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Astron. Astrophys. 354, 35-54 (2000)


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The ROSAT Deep Survey

III. Optical spectral properties of X-ray sources in the Lockman Hole

I. Lehmann 1, G. Hasinger 1, M. Schmidt 2, J.E. Gunn 3, D.P. Schneider 4, R. Giacconi 5, M. McCaughrean 1, J. Trümper 6 and G. Zamorani 7,8

1 Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
2 California Institute of Technology, Pasadena, CA 91125, USA
3 Princeton University Observatory, Peyton Hall, Princeton, NJ 08540, USA
4 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
5 European Southern Observatory, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
6 Max-Planck-Institut für Extraterrestrische Physik, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
7 Osservatorio Astronomico, Via Ranzani 1, 40127 Bologna, Italy
8 Istituto di Radioastronomia del CNR, Via Gobetti 101, 40129 Bologna, Italy

Received 30 August 1999 / Accepted 22 November 1999

Abstract

The ROSAT Deep Survey in the Lockman Hole contains a complete sample of 50 X-ray sources with fluxes in the 0.5-2 keV band larger than 5.5 [FORMULA] 10-15 erg cm-2 s-1. Previous work has provided optical identification of 46 of the 50 X-ray sources; over 75% of the sources are AGNs (Schmidt et al. 1998).

We present now the atlas of optical finding charts and the full description of the spectra, including emission line properties of the optical counterparts, which are important for the object classification.

New optical/infrared observations of three of the four unidentified sources show that one source is an AGN and two sources with an unusually large ratio of X-ray to optical flux have counterparts in the K'-band suggesting that they are obscured AGNs. Furthermore, we found evidence from radio emission that the remaining unidentified source is a powerful radio galaxy (AGN). We thus obtain a 100% completeness.

During the course of our optical identification work, we obtained optical spectra of 83 field galaxies, of which 67 were narrow-emission line galaxies (NELG). We demonstrate that it is highly unlikely that a significant number of NELG are physically associated with X-ray sources.

Key words: surveys – galaxies: active – galaxies: clusters: general – galaxies: quasars: emission lines – galaxies: Seyfert – X-rays: galaxies

Send offprint requests to: I. Lehmann (ilehmann@aip.de)

© European Southern Observatory (ESO) 2000

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