The Cosmic Evolution of Quasar Host Galaxies

We present the results of a near-infrared imaging study of the host galaxies of 17 quasars in the redshift range 1 < z < 2. The observations were carried out at the ESO VLT UT1 8 m telescope under excellent seeing conditions (~0.″4). The sample includes radio-loud (RLQs) and radio-quiet (RQQs) quasars with similar distribution of redshift and optical luminosity. For all the observed objects but one we have been able to derive the global properties of the surrounding nebulosity. The host galaxies of both types of quasars appear to follow the expected trend in luminosity of massive ellipticals undergoing simple passive evolution. However, we find a systematic difference by a factor ~2 in the host luminosity between RLQs and RQQs [RLQ (host) = -27.55 ± 0.12 and RLQ (host) = -26.83 ± 0.25]. Comparison with other samples of quasar hosts at similar and lower redshift indicates that the difference in the host luminosity between RLQs and RQQs remains the same from z = 2 to the present epoch. No significant correlation is found between the nuclear and the host luminosities. Assuming that the host luminosity is proportional to the black hole mass, as observed in nearby massive spheroids, these quasars emit at very different levels (spread ~1.5 dex) with respect to their Eddington luminosity and with the same distribution for RLQs and RQQs. Apart from a factor of ~2 difference in luminosity, the hosts of RLQs and RQQs of comparable nuclear luminosity appear to follow the same cosmic evolution as massive inactive spheroids. Taken together, our results support a view where nuclear activity can occur in all luminous ellipticals without producing a significant change in their global properties and evolution. Quasar hosts appear to be already well formed at z ~ 2, in disagreement with the predictions of models for the joint formation and evolution of galaxies and active nuclei based on the hierarchical structure formation scenario.