The host galaxies of active galactic nuclei

Hubble Space Telescope A-band images of a sample of 10 radio galaxies (RG), 10 radio-loud quasars (RLQ) and 13 radio-quiet quasars (RQQ) at redshift z ≃ 0.2 are presented. The sources comprising the radio-loud sub-samples have been selected to be indistinguishable in the P₅Gₕ₂ ~ z plane. The two quasar sub-samples have been similarly selected to have indistinguishable distributions in the V — z plane. All three sub-samples have been analysed with two-dimensional modelling software, designed to accurately determine the host-galaxy parameters.The modelling results show all of the RG, and all of the quasars with Mᵥ < -23, to have bulge-dominated host galaxies, excluding the hypothesis that host morphology is responsible for the radio-loudness dichotomy. Furthermore, the host galaxies are found to be essentially identical to a de Vaucouleurs (ß = 0.25) law, with 30/33 objects having ß parameters in the range 0.2 < ß < 0.3. The hosts of all of the objects studied are found to be extremely luminous (L ≥ L*), with 25/33 objects having L ≥ 2L*, the hosts of the RQQs are found to be typically 0.4 magnitudes fainter than their radio-loud counterparts. The host galaxies of all three sub-samples are found to be larger than average ellipticals (= 10.5 kpc), with a trend found for the RQQ hosts to be some 20% smaller than those of the RLQs. For the first time it is demonstrated that the hosts of quasars, as well as RG, display a Kormendy relation basically identical to that of inactive ellipticals.In combination with the previous K-band modelling of this sample (Taylor et al. 1996) the first set of reliable optical-infrared colours for a significant sample of AGN host galaxies has been determined. The R - K colours of the hosts in all three xxxiii sub-samples are shown to be consistent with each other, with the close agreement of the median colour of R - K=2.5 with that expected from a ~ 14 Gyr-old stellar population implying that the host galaxies of both RGs and quasars are consistent with having formed at high redshift (z ≥ 3).The nuclear colours of the RGs are shown to be perfectly consistent with those of dust reddened RLQ nuclei. The combined luminosity, scalelength, Kormendy relation and R - K colour results for the RG and RLQ sub-samples clearly demonstrate that they are drawn from the same parent population, in strong support of the orientationbased unification scheme of Barthel (1989). The corresponding results for the full 33 object sample show that, in terms of their global structural parameters, the hosts of powerful AGN are no different to normal massive ellipticals.The host galaxy-black-hole mass correlations of Magorrian et al. (1998) and Franceschini et al. (1998) are combined with the AGN modelling results to investigate the quasar radio-loudness dichotomy. All methods of black-hole mass estimation lead to the conclusion that RLQs harbour black-holes of mass ≥ 10¹⁰M⨀. The RQQ blackhole masses estimated via the Magorrian relation are typically smaller than those of the RLQs (~ 5 x 10⁹M⨀), but show sufficient overlap to require the influence of another physical parameter, possibly black-hole spin, to explain the radio-loudness dichotomy. In contrast, the use of the Franceschini black-hole estimator predicts RQQ black-hole masses a factor of two smaller than those of the RLQs, suggesting that black-hole mass alone may be the crucial factor.The application of the modelling technique to an existing HST survey of 3CR radio galaxies at z ~ 1 shows them to have a scalelength distribution and Kormendy relation indistinguishable from their z ~ 0.2 counterparts. The implied luminosity evolution between z ~ 0.2 and z ~ 0.8 is shown to be consistent with that expected from passive evolution alone in a low density Universe, with a high galaxy formation redshift (z ≥ 3). The lack of evidence for significant merger activity in the redshift range 0.2 < z < 0.8 leads to the conclusion that the traditional passive evolution interpretation of the radio galaxy K - z relation remains acceptable