| We compare the optical spectral types with the X-ray spectral properties for a uniformly selected, highly spectroscopically complete Chandra X-ray selected sample of Active Galactic Nuclei (AGN). Our OPTX sample consists of 1789 sources in the CLANS, CLASXS, and CDF-N Chandra surveys and covers 1.2 square degrees. We confirm that there is significant overlap of the X-ray spectral properties, as determined by the effective photon indices, for different, optical spectral types. Since it is not understood how X-ray and optical classifications relate to the obscuration of the central engine, we strongly advise against a mixed X-ray+optical classification scheme, as it can only complicate the interpretation of X-ray AGN samples. We also find that classic empirical emission line ratio diagnostic diagrams misidentify ∼ 20% of X-ray selected AGNs as star formers. We confirm that there is a large dispersion in the ratio of [OIII] to X-ray luminosities for both our optically obscured and unobscured sources. We postulate that the X-ray selected AGNs that are misidentified by the diagnostic diagrams have low [OIII] luminosities due to the complexity of the structure of the narrow-line region, which causes many ionizing photons from the AGN not to be absorbed. This would mean that the [OIII] luminosity is not a good proxy for the total AGN luminosity. Despite selection effects, we show that the shapes and normalizations of the [OIII] and transformed X-ray luminosity functions show reasonable agreement, suggesting that the [OIII] samples are not finding substantially more AGNs at low redshifts than the hard X-ray samples. We use the Sloan Digital Sky Survey (SDSS) DR7 to define a new optical emission line classification for higher redshifts, based on rest-frame g -- z color, [NeIII]lambda3869, and [OII]lambdalambda3726+3729, to be used for galaxies out to z < 1.4. We find that our high-redshift diagnostic is remarkably robust - it correctly identifies 98% of SDSS AGNs and 96% of SDSS star formers and misidentifies only one of our OPTX Chandra X-ray selected AGNs. |
