Optical spectroscopy of X-ray-selected active galactic nuclei

Optical spectra of 65 x-ray-selected active galactic nuclei (XAGN) from several identification samples are presented. From analysis of their spectra and optical-continuum luminosities, the XAGN are classified as follows: 13 quasars or QSOs, ten narrowline Seyfert 1 galaxies, three Seyfert 1.5s, three Seyfert 2s, one Seyfert 1.9, one H II-region-like galaxy, and three galaxies with very weak emission. The remaining XAGN are most likely Seyfert 1 galaxies. The "red XAGN" of Stocke et al. (1983), which they originally thought might be a new class of low-luminosity object, appear to be a mix of previously known types of objects-Seyfert galaxies and galaxies with weak emission. Their red colors seem to be caused by starlight. X-ray selection may be an efficient way to find narrowline Seyfert 1 galaxies. One possible explanation stems from the possibility that some, or all, narrowline Seyfert 1s are pole-on Seyfert 2 galaxies. In this picture, the material that obscures the broadline region of Seyfert 2 galaxies is in a disk that is optically thick to the broadline- region emission and to x rays. Viewed edge-on, it looks like a Seyfert 2, but along the axis of the disk, x rays can escape, the featureless continuum will be less obscured, and Fe II emission might be visible, all features associated with narrowline Seyfert 1s. A possible spectral discriminant between quasars and QSOs on the one hand, and Seyfert 1 galaxies on the other, is seen. The presence of all of four spectral features-an excess of emission in the UV, a continuum that rises to the blue, broad permitted lines, and the absence of spectral absorption features-in the spectrum of an unknown object strongly suggests that it may be a quasar or QSO, but it is necessary to look for high optical-continuum luminosity to be sure. However, an object without all four features is most likely not a quasar or QSO. Larger samples of quasars, QSOs, and Seyfert 1 galaxies are necessary to confirm this discriminant.