The supermassive black hole of Fornax A

The radio galaxy FornaxA (NGC 1316) is a prominent merger remnant in the outskirts of the Fornax cluster. Its giant radio lobes suggest the presence of a powerful active galactic nuclei (AGN) and thus a central supermassive black hole (SMBH). FornaxA now seems to be in a transition state between active black hole growth and quiescence, as indicated by the strongly declined activity of the nucleus. Studying objects in this evolutionary phase is particularly important in order to understand the link between bulge formation and black hole growth, which is manifested in the M(center dot)-sigma relation between black hole mass and bulge velocity dispersion. So far, a measurement of the SMBH mass has not been possible in Fornax A, as it is enshrouded in dust which makes optical measurements impossible. We present high-resolution adaptive optics assisted integral-field data of Fornax A, taken with SINFONI at the Very Large Telescope in the K band, where the influence of dust is negligible. The achieved spatial resolution is 0.085 arcsec, which is about a fifth of the diameter of the expected sphere of influence of the black hole. The stellar kinematics was measured using the region around the CO bandheads at 2.3 mu m. FornaxA does not rotate inside the inner similar to 3 arcsec. The velocity dispersion increases towards the centre. The weak AGN emission affects the stellar kinematics in the inner similar to 0.06 arcsec only. Beyond this radius, the stellar kinematics appears relaxed in the central regions. We use axisymmetric orbit models to determine the mass of the SMBH in the centre of Fornax A. The three-dimensional nature of our data provides the possibility to directly test the consistency of the data with axisymmetry by modelling each of the four quadrants separately. According to our dynamical models, consistent SMBH masses M(center dot) and dynamical K(s)-band mass-to-light ratios Gamma are obtained for all quadrants, with < M(center dot)> = 1.3 x 10(8) M(circle dot) [rms(M(center dot)) = 0.4 x 10(8) M(circle dot)] and = 0.68 [rms(Gamma) = 0.03], confirming the assumption of axisymmetry. For the folded and averaged data, we find M center dot = 1.5(-0.8)(+ 0.75) x 10(8) M(circle dot) and Gamma = 0.65(-0.05)(+0.075) (3 sigma errors). Thus, the black hole mass of FornaxA is consistent within the error with the Tremaine et al. M(center dot)-sigma relation, but is a factor of similar to 4 smaller than expected from its bulge mass and the Marconi & Hunt relation.