Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004head....8.3802l&link_type=abstract
American Astronomical Society, HEAD meeting #8, #38.02; Bulletin of the American Astronomical Society, Vol. 36, p.1203
Astronomy and Astrophysics
Astronomy
Scientific paper
BL Lac objects are thought to be the beamed counterparts of FRI/FRII radio galaxies (cf. review by Urry & Padovani 1995, PASP, 107,803); at optical wavelengths these objects are dominated by Doppler boosted, synchrotron radiation, the resultant featureless continuum making BL Lacs all but impossible to target in optical surveys. To date, therefore, all BL Lac samples have been initially identified in radio and/or X-ray surveys, thus these objects are naturally found to be emitters at these frequencies.
The first optically selected sample of BL Lac objects was identified from scrutiny of spectra in two redshift surveys (2QZ and 6QZ) using the 2dF and 6dF instruments at Siding Spring, NSW, Australia (Croom et al. 2001, MNRAS, 325, 483; MNRAS, 2004, 349, 1397). Of 52 featureless continuum objects identified, only 14 objects have radio flux densitites > 0.15mJy. Five of these 14 also have detectable X-ray emission.
With optical b J magnitudes in the range 16.4 < b J < 20.0 these candidate BL Lac objects are technically radio quiet. Spectroscopic studies providing redshift information for 13 objects, together with predictions using the quasar evolution model of Croom et al. (2004), indicate a median redshift for the sample (the 2BL) of z=1.25, considerably higher than the median z found for the X-ray selected EMSS BL Lac sample (z=0.3, Morris et al.. 1991, ApJ, 38, 49; Rector et al. 2000, ApJ, 120, 1626) and the radio-selected 1Jy BL Lac Sample (z=0.6, Stickel et al. 1991, ApJ, 374, 431; Rector & Stocke 2001, AJ, 122,565). This raises the question as to why such X-ray-quiet, radio-quiet BL Lacs (lineless QSOs?) have not been found at lower redshifts, and what mechanisms are responsible for the lack of such detectable X-ray and radio emission in these optically selected BL Lacs.
DL acknowledges support from the Science Foundation, University of Sydney.
Boyle Brian J.
Croom Scott M.
Londish D. M.
Sadler Elaine M.
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