New active galactic nuclei detected in ROSAT All Sky Survey galaxies; Part I: Verification of selection strategy

W. Pietsch, Th. Boller, S. Doebereiner, H. -U. Zimmermann, K. Bischoff, W. Kollatschny

Published 1998-01-22Version 1

We present the first results of a program to identify so far unknown active nuclei (AGN) in galaxies. Candidate galactic nuclei have been selected for optical spectroscopy from a cross-correlation of the ROSAT All Sky Survey (RASS) bright source catalog with optical galaxy catalogs. A high X-ray flux has been used as pointer to galaxies with a high probability to contain active nuclei. Only galaxies have been accepted for the program for which no activity was noted in NED. For many of the galaxies no radial velocity was reported before. The optical spectra demonstrate that the galaxies cover a redshift range of 0.014 to 0.13 and that most of them host active nuclei. For 75 percent of the 33 candidates the X-ray emission is caused by the AGN. In addition several of the remaining candidates host Seyfert 2/LINER nuclei that, however, most certainly cannot explain the X-ray emission alone. Three BL Lac objects have been detected serendipitously in galaxy fields that have been followed up by short ROSAT HRI observations to confirm the X-ray galaxy identification with improved position accuracy and point response function. The sources show X-ray to radio flux ratios typical for X-ray selected BL Lac objects. The results presented in the paper prove the selection strategy as very successful to detect previously unknown AGN of all Seyfert 1 types in nearby galaxies encouraging the extension of this program. The detection of new nearby AGN will be used to initiate a detailed investigation of their multi-wavelength properties and a comparison with the more distant AGN population.