Quasar host galaxies
Accretion onto a central black hole now appears to be a common phase in the evolution of normal galaxies. Furthermore, super massive black holes (SMBH) may well have a period of maximum growth (maximum nuclear luminosity) contemporaneous with the bulk of the initial star formation in the bulge. This has led to a renewed interest in AGN properties, especially regarding the host galaxies. Studies of the co-evolution of SMBH and their host spheroids are obviously critical to understanding how and when galaxies in the local Universe formed and evolved.
Comparisons of host galaxies of AGN at high and low redshift constrain host galaxy evolution, which can then be compared to the evolution of normal galaxies.
The QSO phenomenon occurs in the nuclei of massive galaxies and exhibits a strong evolution with the cosmic time. The central engine for the huge energy emission is believed to be a supermassive black hole. Similar massive but inactive black holes have been also detected in nearby early type galaxies. These findings suggest that many normal (inactive) galaxies may have hosted nuclear activity (e.g. quasars) at an early stage of their life and for a relatively short period of time. Studying the link between quasars and their host galaxies may thus yield fundamental clues for understanding the processes of formation and evolution of both the active nuclei and the galaxies. Physical insight on the evolution of spheroidal galaxies and black holes with cosmic epoch can be obtained by the characterization of high redshift quasar hosts, assuming that the relation between the central black hole mass and the mass of the host galaxy remains valid at high redshifts.
Luminous active nuclei may in fact trace the massive spheroidal galaxies at high redshift. The study of their hosts offers an unique way to determine how the luminosity, scale-length and morphology of massive galaxies vary between the peak of quasar activity (at z = 2 - 3) and the present epoch. These studies are carried out at OAPd using VLT high resolution near-IR imaging (see example in Figure 1) and also with Adaptive Optics. An example of these results is shown in Figure 2 where the cosmic evolution of RLQ hosts is given for objects up to z = 3.
Altogether these observations describe a general trend where the host luminosity increases by ∼ 1.5 mag from present epoch up to z ∼ 3. On average, this trend is consistent with that of galaxies of luminosity ∼5L* undergoing passive stellar evolution. The predominance of an old, evolved stellar population is also indicated by spectroscopic studies of low redshift quasars. Remarkably, both RG and RLQ hosts follow a similar trend of the luminosity up to redshift z ∼ 3. This is suggestive of a common origin of the parent galaxies, it also shows both types of radio loud galaxies follow the same evolutionary trend of inactive massive spheroids. The available data indicate therefore that QSO host galaxies are already well formed at z ∼ 3. Since this epoch they follow a luminosity evolution consistent with stellar passive evolution. This does not exclude that in some cases episodes of new star formation may occur. The inferred picture, however, is that there is no decrease in mass at early epochs up to z ∼ 3.
Full discussion of these results is given in Falomo et al (2008), Kotilainen et al. (2009).
People: Renato Falomo
Collaboration: A. Treves (Insubria), J. Kotilainen (Tuorla,FIN), R. Scarpa (GTC)
Publications: Kotilainen et al. (2007), ApJ 660,1039; Hyvönen et al. (2007), A&A 476,723;
Falomo et al. (2008),ApJ 673,694; Kotilainen et al. (2009), ApJ