The CLASS BL Lac sample: The Radio Luminosity Function
  This paper presents a new sample of BL Lac objects selected from a deep (30
mJy) radio survey of flat spectrum radio sources (the CLASS blazar survey,
henceforth CBS). The sample is one of the largest well defined samples in the
low power regime with a total of 130 sources of which 55 satisfy the
'classical' optical BL Lac selection criteria, and the rest have
indistinguishable radio properties. The primary goal of this study is to
establish the Radio Luminosity Function (RLF) on firm statistical ground at low
radio luminosities where previous samples have not been able to investigate.
The gain of taking a peek at lower powers is the possibility to search for the
flattening of the LF which is a feature predicted by the beaming model but
which has remained elusive to observational confirmation. In this study we
extend for the first time the BL Lac RLF down to very low radio powers ~10^22
W/Hz, ie, two orders of magnitude below the RLF currently available in the
literature. In the process we confirm the importance of adopting a broader, and
more physically meaningful set of classification criteria to avoid the
systematic missing of low luminosity BL Lacs. Thanks to the good statistics we
confirm the existence of weak but significant positive cosmological evolution
for the BL Lac population, and we detect, for the first time the flattening of
the RLF at ~10^25 W/Hz in agreement with the predictions of the beaming model.
