Imaging redshifts of BL Lac objects

The HST snapshot imaging survey of 110 BL Lac objects (Urry et al. 2000) has clearly shown that the host galaxies are massive and luminous ellipticals. The dispersion of the absolute magnitudes is sufficiently small, so that the measurement of the galaxy brightness becomes a valuable way of estimating their distance. This is illustrated constructing the Hubble diagram of the 64 resolved objects with known redshift. By means of this relationship we estimate the redshift of five resolved BL Lacs of the survey, which have no spectroscopic z. The adopted method allows us also to evaluate lower limits to the redshift for 13 objects of still unknown z, based on the lower limit on the host galaxy magnitude. This technique can be applied to any BL Lac object for which an estimate or a lower limit of the host galaxy magnitude is available. Finally we show that the distribution of the nuclear luminosity of all the BL Lacs of the survey, indicates that the objects for which both the redshift and the host galaxy are undetected, are among the most luminous, and possibly the most highly beamed.