"Revealing a Population of Heavily Obscured Active Galactic Nuclei at
  z=0.5-1 in the Chandra Deep Field-South
  (abridged) We identify a numerically significant population of heavily
obscured AGNs at z~0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended
Chandra Deep Field-South by selecting 242 X-ray undetected objects with
infrared-based star formation rates (SFRs) substantially higher (a factor of
3.2 or more) than their SFRs determined from the UV after correcting for dust
extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S
region using the 4 Ms Chandra data reveals a hard X-ray signal with an
effective power-law photon index of Gamma=0.6_{-0.4}^{+0.3}, indicating a
significant contribution from obscured AGNs. Based on Monte Carlo simulations,
we conclude that 74+-25% of the selected galaxies host obscured AGNs, within
which ~95% are heavily obscured and ~80% are Compton-thick (CT; NH>1.5x10^{24}
cm^{-2}). The heavily obscured objects in our sample are of moderate intrinsic
X-ray luminosity [ ~ (0.9-4)x10^{42} erg/s in the 2-10 keV band]. The space
density of the CT AGNs is (1.6+-0.5)x10^{-4} Mpc^{-3}. The z~0.5-1 CT objects
studied here are expected to contribute ~1% of the total XRB flux in the 10-30
keV band, and they account for ~5-15% of the emission in this energy band
expected from all CT AGNs according to population-synthesis models. In the 6--8
keV band, the stacked signal of the 23 heavily obscured candidates accounts for
<5% of the unresolved XRB flux, while the unresolved ~25% of the XRB in this
band can probably be explained by a stacking analysis of the X-ray undetected
optical galaxies in the CDF-S (a 2.5 sigma stacked signal). We discuss
prospects to identify such heavily obscured objects using future hard X-ray
observatories.
