Abstract
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 ~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 FX/Fopt, 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 ~ 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 σ, 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 ~ 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.
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