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 (~0 5) velocity fields from both ionized and molecular gas (Paβ, [Fe II], Brγ, 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 Paα feature previously revealed by HST /NICMOS imaging; (2) a ringlike system with a ~6'' 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 ° ) is M BH = 2 × 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.