Spin orientation of supermassive black holes in active galaxies

Accretion of gas onto a central supermassive black hole is generally accepted to be the source of the emitted energy in active galactic nuclei. The broad emission lines we observe in their optical spectra are probably formed in the wind of an accretion disk at distances of light days to light years from the central black hole. The variable fraction of the emission lines originates at typical distances of only 1 to 50 light days from the central supermassive black hole. We derived a central black hole mass of M_{ orbital}=1.8(plus-minus) 0.4* 10^{7}M_{odot} in the Seyfert galaxy Mrk 110 assuming the broad emission lines are generated in gas clouds orbiting within an accretion disk. This figure depends on the inclination angle of the accretion disk. Here we report on the detection of gravitational redshifted emission in the variable fraction of the broad emission lines. We derive a central black hole mass of M_{ grav}=14.0(plusminus) 3.0* 10^{7}M{odot}. These measurements are independent on the orientation of the accretion disk. The comparison of both black hole mass estimates allows to determine the projection of the central accretion disk angle i to 21(plus-minus)5 deg in Mrk 110 and therefore the orientation of the spin axis of the central black hole.