GOODS-Herschel: Ultra-deep XMM-Newton observations reveal
  AGN/star-formation connection
  Models of galaxy evolution assume some connection between the AGN and star
formation activity in galaxies. We use the multi-wavelength information of the
CDFS to assess this issue. We select the AGNs from the 3Ms XMM-Newton survey
and measure the star-formation rates of their hosts using data that probe
rest-frame wavelengths longward of 20 um. Star-formation rates are obtained
from spectral energy distribution fits, identifying and subtracting an AGN
component. We divide the star-formation rates by the stellar masses of the
hosts to derive specific star-formation rates (sSFR) and find evidence for a
positive correlation between the AGN activity (proxied by the X-ray luminosity)
and the sSFR for the most active systems with X-ray luminosities exceeding
Lx=10^43 erg/s and redshifts z~1. We do not find evidence for such a
correlation for lower luminosity systems or those at lower redshifts. We do not
find any correlation between the SFR (or the sSFR) and the X-ray absorption
derived from high-quality XMM-Newton spectra either, showing that the
absorption is likely to be linked to the nuclear region rather than the host,
while the star-formation is not nuclear. Comparing the sSFR of the hosts to the
characteristic sSFR of star-forming galaxies at the same redshift we find that
the AGNs reside mostly in main-sequence and starburst hosts, reflecting the AGN
- sSFR connection. Limiting our analysis to the highest X-ray luminosity AGNs
(X-ray QSOs with Lx>10^44 erg/s), we find that the highest-redshift QSOs (with
z>2) reside predominantly in starburst hosts, with an average sSFR more than
double that of the "main sequence", and we find a few cases of QSOs at z~1.5
with specific star-formation rates compatible with the main-sequence, or even
in the "quiescent" region. (abridged)
