Hidden AGNs in Early-Type Galaxies
  We present a stacking analysis of the complete sample of Early Type Galaxies
(ETGs) in the \textit{Chandra} COSMOS (C-COSMOS) survey, to explore the nature
of the X-ray luminosity in the redshift and stellar luminosity ranges
\(0<z<1.5\) and \({10}^{9}<L_K/L_{\astrosun}<{10}^{13}\). Using established
scaling relations, we subtract the contribution of X-ray binary populations, to
estimate the combined emission of hot ISM and AGN. To discriminate between the
relative importance of these two components, we (1) compare our results with
the relation observed in the local universe \(L_{X,gas}\propto L_K^{4.5}\) for
hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of
each stacked bin. We find two regimes where the non-stellar X-ray emission is
hard, consisten t with AGN emission. First, there is evidence of hard, absorbed
X-ray emission in stacked bins including relatively high z (\(\sim 1.2\)) ETGs
with average high X-ray luminosity (\(L_{X-LMXB}\gtrsim 6\times{10}^{42}\mbox{
erg}/\mbox{s}\)). These luminosities are consistent with the presence ofhighly
absorbed "hidden" AGNs in these ETGs, which are not visible in their optical-IR
spectra and spectral energy distributions. Second, confirming the early
indication from our C-COSMOS study of X-ray detected ETGs, we find
significantly enhanced X-ray luminoaity in lower stellar mass ETGs
(\(L_K\lesssim{10}^{11}L_{\astrosun}\)), relative to the local
\(L_{X,gas}\propto L_K^{4.5}\) relation. The stacked spectra of these ETGs also
suggest X-ray emission harder than expected from gaseous hot halos. This
emission is consistent with inefficient accretion
\({10}^{-5}-{10}^{-4}\dot{M}_{Edd}\) onto \(M_{BH}\sim
{10}^{6}-{10}^{8}\,M_{\astrosun}\).
