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C. Cardamone, E. C. Moran (Wesleyan University)
Since the Chandra\/ Deep Field surveys (CDFs) have resolved the majority of the hard (2--10 keV) X-ray background (XRB), our focus has turned to the nature of the faint X-ray galaxies detected in these surveys. It was anticipated that type 2 Seyfert galaxies would make a dominant contribution to the hard XRB. However, follow-up studies have shown that a significant number of the hard X-ray sources in the CDFs appear to be distant ``normal'' galaxies with starlight-dominated optical spectra. This suggests that the X-ray galaxy population may have evolved. To investigate this possibility, we have compared the X-ray--to--optical flux ratios (F\rm X/F\rm opt) of the hard X-ray galaxies found in the CDFs to those that nearby Seyfert 2s would exhibit at comparable redshifts. Accounting for redshift and selection effects, we find the F\rm X/F\rm opt ratios of hard X-ray galaxies in the CDFs are consistent with those of nearby Seyfert 2s. The spectroscopic differences between the distant and local galaxies may result from the limitations of ground-based optical observing. Due to the small angular extent of the CDFs galaxies, only their integrated spectra can be obtained from the ground. Starlight from the host galaxy may overwhelm their nuclear emission, hiding their Seyfert 2 emission-line signatures.
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Bulletin of the American Astronomical Society, 36 #2
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