GOODS-Herschel: The far-infrared view of star formation in AGN host
  galaxies since z~3
  We present a study of the infrared properties of X-ray selected, moderate
luminosity (Lx=10^{42}-10^{44}ergs/s) active galactic nuclei (AGNs) up to z~3,
to explore the links between star formation in galaxies and accretion onto
their central black holes. We use 100um and 160um fluxes from GOODS-Herschel
-the deepest survey yet undertaken by the Herschel telescope- and show that in
>94 per cent of cases these fluxes are dominated by the host. We find no
evidence of any correlation between the X-ray and infrared luminosities of
moderate AGNs at any redshift, suggesting that star-formation is decoupled from
nuclear (AGN) activity. The star formation rates of AGN hosts increase strongly
with redshift; by a factor of 43 from z<0.1 to z=2-3 for AGNs with the same
X-ray luminosities. This increase is consistent with the factor of 25-50
increase in the specific star formation rates (SSFRs) of normal, star-forming
(main-sequence) galaxies. Indeed, the average SSFRs of AGN hosts are only
marginally (20 per cent) lower than those of main-sequence galaxies, with this
small deficit being due to a fraction of AGNs residing in quiescent (low-SSFR)
galaxies. We estimate 79+/-10 per cent of moderate AGNs are hosted in
main-sequence galaxies, 15+/-7 per cent in quiescent galaxies and <10 per cent
in strongly starbursting galaxies. The fractions of all main sequence galaxies
at z<2 experiencing a period of moderate nuclear activity is strongly dependent
on galaxy stellar mass (Mstars); rising from a few per cent at
Mstars~10^{10}Msun to >20 per cent at Mstars>10^{11}Msun. Our results indicate
that it is galaxy stellar mass that is most important in dictating whether a
galaxy hosts a moderate luminosity AGN. We argue that the majority of moderate
nuclear activity is fuelled by internal mechanisms rather than violent mergers,
suggesting that disk instabilities could be an important AGN feeding mechanism.
