2001ApJ...549..915M

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 H₂) 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<sub>2</sub> 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<sub>☉</sub>/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<sub>BH</sub>=2<sup>+3.0</sup><sub>–1.4</sub>x10<sup>8</sup> M<sub>☉</sub>. The ringlike system is probably characterized by noncircular motions; a ``figure of 8'' pattern observed in the H₂ position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.