Infrared imaging of the host galaxies of radio-loud and radio-quiet quasars

We present initial results from the first deep near-infrared imaging study of the host galaxies of an extensive sample of low-redshift (z < 0.4) radio-loud and radio-quiet quasars. To minimize selection effects, the radio-loud and radio-quiet subsamples have been selected to be indistinguishable in terms of their distributions on the V–z plane, and classification as radio-loud or radio-quiet is defined in terms of 5-GHz luminosity. We discuss the problems associated with accurate determination of the infrared point spread function (PSF) and our adopted solution of an a posteriori selection from a ‘library’ of observationally determined PSFs. Images reaching surface brightness levels of $${\mu}_{K} = 22$$ mag arcsec^–2 of the quasar hosts, before and after subtraction of the nuclear component, are then presented. After removal of the nuclear component, the host galaxies of both classes of quasar are found to be good ‘standard candles’ at K, displaying a K–z relation essentially identical to that already established for radio galaxies. In addition, we find no evidence that radio-loud and radio-quiet hosts differ in terms of K-band luminosity; both appear to be selected solely from the top of the K-band galaxy luminosity function. We briefly compare the dimensions and near-infrared morphologies of the radio-loud and radio-quiet hosts and, in the case of radio-loud quasars, investigate the relationship between near-infrared and radio morphology. Finally, we give a preliminary discussion of the significance of the universal incidence of apparent companion objects in our images. A detailed analysis of the luminosity profiles and companion statistics derived from the images presented here will be the subject of subsequent papers.