The Cosmic Evolution of Fermi BL Lacertae Objects

Fermi-LAT has provided the largest sample of gamma-ray selected blazars to date. We use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by Fermi-LAT to determine the luminosity function of this class of blazars and its evolution with cosmic time. To make it possible, we have obtained redshift constraints for 206 out of the 211 BL Lacs making it the largest and most complete sample of BL Lacs available in the literature. 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 show an exception, with strong negative evolution and number density increasing for redshift ~ 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 strengthening the genetic link between the 2 blazar subclasses. Finally we discuss, for BL Lacs, the known correlation between luminosity and photon spectral index which has implications for the so called `blazar sequence'.