Cosmic evolution of galaxies

Executive summary

Today there is increasing evidence that the evolution of massive galaxies is linked to their nuclear activity. This evidence is supported by: 1) the similarities between the evolution of the QSO luminosity density and the star formation rate; 2) the tight correlation between the black hole mass and the bulge stellar velocity dispersion.

These results imply that the creation and evolution of a BH (presumably associated to a QSO) is intimately connected to that of the galaxy bulge. The hierarchical galaxy evolution models can explain these results as a consequence of the galaxy merger processes that drive the gas into the nuclear regions and trigger both star formation and AGN activity. However, some additional ingredients need to be considered in the formation and evolution of galaxies, in particular if we want to understand the "downsizing effect". In fact, the feedback processes and outflows from the starbursts and the AGNs can have a strong effect in the evolution of galaxies. Outflows can drive hot ionized gas out of the central regions of the galaxies and effectively quench star formation. If so, outflows can be the key ingredient in the hierarchical models to explain why the most massive galaxies seem to be formed at high-redshift and they have the oldest stellar population.