Hubble Space Telescope imaging in the Chandra Deep Field-South.: III.: Quantitative morphology of the 1 million second Chandra counterparts and comparison with the field population

We present quantitative morphological analyses of 37 Hubble Space Telescope (HST)/WFPC2 counterparts of X-ray sources in the 1 Ms Chandra Deep Field - South (CDFS). We investigate (1) one-dimensional surface brightness profiles via isophotal ellipse fitting; (2) two-dimensional, point-spread function convolved, bulge+ disk+ nucleus profile fitting; (3) asymmetry and concentration indices compared with all similar to 3000 sources in our three WFPC2 fields; and (4) near-neighbor analyses comparing local environments of X-ray sources versus the field control sample. Significant nuclear point-source optical components appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow range of F-X/F-opt,F-nuc consistent with the several HST-unresolved X-ray sources (putative type 1 active galactic nuclei [AGNs]) in our fields. We infer that roughly half of the HST/WFPC2 counterparts host unobscured AGNs, comparable to analogous low-redshift AGN samples and suggesting no steep decline in the type 1/ type 2 ratio out to the redshifts z similar to 0.5-1 typical of our sources. The concentration indices of the CDFS counterparts are clearly larger on average than those of the field distribution, at 5 sigma, suggesting that the strong correlation between central black hole mass and host galaxy properties ( including concentration index) observed in nearby galaxies is already evident by z similar to 0.5-1. By contrast, the asymmetry index distribution of the 21 resolved CDFS sources at I < 23 is indistinguishable from that of the I < 23 field. Moreover, the frequency of I < 23 near neighbors around the CDFS counterparts is not significantly different from that of the field sample. These results, combined with previous similar findings for local samples, suggest that recent merger/interaction history is not a good indicator of AGN activity over a substantial range of look-back time.