Optically Identified BL Lacertae Objects From the Sloan Digital Sky Survey
NAVAL OBSERVATORY WASHINGTON DC
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We present a sample of 386 BL Lacertae candidates identified from 2860 degexp 2 of the Sloan Digital Sky Survey SDSS spectroscopic database. The candidates are primarily selected to have quasi-featureless optical spectra and low proper motions as measured from SDSS and USNO-B positions however, our ability to separate Galactic from extragalactic quasi-featureless objects QFOs on the basis of proper motion alone is limited by the lack of reliable proper motion measurements for faint objects. Fortunately, high proper motion QFOs, mostly DC white dwarfs, populate a well-defined region of color space, approximately corresponding to blackbodies with temperatures in the range 7000-12,000 K. QFOs with measurable redshifts or X-ray or radio counterparts i.e., evidence of an extragalactic active galactic nucleus nature loosely follow a track in color space that corresponds to power-law continua plus host galaxy starlight, with typical power-law slopes in the range 1alphasub opt 2 fv varies as upsilon Zexp -alpha. Based largely on this remarkably clean color separation, we subdivide the sample into 240 probable candidates and 146 additional less probable likely stellar candidates. The probable BL Lac candidates have multiwavelength properties consistent with the range of previously known BL Lac objects, with an apparent preponderance of objects with synchrotron peaks at relatively high energies HBLXBL type. The majority of the 154 objects with measurable redshifts have z 1, with a median of 0.45 there are also a handful of high-redshift objects extending up to z 503. We identify a small number of potential radio-quiet BL Lac candidates, although more sensitive radio observations are needed to confirm their radio-quiet nature.