The Serendipitous Extragalactic X‐Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy

Abstract

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 deg2 of sky to 2-10 keV fluxes of 1 × 10-14 ergs cm-2 s-1, and 2 deg2 for fluxes of 3 × 10-14 ergs cm-2 s-1. Our spectra reach to R-band magnitudes of 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 1022 cm-2, independent of unobscured luminosity. We classify 168 sources as emission-line galaxies; all are X-ray-luminous (LX > 1041 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] λ3426 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 (LX > 1044 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.