Velocity Dispersion and Black Hole Mass in the Blazar Markarian

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 Markarian 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• −σ⋆ correlation defined for local galaxies, then its central black hole has a mass of (0.9 − 3.4)× 109 M⊙. This is significantly larger than some previous estimates for the central mass in Mrk 501 that have been based on models for its nonthermal emission. The host galaxy luminosity implies a black hole of ∼ 6×108 M⊙, but this is not in severe conflict with the mass derived from σ⋆ because the M• −Lbulge 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/LEdd ≈ 10−4. Further applications of the M• −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. Subject headings: BL Lacertae objects: individual (Mrk 501) — galaxies: active — galaxies: elliptical and lenticular — galaxies: kinematics and dynamics — galaxies: nuclei