The ROSAT Deep Survey: VI. X-ray sources and Optical identifications of the Ultra Deep Survey

TL;DR

This paper presents the ROSAT Ultra Deep Survey, extending previous X-ray observations to fainter flux levels, with high identification completeness, revealing diverse AGN types, galaxy groups, and potential obscured quasars, informing X-ray background models.

Contribution

It provides the deepest X-ray survey to date with nearly complete optical identifications, including potential obscured AGNs, advancing understanding of X-ray source populations and the cosmic X-ray background.

Findings

94 X-ray sources identified, 90% spectroscopic completeness

Majority are broad emission line AGNs, with significant type II AGNs

Detection of potential obscured AGNs at high redshift

Abstract

We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the ROSAT Deep Survey (RDS) in the Lockman Hole. The UDS reaches a flux level of 1.2 x 10E-15 erg/cm2/s in 0.5-2.0 keV energy band, a level ~4.6 times fainter than the RDS. We present nearly complete spectroscopic identifications (90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra. The majority of the sources (57) are broad emission line AGNs (type I), whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement (type II AGNs) indicating significant optical absorption. The second most abundant class of objects (10) are groups and clusters of galaxies (~11%). Further we found five galactic stars and one ''normal'' emission line galaxy. Eight X-ray sources remain spectroscopically unidentified. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN in the range of 1.2 < z < 2.7. These objects could belong to the long-sought population of type 2 QSOs, which are predicted by the AGN synthesis models of the X-ray background. Finally, we discuss the optical and soft X-ray properties of the type I AGN, type II AGN, and groups and clusters of galaxies, and the implications to the X-ray backround.