Stellar Velocity Dispersion and Black Hole Mass in the Blazar Markarian 501

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'' × 3 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) × 10 9 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 ~6 × 10 8 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.