Radio-loud active galaxies in the northern ROSAT All-Sky Survey. III. New spectroscopic identifications from the Rass-Green bank BL Lacertae survey

We present new spectroscopic identifications for 169 objects in the RASS-Green Bank (RGB) catalog of radio- and X-ray-emitting active galactic nuclei (AGNs). The data presented here significantly increase the fraction of bright RGB objects with classifications. Specifically, we report and discuss the classification of 57 radio-loud quasars, eight radio-quiet quasars (QSOs), 53 BL Lacs, 32 broad-line radio galaxies, five narrow-line radio galaxies, three Seyfert I galaxies, and II galaxies or galaxies in clusters. Over 78% of the identifications we present here are the first published classifications for these sources. The observations we report were undertaken as part of our targeted search program to identify a new, large unbiased sample of BL Lac objects, and we therefore discuss the BL Lac sample extensively. Unlike many previous surveys, we impose no selection criteria based on optical morphology, color, or broadband spectral energy distribution. Our classifications are based solely on a carefully defined set of self-consistent spectroscopic classification criteria. These criteria are then carefully evaluated, and particular attention is paid to issues involving the classification and description of BL Lacs. The criteria yielded BL Lac classifications for 53 RGB objects, 38 of which were are newly discovered BL Lacs. We show these RGB BL Lacs exhibit transitional properties between normal galaxies and BL Lacs discovered in previous radio and X-ray surveys. We briefly discuss the broadband flux distributions of these new RGB BL Lacs and the range of Ca II H and K break contrasts (Br-4000) they exhibit. We show that there is no clear separation in Br-4000 between BL Lacs and galaxies detected in the RGB survey, with the distribution of break strengths varying smoothly between 0% and 50%. We also present and use a simple method based on the break strength to estimate the contribution of both the host galaxy and AGN to the 4000 Angstrom flux. We also show that the newly discovered RGB BL Lacs reside in a "zone of avoidance" in the log (S-x/S-r) versus log (S-o/S-r) diagram. This has important implications for BL Lac search strategies since it shows that RASS BL Lac samples will be severely incomplete if candidates are chosen only from among those objects with the highest S-x/S-r flux ratios.