Spectroscopy of The Largest Ever Gamma-ray Selected BL Lac Sample
  We report on spectroscopic observations covering most of the 475 BL Lacs in
the 2nd Fermi LAT catalog of AGN. 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 z from 0.23 to
0.33 and include redshifts as large as z=2.471. Spectroscopic redshift minima
from intervening absorbers have ~ z= 0.70, 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^{8.5-9},
substantially larger than the mean of optical AGN and LAT Flat Spectrum Radio
Quasar samples. A slow increase in M_\bullet with z may be due to selection
bias. We find that the power-law dominance of the optical spectrum extends to
extreme values, but this does not strongly correlate with the gamma-ray
properties, suggesting that strong beaming is the primary cause of the range in
continuum dominance.
