Cosmic evolution of quasar host galaxies

The quasar phenomenon occurs in the nuclei of massive galaxies and exhibits a strong evolution with cosmic time. The engine for their huge energy is believed to be a supermassive black hole. Similar massive but inactive black holes have been detected in nearby early-type galaxies, suggesting that many normal (inactive) galaxies may have had nuclear activity (e.g. quasars) at an early stage of their life, for a relatively short period. Luminous quasars may therefore trace distant massive spheroidal galaxies. Studying the link between distant quasars and their host galaxies from the peak of quasar activity to the present epoch thus yields fundamental clues for understanding the formation and evolution of both galaxies and their black holes.

Using the European Southern Observatory's (ESO) Very Large Telescope (VLT) and its high resolution ISAAC near-infrared imager, a team lead by Jari Kotilainen at Tuorla Observatory has observed a sample of 16 quasar host galaxies at the epoch around the peak of the quasar activity (2 < z < 3). Comparison of the new data with those in literature for lower redshift hosts, allowed to trace the cosmological luminosity evolution of the host galaxies up to z = 3. The luminosity trend of quasar hosts with cosmic epoch, where the host luminosity increases by ~1.5 mag from the present epoch up to z ~3, remarkably resembles that observed for massive inactive galaxies, suggesting similar star formation history (see Figure). In particular, both quasar host galaxies and massive inactive galaxies are fully assembled already at the peak age of the quasar activity. Since this epoch they follow luminosity evolution that is consistent with a passively evolving stellar population generated in a burst at very high redshift. The predominance of an old, evolved stellar population is also indicated by spectroscopic studies of low redshift quasars. The inferred picture is that there is no significant decrease in mass at early epochs. This situation is dramatically different from less massive galaxies which have substantial mass growth by dry merging at all epochs. This result is of key importance for testing the models of joint formation and evolution of galaxies and their active nuclei.

The luminosity evolution of the host galaxies of quasars. Circles are median averages at z = 0.5, 1.5, and 2.5, while the dots refer to individual hosts. The upper panel shows the number of targets per bin. Solid lines refer to the model evolution of a passively evolving stellar population, where the redshift of the burst is z=3, z=5 and z=10, from top to bottom. The shaded area represents the evolution of the knee of inactive galaxy luminosity function, extending from M* to M*-1.