Evolutionary Paths for Galaxies and AGNs: New Insights by the Spitzer Space Telescope
We compare the history of the galaxy mass build-up, as inferred from near-IR observations, and the Star Formation Rate of massive stars in the comoving volume traced by deep extensive far-IR surveys, both possible now with the Spitzer Space Telescope. These two independent and complementary approaches to the history of galaxy formation consistently indicate that a wide interval of cosmic epochs between z~0.7 to z~2 brackets the main evolutionary phases. The rate of the integrated galaxy mass growth indicated by the IR-based comoving SFR appears consistent with the observed decrease of the stellar mass densities with redshift. There are also indications that the evolution with z of the total population depends on galaxy mass, being stronger for moderate-mass, but almost absent up to z=1.4 for high-mass galaxies, thus confirming previous evidence for a "downsizing" effect in galaxy formation. The most massive galaxies appear already mostly in place by z~1. Although a precise matching of this galaxy build-up with the growth of nuclear super-massive black-holes is not possible with the present data (due to difficulties for an accurate census of the obscured AGN phenomenon), some preliminary indications reveal a similar mass/luminosity dependence for AGN evolution as for the hosting galaxies.