Vlt Diffraction Limited Imaging and Spectroscopy in the Nir: Weighing the Black Hole in Centaurus a with Naco

We present high spatial resolution near-infrared spectra and images of the nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. The adaptive optics corrected data have a spatial resolution of 0. 06 (FWHM) in Kand 0. 11 in H-band, four times higher than previous studies. The mean velocities and velocity dispersions of the ionized gas ([FeII]) are mapped along four slit positions. The [FeII] emission line width decreases from a central value of σ ≃ 320 km s to σ ≃ 120 km s at r ≃ 0. 5, and exceeds the mean rotation within this radial range. The observed gas motions suggest a kinematically hot disk which is orbiting a central object and is oriented nearly perpendicular to the nuclear jet. We model the central rotation and velocity dispersion curves of the [FeII] gas orbiting in the combined potential of the stellar mass and the (dominant) black hole. Our physically most plausible model, a dynamically hot and geometrically thin gas disk, yields a black hole mass of Mbh = 6.1 +0.6 −0.8 × 10M⊙. As the physical state of the gas is not well understood, we also consider two limiting cases: first a cold disk model, which completely neglects the velocity dispersion, but is in line with many earlier gas disk models; it yields an Mbh estimate that is almost two times lower. The other extreme case is to model a spherical gas distribution in hydrostatic equilibrium through Jeans equation. Compared to the hot disk model the best-fit black hole mass increases by a factor of 1.5. This wide mass range spanned by the limiting cases shows how important the gas physics is even for high resolution data. Our overall best-fitting black hole mass is a factor of 2-4 lower than previous measurements. A substantially lower Mbh estimate when using higher resolution kinematics was also found for many other black hole mass measurements as HST data became available. With our revised Mbh estimate, Cen A’s offset from the Mbh-σ relation is significantly reduced; it falls above this relation by a factor of ∼ 2, which is close to the intrinsic scatter of this relation. Subject headings: black holes: general —galaxies: individual(NGC 5128)