AGNPROBES · Active Galactic Nuclei as probes of galaxy evolution

Active Galactic Nuclei as probes of galaxy evolution

One of the most important questions facing modern astrophysics is the question of how galaxies form and evolve. This project addresses this question by investigating how black hole mass and stellar mass scale in a particular type of galaxies where the central supermassive black hole is still actively accreting matter. Galaxies with actively accreting supermassive black holes are picked especially for their ability to act as beacons, allowing us to probe further back into the history of galaxy formation. The discovery that the mass of the black hole ("M") in galaxy centres scales with the velocity dispersion ("sigma") of the stars in the galaxy bulge (the central "hub" of a galaxy) strongly suggests that there is a causal link between the bulge and the supermassive black hole: physical processes on the scale of 1/10,000 parsecs are related to bulk properties of the host galaxy on much larger scales of 100-1000 parsecs. Probably every galaxy in the Universe contains a supermassive (a million to a billion solar masses) black hole, and in many cases the black hole is dormant and does not reveal any information about its physical properties. Some galaxies contain actively accreting black holes, called Active Galactic Nuclei (AGN), and it is through this activity that we can obtain physical information about black holes. Galaxies with actively accreting black holes are therefore objects appropriate for such studies. This project uses AGN to study the relation between black holes and bulges in two different types of AGN, red quasars and post-starburst quasars. These two types of AGN were selected because they are obsured by dust. The dust blocks out some of the bright quasar light at wavelengths that are important for studying the bulge, thus allowing us to study both black hole mass and stellar velocity dispersion simultaneously and directly. Red quasars and post-starburst quasars are also galaxies where there is recent or on-going star formation building up the stars in the bulge. The project therefore aims to address the relation between the mass of the black hole mass and the bulge mass (i.e. stellar velocity dispersion) in these galaxies. For the sample of red quasars we found that there is an offset between the quasars and local galaxies in the sense that the red quasars have lower velocity dispersion and/or more massive black holes that local galaxies. Our sample has a redshift range 0.14 < z < 0.37, and the result is in agreement with other, similar studies at comparable redshifts. This could indicate an unusually rapid growth in the host galaxies since z ~ 0.2, if these objects develop to become like the galaxies on the relation in the local universe. A direct comparison might not be straightforward, however, since the z>0.1 AGN have significantly higher black hole masses than those of local AGN. Furthermore, using several samples of local and higher-z AGNs, we find a striking trend of an increasing offset with respect to the local M-sigma relation as a function of AGN luminosity, with virtually all high-luminosity objects above the relation. Given the relatively small number of AGNs at z > 0.1 for which there are direct measurements of stellar velocity dispersions, it is impossible at present to determine whether there truly is evolution in the M-sigma relation with redshift. Larger, carefully selected samples of AGNs are necessary to disentangle the dependence on mass, luminosity, accretion rates, and redshift. The relation between black hole mass and host galaxy properties was also studied for a of post-starburst quasars at z~0.3. Spectra of these objects simultaneously display features from the active nucleus including broad emission lines and a host galaxy Balmer absorption series indicative of the post-starburst stellar population. These are the first measurements of stellar velocity dispersion in such objects, and we have therefore significantly increased the number of directly measured non-local objects on the M-sigma diagram. The post-starburst quasars in our sample fall on or above the locally defined M-sigma relation, a result that is consistent with previous studies of samples at z > 0.1. However, they are generally consistent with the black hole mass - bulge luminosity relation. Furthermore, their location on the Faber-Jackson relation suggests that some of the bulges may be dynamically peculiar. The outcome of this project has contributed toward a better understanding of the dynamical evolution and growth of galaxies in the Universe, as well as to the co-evolution of black holes and stellar bulges. In particular the study has pushed the enquires to higher redshifts, and thus laid the ground for further work utilizing AGN activity to address the M-sigma relation at earlier times in the history of the universe.

Data: CORDIS, © European Union