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Astron. Astrophys. Suppl. Ser. 134, 221-240

Radio-loud ROSAT sources near the North Ecliptic Pole

W. Brinkmann1 - M. Chester2 - R. Kollgaard3 - E. Feigelson4 - W. Voges1 - P. Hertz5

Send offprint request: W. Brinkmann,

1 - Max-Planck Institut für extraterrestrische Physik, P.O. Box 1603, D-85740 Garching, Germany
2 - Dept. of Physics, Bucknell University, Lewisburg, PA 17837, U.S.A.
3 - Fermi National Accelerator Laboratory, Batavia, IL 60510, U.S.A.
4 - Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, U.S.A.
5 - E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington DC 20375-5320, U.S.A.

Received March 6; accepted August 6, 1998


A deep and large-area survey of the North Ecliptic Pole region was made with the ROSAT All-Sky Survey and the VLA to elucidate the population of radio-loud extragalactic objects. A region of 29.3 square degrees was surveyed with sensitivities around $5 \ 10^{-14}$ erg s-1 cm-2 in the soft X-ray band and 1 mJy at 1.5 GHz. Optical counterparts were sought on digitized Schmidt plates from POSS-I and II. Seventy-four reliable RASS-VLA sources were found.

The sample is a heterogeneous mixture of Seyfert galaxies, quasars, BL Lac objects, galaxy clusters and groups containing a radio galaxy. Optical magnitudes range from $B \simeq 14$ to B > 22. Three results are noteworthy: (1) the bimodality in the ratio of radio to optical emission seen in optically- and X-ray-selected AGN samples is also evident in the ROSAT-VLA objects; (2) X-ray and radio selection is an effective method for locating poor galaxy clusters and groups; and (3) a considerable population of optically faint but X-ray/radio-bright objects is present. This last group may be either distant clusters with radio galaxies or "red quasars".

Key words: galaxies: active -- clusters: general -- quasars: general -- X-rays: general -- radio continuum: galaxies

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