GOODS-Herschel: radio-excess signature of hidden AGN activity in distant
  star-forming galaxies
  We present here a new spectral energy distribution (SED) fitting approach
that we adopt to select radio-excess sources amongst distant star-forming
galaxies in the GOODS-Herschel (North) field and to reveal the presence of
hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies
with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest
Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to
date, we have robustly identified a sample of 51 radio-excess AGN (~1300
deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower
far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming
galaxies (q~2.2). We find that ~45% of these radio-excess sources have a
dominant AGN component in the mid-IR band, while for the remainders the excess
radio emission is the only indicator of AGN activity. The fraction of
radio-excess AGN increases with X-ray luminosity reaching ~60% at
Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN
population. However, almost half (24/51) of these radio-excess AGN are not
detected in the deep Chandra X-ray data, suggesting that some of these sources
might be heavily obscured. We also find that the specific star formation rates
(sSFRs) of the radio-excess AGN are on average lower that those observed for
X-ray selected AGN hosts, indicating that our sources are forming stars more
slowly than typical AGN hosts, and possibly their star formation is
progressively quenching.
