Large sample of BL Lac objects from the SDSS and first

We present a large sample of 501 radio-selected BL Lac candidates from a combination of Sloan Digital Sky Survey (SDSS) Data Release 5 optical spectroscopy and the Faint Images of the Radio Sky at Twenty cm ( FIRST) radio survey; this is one of the largest BL Lac samples yet assembled, and each object emerges with homogeneous data coverage. Each candidate is detected in the radio from FIRST and confirmed in SDSS optical spectroscopy to have (1) no emission feature with measured rest-equivalent width larger than 5 angstrom and (2) no measured Ca II H/K depression larger than 40%. We subdivide our sample into 426 higher-confidence candidates and 75 lower-confidence candidates. We argue that contamination from other classes of objects that formally pass our selection criteria is small, and we identify a few very-rare radio active galactic nuclei (AGNs) with unusual spectra that are probably related to broad absorption line quasars. About one-fifth of our sample were known BL Lac objects prior to the SDSS. A preliminary analysis of the sample generally supports the standard beaming paradigm. While we recover sizable numbers of low-energy and intermediate-energy cutoff BL Lac objects (LBLs and IBLs, respectively), there are indications of a potential bias toward recovering high-energy cutoff BL Lac objects (HBLs) from SDSS spectroscopy. Such a large sample may eventually provide new constraints on BL Lac unification models and their potentially peculiar cosmic evolution; in particular, our sample contains a significant number of higher redshift objects, a sub-population for which the standard paradigm has yet to be rigorously constrained.