HIDDEN ACTIVE GALACTIC NUCLEI IN EARLY-TYPE GALAXIES

We present a stacking analysis of the complete sample of early-type galaxies (ETGs) in the 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-circle dot < 10(13). Using established scaling relations, we subtract the contribution of X-ray binary populations to estimate the combined emission of hot ISM and active galactic nuclei (AGNs). 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,L-gas proportional to 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, consistent with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z (similar to 1.2) ETGs with average high X-ray luminosity (LX-LMXB greater than or similar to 6 x 10(42) ergs s(-1)). These luminosities are consistent with the presence of highly 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 luminosity in lower stellar mass ETGs (L-K less than or similar to 10(11) L-circle dot), relative to the local L-X,L-gas proportional to 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) (M) over dot(Edd) onto M-BH similar to 10(6) - 10(8) M-circle dot.