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Astron. Astrophys. 319, 7-17 (1997)

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Deep radio observations of the "Lockman Hole"

H.R. , de Ruiter 1, 2, G. Zamorani 1, 2, P. Parma 2, G. Hasinger 3, G. Hartner 4, J. Trümper 4, R. Burg 5, R. Giacconi 6 and M. Schmidt 7

1 Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
2 Istituto di Radioastronomia del CNR, Via Gobetti, 101, I-40129 Bologna, Italy
3 Astrophysikalisches Institut, An der Sternwarte 16, D-14482 Potsdam, Germany
4 Max Planck Institut für Extra-terrestrische Physik, D-8046 Garching, Germany
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
6 European Southern Observatory, Karl Schwarzschildstr. 2, D-8046 Garching, Germany
7 California Institute of Technology, 105-24 Robinson Lab., Pasadena, CA 91125, USA

Received 1 March 1996 / Accepted 27 June 1996


A deep radio image of the "Lockman Hole" is presented. The observations, done with the C-configuration of the Very Large Array at a wavelength of 20 cm, are part of a wider study of this field in X-rays with the ROSAT satellite. Optical follow up will provide a sample of objects observed down to very faint flux limits in all three wavelength windows.

The radio data reduction is described and a catalog of 149 radio sources with peak flux densities at least four times the local RMS noise is presented. The limiting peak flux density in the central part of the field is [FORMULA] 120 µJy. The sources have been extracted from a circular area of [FORMULA] radius around the field center.

Radio source counts are briefly discussed and shown to be in good agreement with previous results at similar flux densities.

All radio sources have been searched for optical counterparts on POSS prints and an optical candidate has been found for [FORMULA] 30 % of the radio sources. The optical identification rate increases at fluxes [FORMULA]  mJy and the increase is mainly due to identifications with relatively bright galaxies ([FORMULA]).

Cross correlation with the X-ray source list, which contains 135 sources in the same area, yields 16 possible radio/X-ray associations. After correcting for expected spurious associations, we estimate that [FORMULA]  % of the X-ray sources are associated with radio sources at these fluxes. Deep CCD frames provide optical counterparts for most of the radio/X-ray sources. On the basis of partial spectroscopy, optical morphology and correlations between radio, optical and X-ray fluxes, we conclude that although the majority ([FORMULA]  %) of the radio/X-ray associations are identified with AGNs, a non-negligible fraction of them ([FORMULA]  %) appears to be associated with normal galaxies. Most of these galaxies have low fluxes, both in radio and in X-rays and they may partly represent the bright X-ray tail of the same population that is responsible for the upturn in the radio source counts below 1 mJy.

Key words: radio continuum: general – X-rays: general

Send offprint requests to: H.R. de Ruiter

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998