6.2 Unification of BL Lac Objects and FR I Galaxies

The unification of low-luminosity radio-loud AGN can be tested in the same way, by comparing the observed number densities of BL Lac objects and FR I radio galaxies. We derive the FR I luminosity function (Padovani and Urry 1990; Urry et al. 1991a), beam it according to the prescription outlined in Sec. 5.6 and fit it to the observed BL Lac luminosity functions (Padovani and Urry 1990; Urry et al. 1991a; Celotti et al. 1993). The free parameters are , the bulk Lorentz factor of the jet, and f, the fractional luminosity of the jet. We further constrain these two parameters either from R, the ratio of beamed to unbeamed flux (that is, the core to extended flux in high resolution radio maps), as was done for quasars and FR IIs (Sec. 6.1), or from the ratio of maximum BL Lac luminosity to maximum FR I luminosity. The former approach is used for the radio comparison (Sec. 6.2.4) and the latter approach is used for the X-ray comparison (Sec. 6.2.3) because X-ray spatial resolution is insufficient to measure R. Some separation of beamed and unbeamed components is now becoming possible for radio galaxies with ROSAT (Worrall and Birkinshaw 1994) and in the future, we expect X-ray measurements of R for large samples of BL Lacs will be possible with AXAF.

6.2.1 X-Ray and Radio Samples of BL Lac Objects

Sizable well-defined samples of BL Lac objects are now available. X-ray surveys have produced a number of samples, including the HEAO-1 A-2 all-sky survey (only 5 BL Lac objects; Piccinotti et al. 1982), the EMSS (36 BL Lacs; Stocke et al. 1991), and the EXOSAT High Galactic Latitude Survey (HGLS: 11 BL Lacs; Giommi et al. 1991). The last two go down to fairly low X-ray fluxes, ~ 2 x 10^-13 erg cm^-2 s^-1 in the 0.3-3.5 keV band. Redshifts are available for most EMSS BL Lacs.

The 1 Jy sample of BL Lac objects, a radio-selected sample of size comparable to the EMSS X-ray-selected sample, has also become available recently (Stickel et al. 1991). These BL Lac objects were selected from a catalog of 518 radio sources with 5 GHz fluxes 1 Jy on the basis of their flat radio spectra (_r 0.5), weak or absent emission lines (rest-frame equivalent widths W 5 Å), and optical magnitudes (brighter than V = 20 mag). Of the 34 1 Jy BL Lacs in the complete sample, 26 have redshift information from emission lines (Stickel et al. 1991; Stickel et al. 1994), 4 have lower limits to their redshifts from absorption lines, while in 4 objects neither emission or absorption lines have been detected but a lower limit of z > 0.2 can be estimated from their stellar appearance (Stickel et al. 1993).