IAU02757
Presentation type requested: POSTER
AN OPTICALLY SELECTED SAMPLE OF BL LAC OBJECTS
Diana Londish
University of Sydney
We have identified a sample of 48 candidate BL Lac objects selected at optical wavelengths from the 2dF Quasar Redshift Survey (2QZ). The sample was selected from 6772 obects in the 2QZ satisfying our selection criteria (namely b(J) magnitude between 18.25 and 20.0 and spectra with minimum signal-to- noise ratio of 10), and is the first BL Lac sample unbiased with respect to the objects' X-ray and radio flux levels. Significantly 80 percent of objects are undetected at the 0.2mJy level, reportedly the lowest radio flux density of hitherto known BL Lac objects. The number-magnitude relation computed for the sample shows a strongly evolving population, similar to that of the 2QZ QSOs, and markedly different from the negative evolution reported for low luminosity BL Lacs. Using the 2QZ quasar evolution model we compute a median redshift of 1.1 for the sample; redshift information from weak lines and intervening absorbers found in 12 objects supports this prediction. These results therefore suggest that samples selected initially at X-ray frequencies are biased against high redshift objects, and this in turn has led to the erroneous conclusion of negative evolution for low luminosity BL lac objects.
Croom Scott M, Boyle Brian J, Sadler Elaine M
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