On the cosmological evolution of BL Lacertae objects

We study the cosmological evolution of BL Lac objects by applying the V-e/V-a analysis to a new sample of 55 objects, presented for the first time in this paper. This sample has been selected from the 239 sources with the brightest X-ray flux (greater than or equal to4 x 10(-13) ergs s(-1) cm(-2)) and relatively bright optical counterparts (B less than or equal to 20.5) among the similar to1600 objects included in the REX survey. The uniform distribution of the Ve/Va values found in the sample suggests that BL Lac objects are not affected by a strong cosmological evolution, in contrast with the behavior observed in the emission-line active galactic nuclei. The V-e/V-a analysis applied to the subsample of the high-energy-peaked BL Lac objects (HBLs) does not reveal a significant departure from a uniform distribution. This result suggests either that the cosmological evolution is less extreme than that previously found in other samples or that it is carried out only by a minority of objects, namely, by the most extreme HBLs not selected in our sample.