EDP Sciences
Free Access
Volume 371, Number 3, June I 2001
Page(s) 833 - 857
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20010419
Published online 15 June 2001

A&A 371, 833-857 (2001)
DOI: 10.1051/0004-6361:20010419

The ROSAT Deep Survey

VI. X-ray sources and Optical identifications of the Ultra Deep Survey
I. Lehmann1, 2, G. Hasinger1, M. Schmidt3, R. Giacconi4, J. Trümper5, G. Zamorani6, 7, J. E. Gunn8, L. Pozzetti7, D. P. Schneider2, T. Stanke9, G. Szokoly1, D. Thompson3 and G. Wilson10

1  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: ilehmann@aip.de; ghasinger@aip.de, gpszokoly@aip.de
2  Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
    e-mail: ingo@astro.psu.edu, dps@astro.psu.edu
3  California Institute of Technology, Pasadena, CA 91125, USA
    e-mail: mxs@deimos.caltech.edu, djt@mop.caltech.edu
4  Associated Universities, Inc. 1400 16th Street, NW, Suite 730, Washington, DC 20036, USA
    e-mail: giacconi@aui.edu
5  Max-Planck-Institut für extraterrestrische Physik, Karl-Schwarzschild-Str. 1, 85748 Garching bei München, Germany
    e-mail: jtrumper@mpe-garching.mpg.de
6  Istituto di Radioastronomia del CNR, via Gobetti 101, 40129, Bologna, Italy
    e-mail: zamorani@astbo3.bo.astro.it
7  Osservatorio Astronomico, Via Ranzani 1, 40127, Bologna, Italy
    e-mail: lucia@bo.astro.it
8  Princeton University Observatory, Peyton Hall, Princeton, NJ 08540, USA
    e-mail: jeg@astro.princeton.edu
9  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53010 Bonn, Germany
    e-mail: tstanke@mpifr-bonn.mpg.de
10  Brown University, Physics Department, Providence, RI 02912, USA
    e-mail: gillian@het.brown.edu

(Received 22 December 2000 / Accepted 19 March 2001 )

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 $1.2 \le z \le 2.7$. 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.

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

Offprint request: I. Lehmann, ilehmann@aip.de

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© ESO 2001

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