Peering through the dust: Evidence for a supermassive Black Hole at the nucleus of Centaurus A from VLT IR spectroscopy

Abstract

We used the near infrared spectrometer ISAAC at the ESO &#39;Very Large Telescope&#39; to map the velocity field of Centaurus A (NGC 5128) at several position angles and locations in the central 20&#34; of the galaxy. The high spatial resolution (~0.5&#34;) velocity fields from both ionized and molecular gas (PaBeta, [FeII], BrGamma, and H2) are not compromised by either excitation effects or obscuration. We identify three distinct kinematical systems: (i) a rotating &#39;nuclear disk&#39; of ionized gas, confined to the inner 2&#34;, the counterpart of the PaAlpha feature previously revealed by HST/NICMOS imaging; (ii) a ring-like system with a ~6&#34; inner radius detected only in H2, likely the counterpart of the 100pc-scale structure detected in CO by other authors; (iii) 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 Msun/LsunK) and point-like at the spatial resolution of the data (R<0.25&#34; ~4pc). We interpret this mass concentration as a supermassive black hole. Its dynamical mass based on the line velocities and disk inclination (i>15deg) is M=2(+3.0;-1.4) 10^8 Msun. The ring-like system is probably characterized by non-circular motions; a &#39;figure-of-8&#39; pattern observed in the H2 position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.