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 ~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,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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.