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We present a large sample of 501 radio-selected BL Lac candidates from thecombination of the Sloan Digital Sky Survey (SDSS) Data Release 5 opticalspectroscopy and from the Faint Images of the Radio Sky at Twenty-Centimeters(FIRST) radio survey; this is one of the largest BL Lac samples yet assembled,and each object emerges with homogeneous data coverage. Each candidate isdetected in the radio from FIRST and confirmed in SDSS optical spectroscopy tohave: (1) no emission feature with measured rest equivalent width larger than 5Angstroms; and (2) no measured Ca II H/K depression larger than 40%. Wesubdivide our sample into 426 higher confidence candidates and 75 lowerconfidence candidates. We argue that contamination from other classes ofobjects that formally pass our selection criteria is small, and we identify afew very rare radio AGN with unusual spectra that are probably related to broadabsorption line quasars. About one-fifth of our sample were known BL Lacs priorto the SDSS. A preliminary analysis of the sample generally supports thestandard beaming paradigm. While we recover sizable numbers of low-energy andintermediate-energy cutoff BL Lacs (LBLs and IBLs, respectively), there areindications of a potential bias toward recovering high-energy cutoff BL Lacs(HBLs) from SDSS spectroscopy. Such a large sample may eventually provide newconstraints on BL Lac unification models and their potentially peculiar cosmicevolution; in particular, our sample contains a significant number of higherredshift objects, a sub-population for which the standard paradigm has yet tobe rigorously constrained.

A Large Sample of BL Lacs from SDSS and FIRST