FANAROFF-RILEY-I GALAXIES AS THE PARENT POPULATION OF BL LACERTAE OBJECTS .1. X-RAY CONSTRAINTS

We investigate the hypothesis that BL Lacertae objects are a special subset of Fanaroff-Riley type I radio galaxies, namely, those dominated by beamed emission from a relativistic jet aligned with the line of sight, by looking at the relative number densities. The calculation depends primarily on observed quantities, including the X-ray luminosity function of Fanaroff-Riley type I galaxies, which we derive. In addition, the bulk relativistic velocity of the jet is constrained by fitting to the known number counts derived from complete X-ray samples. Our model predicts the X-ray luminosity function of BL Lacertae objects (and also limits its evolution), the shape of the X-ray counts below currently observed fluxes, and the velocity of the X-ray emitting jet (γ ≈ 3). With Fanaroff-Riley type I galaxies as the parent population one can explain the flat luminosity function, the observed X-ray number counts, and the partial redshift distribution of BL Lacertae objects. The available data are consistent with the beaming hypothesis, although they cannot prove it. A definite test will be possible with the upcoming ROSAT surveys.