AGN black hole mass derived from the gravitational redshift in optical lines

We determined a central black hole mass of M_virial=1.8± 0.4× 10⁷ M_⊙ in the Seyfert galaxy Mrk 110 with reverberation mapping methods - i.e. from integrated optical line profile variations as well as from velocity-delay maps. But this black hole mass depends on the unknown orientation of the central accretion disk. Here we report on the detection of gravitational redshifted emission in the variable fraction of all broad optical emission lines in Mrk 110. We derive a central black hole mass of M_grav=14.0± 3.0× 10⁷ M_⊙ with this method. This value is independent on the orientation of the accretion disk. A comparison of both black hole mass estimates allows to determine the central accretion disk inclination angle i to 21° in Mrk 110, and therefore the orientation of the spin axis of the central black hole.