Quantitative evaluation of the hypothesis that BL Lacertae objects are   QSO remnants
  We evaluate with numerical simulations the hypothesis that BL Lacertae objects are the remnants of Quasi Stellar Objects. This hypothesis is based on their highly peculiar redshift evolution. They have a comoving space density that increases with decreasing redshift, contrary to all other active galactic nuclei. We assume that relativistic jets are below detection in young radio-quiet quasars and increase in strength with cosmic time so that they eventually are detected as BL Lacertae objects. Our numerical simulations fit very well the observed redshift distributions of BL Lacertae objects. There are strong indications that only the high-synchrotron-peaked BL Lacertae objects could be QSO remnants. 
E.F. Borra
astro-ph.HE