The black hole mass of BL Lacertae objects from the stellar velocity dispersion of the host galaxy

The correlation between black hole mass M-BH and stellar velocity dispersion sigma in nearby elliptical galaxies MBH affords us a novel way to determine M-BH in active galaxies. We report on measurements of sigma from optical spectra MBH of seven BL Lacertae host galaxies. The derived values of sigma are in the range of 160-290 km s(-1) corresponding to M-BH of 5 x 10(7)-1 x 10(9) M-.. The average ratio of M-BH to the host galaxy mass is 1.4 x 10(-3), consistent with that estimated in other active and inactive galaxies. The velocity dispersions and the derived values of M-BH of the BL Lac objects are similar to those obtained for low-redshift radio galaxies, in good agreement with MBH the predictions of the unified models for radio-loud active galaxies.