AGN host galaxy mass function in COSMOS: is AGN feedback responsible for
  the mass-quenching of galaxies?
  We investigate the role of supermassive black holes in the global context of
galaxy evolution by measuring the host galaxy stellar mass function (HGMF) and
the specific accretion rate i.e., lambda_SAR, distribution function (SARDF) up
to z~2.5 with ~1000 X-ray selected AGN from XMM-COSMOS. Using a maximum
likelihood approach, we jointly fit the stellar mass function and specific
accretion rate distribution function, with the X-ray luminosity function as an
additional constraint. Our best fit model characterizes the SARDF as a double
power-law with mass dependent but redshift independent break whose low
lambda_SAR slope flattens with increasing redshift while the normalization
increases. This implies that, for a given stellar mass, higher lambda_SAR
objects have a peak in their space density at earlier epoch compared to the
lower lambda_SAR ones, following and mimicking the well known AGN cosmic
downsizing as observed in the AGN luminosity function. The mass function of
active galaxies is described by a Schechter function with a almost constant
Mstar* and a low mass slope alpha that flattens with redshift. Compared to the
stellar mass function, we find that the HGMF has a similar shape and that, up
to log((Mstar/Msun)~11.5 the ratio of AGN host galaxies to star forming
galaxies is basically constant (~10%). Finally, the comparison of the AGN HGMF
for different luminosity and specific accretion rate sub-classes with the
phenomenological model prediction by Peng et al. (2010) for the "transient"
population, i.e. galaxies in the process of being mass-quenched, reveals that
low-luminosity AGN do not appear to be able to contribute significantly to the
quenching and that at least at high masses, i.e. Mstar>10^(10.7) Msun ,
feedback from luminous AGN (log(Lbol)>~46 [erg/s]) may be responsible for the
quenching of star formation in the host galaxy.
