Spectroscopic redshifts of BL Lac objects

Including archival measurements (correcting several erroneous literature values) we now have spectroscopic redshifts for 44% of the BL Lacs. We establish firm lower redshift limits via intervening absorption systems and statistical lower limits via searches for host galaxies for an additional 51% of the sample leaving only 5% of the BL Lacs unconstrained.

The new redshifts raise the median spectroscopic {overline}{Z} from 0.23 to 0.33 and include redshifts as large as z=2.471. Spectroscopic redshift minima from intervening absorbers have {overline}{Z}, showing a substantial fraction at large z and arguing against strong negative evolution. We find that detected BL Lac hosts are bright ellipticals with black hole masses M_{bullet}_~10^8.5^-10^9^, substantially larger than the mean of optical AGNs and LAT Flat Spectrum Radio Quasar samples.