The Serendipitous Extragalactic X-ray Source Identification (SEXSI) program. III. Optical spectroscopy

We present the catalog of 477 spectra from the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program, a survey designed to probe the dominant contributors to the 2-10 keV cosmic X-ray background. Our survey covers 1 deg(2) of sky to 2-10 keVfluxes of 1 x 10(-14) ergs cm(-2) s(-1), and 2 deg(2) for fluxes of 3 x 10(-14) ergs cm(-2) s(-1). Our spectra reach to R-band magnitudes of less than or similar to 24 and have produced identifications and redshifts for 438 hard X-ray sources. Typical completeness levels in the 27 Chandra fields studied are 40%-70%. The vast majority of the 2-10 keV selected sample are active galactic nuclei (AGNs) with redshifts between 0.1 and 3; our highest redshift source lies at z = 4.33. We find that few sources at z < 1 have high X-ray luminosities, reflecting a dearth of high-mass, high-accretion-rate sources at low redshift, a result consistent with other recent wide-area surveys. We find that half of our sources show significant obscuration, with N-H > 10(22) cm(-2), independent of unobscured luminosity. We classify 168 sources as emission-line galaxies; all are X-ray-luminous (L-X > 10(41) ergs s(-1)) objects with optical spectra lacking both high-ionization lines and evidence of a nonstellar continuum. The redshift distribution of these emission-line galaxies peaks at a significantly lower redshift than does that of the sources we spectroscopically identify as AGNs. We conclude that few of these sources, even at the low-luminosity end, can be powered by starburst activity. Stacking spectra for a subset of these sources in a similar redshift range, we detect [Ne V] lambda 3426 emission, a clear signature of AGN activity, confirming that the majority of these objects are Seyfert 2 galaxies in which the high-ionization lines are diluted by stellar emission. We find a total of 33 objects lacking broad lines in their optical spectra that have quasar X-ray luminosities (L-X > 10(44) ergs s(-1)), the largest sample of such objects identified to date. In addition, we explore 17 AGNs associated with galaxy clusters and find that the cluster-member AGN sample has a lower fraction of broad-line AGNs than does the background sample.