Radio stacking reveals evidence for star formation in the host galaxies
  of X-ray selected active galactic nuclei at z<1
  Nuclear starbursts may contribute to the obscuration of active galactic
nuclei (AGNs). The predicted star formation rates are modest, and, for the
obscured AGNs that form the X-ray background at z < 1, the associated faint
radio emission lies just beyond the sensitivity limits of the deepest surveys.
Here, we search for this level of star formation by studying a sample of 359
X-ray selected AGNs at z < 1 from the COSMOS field that are not detected by
current radio surveys. The AGNs are separated into bins based on redshift,
X-ray luminosity, obscuration, and mid-infrared characteristics. An estimate of
the AGN contribution to the radio flux density is subtracted from each radio
image, and the images are then stacked to uncover any residual faint radio flux
density. All of the bins containing 24 micron-detected AGNs are detected with a
signal-to-noise >3sigma in the stacked radio images. In contrast, AGNs not
detected at 24 microns are not detected in the resulting stacked radio images.
This result provides strong evidence that the stacked radio signals are likely
associated with star formation. The estimated star formation rates derived from
the radio stacks range from 3 solar masses per year to 29 solar masses per
year. Although it is not possible to associate the radio emission with a
specific region of the host galaxies, these results are consistent with the
predictions of nuclear starburst disks in AGN host galaxies.
