A NEAR-IR STUDY OF THE HOST GALAXIES OF RADIO-QUIET QUASARS, RADIO-LOUD QUASARS AND RADIO GALAXIES

We present the results of a detailed analysis of the host galaxies of powerful active galactic nuclei (AGN) as revealed by deep near-infrare d imaging. New K-band images have been obtained of a sample of radio g alaxies (RGs) selected to match the radio-loud quasar (RLQ) and radio- quiet quasar (RQQ) samples for which comparable images were presented by Dunlop et al. The images of all three samples have been analysed us ing detailed 2-dimensional modelling to extract the parameters of the host galaxies. The low nuclear:host ratios displayed by quasars at 2.2 mu m, combined with the consistent depth of our images, have enabled us to determine reliably the luminosities, scalelengths and even morph ological types of the vast majority of the host galaxies. In fact, tes ts of the repeatability of our galaxy model fitting with increasing nu clear:host ratio indicate that, with ground-based seeing, it is only a t near-infrared wavelengths that the properties of quasar host galaxie s can be determined with acceptable reliability. The statistical match ing of our samples, combined with consistent application of the same m odelling procedure to all sources, has enabled us to perform the most reliable comparison of the host galaxies of RQQs, RLQs and RGs underta ken to date. The picture that emerges is one of relative uniformity, i n contrast to the results of severer optical studies. We find that the host galaxies of all three classes of AGN are large luminous galaxies with, on average, very similar luminosities. All of the host galaxies have a K-band luminosity L greater than or equal to L and all have a half-light radius r(1/2)greater than or equal to 10 kpc. In addition, the host galaxies of all three classes of AGN display a mu(1/2)-r(1/2 ) relation identical in both slope and normalization to that displayed by brightest cluster galaxies. With regard to morphological type, we find that essentially all the radio galaxies and RLQ hosts are best de scribed by a de Vaucouleurs r(1/4) law, but that slightly more than ha lf of the RQQs appear to lie in galaxies which are dominated by an exp onential disc. Those RQQs which have elliptical hosts are in general m ore luminous than those which reside in discs. Our results provide cle ar support for the unification of RLQs and RGs via orientation, and su ggest that a significant fraction of the RQQ population may at least b e capable of producing powerful radio emission. Finally, our modelling indicates that the majority of the radio galaxies in our sample conta in nuclear flux at IC in excess of that expected from the best-fitting r(1/4)- law model. These unresolved components may simply be indicati ve of central cusps in the starlight, but their colours and magnitudes appear consistent with dust-reddened quasars. Removal of these nuclea r contributions shifts the low-redshift end of the radio galaxy K-z re lation faintward by 0.5 mag, a result which has implications for the a ssessment of cosmological evolution via the K-z relation for radio gal axies.