2003ApJ...595..624F

We report on measurements of the stellar velocity dispersion (σ) from the optical spectra of the host galaxies of four BL Lac objects. Together with our earlier results on seven BL Lac objects (Falomo and coworkers) and with the previously derived photometric and structural properties, these data are used to construct the fundamental plane (FP) of the BL Lac hosts. We find that the BL Lac objects follow the same FP as low-redshift radio galaxies and inactive luminous elliptical galaxies, in agreement with similar results presented by Barth and coworkers. This indicates that the photometric, structural, and kinematical properties of the host galaxies of BL Lac objects are indistinguishable from those of inactive massive elliptical galaxies. Using the correlation between black hole mass (M_BH) and σ in nearby elliptical galaxies, we derive the masses of the central black hole in BL Lac objects. These masses, in the range from 6x10⁷ to 9x10⁸ M_☉, are consistent with the values derived from the bulge luminosity and appear to be linearly correlated with the mass of the galaxies (M_BH~0.001M_bulge).