Hard X-Ray-emitting Active Galactic Nuclei Selected by the Chandra Multiwavelength Project

We present X-ray and optical analysis of 188 active galactic nuclei (AGN) identified from 497 hard X-ray (f2.0-8.0 keV > 2.7 × 10-15 ergs cm-2 s-1) sources in 20 Chandra fields (1.5 deg2) forming part of the Chandra Multiwavelength Project. These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 2-8 keV cosmic X-ray background. Brighter than our optical spectroscopic limit, we achieve a reasonable degree of completeness (77% of X-ray sources with counterparts r′ < 22.5 have been classified): broad emission-line AGNs (62%), narrow emission-line galaxies (24%), absorption line galaxies (7%), stars (5%), or clusters (2%). We find that most X-ray unabsorbed AGNs (NH < 1022 cm-2) have optical properties characterized by broad emission lines and blue colors, similar to optically selected quasars from the Sloan Digital Sky Survey but with a slightly broader color distribution. However, we also find a significant population of redder (g′ - i′ > 1.0) AGNs with broad optical emission lines. Most of the X-ray-absorbed AGNs (1022 cm-2 < NH < 1024 cm-2) are associated with narrow emission-line galaxies, with red optical colors characteristically dominated by luminous, early-type galaxy hosts rather than from dust reddening of an AGN. We also find a number of atypical AGNs; for instance, several luminous AGNs show both strong X-ray absorption (NH > 1022 cm-2) and broad emission lines. Overall, we find that 81% of X-ray-selected AGNs can be easily interpreted in the context of current AGN unification models. Most of the deviations seem to be due to an optical contribution from the host galaxies of the low-luminosity AGNs.

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