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.