2002ApJ...566L..13B

The recently discovered correlation between black hole mass and stellar velocity dispersion provides a new method to determine the masses of black holes in active galaxies. We have obtained optical spectra of Mrk 501, a nearby γ-ray blazar with emission extending to TeV energies. The stellar velocity dispersion of the host galaxy, measured from the calcium triplet lines in a 2''x3".7 aperture, is 372±18 km.s^–1. If Mrk 501 follows the M_BH-σ_* correlation defined for local galaxies, then its central black hole has a mass of (0.9-3.4)x10⁹ M_☉. This is significantly larger than some previous estimates for the central mass in Mrk 501 that were based on models for its nonthermal emission. The host galaxy luminosity implies a black hole of ∼6x10⁸ M_☉, but this is not in severe conflict with the mass derived from σ_* because the M_BH-L_bulge correlation has a large intrinsic scatter. Using the emission-line luminosity to estimate the bolometric luminosity of the central engine, we find that Mrk 501 radiates at an extremely sub-Eddington level of L/L_Edd~10^–4. Further applications of the M_BH-σ_* relation to radio-loud active galactic nuclei may be useful for interpreting unified models and understanding the relationship between radio galaxies and BL Lac objects.