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

Creators: Lehmann, I.; Hasinger, G.; Schmidt, M.; Giacconi, R.; Trümper, J.; Zamorani, G.; Gunn, J. E.; Pozzetti, L.; Schneider, D. P.; Stanke, T.; Szokoly, G.; Thompson, D.; Wilson, G.

Abstract

The ROSAT Deep Surveys in the direction of the Lockman Hole are the most sensitive X-ray surveys performed with the ROSAT satellite. About 70-80% of the X-ray background has been resolved into discrete sources at a flux limit of ~10^(-15) erg cm^(-2) s^(-1) in the 0.5-2.0 keV energy band. A nearly complete optical identification of the ROSAT Deep Survey (RDS) has shown that the great majority of sources are AGNs. We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the RDS in the Lockman Hole. The Ultra Deep Survey reaches a flux level of 1.2 10^(-15) erg cm^(-2) s^(-1) 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. We see no evidence for any change in population from the RDS survey to the UDS survey. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN. Their photometric redshifts, assuming that the spectral energy distribution (SED) in the optical/near-infrared is due to stellar processes, are 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 implication to the X-ray background.

Additional Information

© 2001 ESO. Article published by EDP Sciences. Received 22 December 2000; Accepted 19 March 2001; Published online 15 June 2001. The ROSAT project is supported by the Bundesministerium für Bildung, Forschung und Technologie (BMBF/DFR) and the Max-Planck-Gesellschaft. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank J. Oke and J. Cohen for their efforts in the contruction of LRIS, and B. Schaefer for organizing service observing. I. L. was visiting astronomer at the German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institut for Astronomy, Heidelberg, jointly with the Spanish National Commission for Astronomy. I. L. thanks for the support by the DFG travel grant (HA 1850/11-1). Partial support for this work was provided by National Science Foundation grant AST99-00703 and by NASA through grant number GO-08194.01-97A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555 (DPS). G. Z. acknowledges partial support by the Italian Space Agency (ASI) under ASI contract ARS-98-119 and by the Italian Ministry for University and Research (MURST) under grant Cofin98-02-32. Part of this work was supported by the German Deutsches Zentrum für Luft- und Raumfahrt, DLR project numbers 50 OX 9801 and 50 OR 9908. We thank our referee R. S. Warwick for his helpful comments and suggestions.

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