The Large Bright Quasar Survey VII. The LBQS and FIRST Surveys

Abstract

The source catalogue for the LBQS and the FIRST Survey are compared in their regions of overlap. In the 270 deg^2 common to both surveys the LBQS contains \~100,000 stellar and ~40,000 non-stellar objects, while the FIRST catalogue contains ~25,000 sources. Cross-correlation of these lists yields 67 positional coincidences between known LBQS quasars and FIRST sources and an additional 19 stellar and 149 non-stellar positional coincidences with the radio catalogue. Spectroscopy of all the stellar matches and two-thirds of the non-stellar matches produces eight new quasars. One BL Lac object, previously misclassified during the LBQS survey is also identified. The straightforward fractional incompleteness of the LBQS determined from this sample is 13+/-4%, in good agreement with the published estimate of 10%. The distributions of the ratio of radio-to-optical power, apparent magnitude and spectroscopic properties for the new objects are consistent with those of the 67 LBQS-FIRST objects previously known. The consistency of the optical and radio properties of the new objects with those of the known quasars thus supports the conclusion that no new population of objects, constituting more than ~7% of quasars detected by FIRST, has eluded the LBQS optical selection techniques. The percentage of radio-detected quasars in the LBQS catalogue is found to be 12+/-2%, considerably smaller than the value of 25% advocated by White et al. (2000) based on the FBQS. Apparent differences in the form of the number-redshift relation for the LBQS and FBQS samples are shown to arise in large part from the very different optical passbands employed.