Peering through the dust: Evidence for a supermassive black hole at the nucleus of Centaurus A from VLT infrared spectroscopy

We used the near-infrared spectrometer ISAAC at the ESO Very Large Telescope to map the velocity field of Cen A (NGC 5128) at several position angles and locations in the central 20 of the galaxy. The high spatial resolution (similar to0.5) velocity fields from both ionized and molecular gas (Pa beta, [Fe II], Br gamma, and H-2) are not compromised by either excitation effects or obscuration. We identify three distinct kinematical systems: (1) a rotating nuclear disk of ionized gas, confined to the inner 2, the counterpart of the Pa alpha feature previously revealed by HST/NICMOS imaging; (2) a ringlike system with a similar to6 inner radius detected only in H-2 likely the counterpart of the 100 pc scale structure detected in CO by other authors; (3) a normal extended component of gas rotating in the galactic potential. The nuclear disk is in Keplerian rotation around a central mass concentration, dark (M/L > 20 M-./L-K.) and pointlike at the spatial resolution of the data (R < 0.25-4 pc). We interpret this mass concentration as a supermassive black hole. Its dynamical mass based on the line velocities and disk inclination (i > 15 degrees) is M-BH = 2(-1.4)(+3.0) x 10(8) M.. The ringlike system is probably characterized by noncircular motions; a figure of 8 pattern observed in the H-2 position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.