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

The black hole mass of BL Lac objects from the stellar velocity dispersion of the host galaxy ABSTRACT The correlation between black hole mass M BH and stellar velocity dispersion σ in nearby elliptical galaxies affords a novel way to determine M BH in active galaxies. We report on measurements of σ from optical spectra of 7 BL Lac host galaxies. The derived values of σ are in the range of 160 – 290 km s −1 corresponding to M BH of 5 × 10 7 to 1 × 10 9 M ⊙. The average ratio of M BH to the host galaxy mass is 1.4 × 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 Lacs are similar to those obtained for low redshift radio galaxies, in good agreement with the predictions of the unified models for radio–loud active galaxies. Subject headings: BL Lacertae objects – galaxies:active – galaxies: elliptical and lenticular – galaxies: kinematics and dynamics – galaxies:nuclei – 2 –