of galaxy therefore appears to be the most common host for a luminous quasar.