The Cosmic Evolution of Fermi BL Lacertae Objects
  Fermi has provided the largest sample of gamma-ray selected blazars to date.
In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac)
objects detected by it Fermi during its first year of operation. We have
obtained redshift constraints for 206 out of the 211 BL Lacs in our sample
making it the largest and most complete sample of BL Lacs available in the
literature. We use this sample to determine the luminosity function of BL Lacs
and its evolution with cosmic time. We find that for most BL Lac classes, the
evolution is positive with a space density peaking at modest redshift (z~1.2).
The low-luminosity, high-synchrotron peaked (HSP) BL Lacs are an exception,
showing strong negative evolution, with number density increasing for
z$\lesssim$0.5. Since this rise corresponds to a drop-off in the density of
flat-spectrum radio quasars (FSRQs), a possible interpretation is that these
HSPs represent an accretion-starved end-state of an earlier merger-driven
gas-rich phase. We additionally find that the known BL Lac correlation between
luminosity and photon spectral index persists after correction for the
substantial observational selection effects with implications for the so called
`blazar sequence'. Finally, estimating the beaming corrections to the
luminosity function, we find that BL Lacs have an average Lorentz factor of
$\gamma=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the
jet axis.
