VLT DIFFRACTION-LIMITED IMAGING AND SPECTROSCOPY IN THE NIR: WEIGHING THE BLACK HOLE IN CENTAURUS A WITH NACO

摘要

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 K and 0.11″ in H band, 4 times higher than previous studies. The kinematics of the ionized gas ([Fe Ⅱ]) are mapped along four slit positions. The observed gas motions suggest a kinematically hot disk that 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 [Fe Ⅱ] gas 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 M_(bh) = 6.1_(-0.8)~(+0.6) x 10~7 solar mass. 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 M_(bh) estimate that is almost 2 times lower. Second, a spherical gas distribution in hydrostatic equilibrium via the Jeans equation; 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 M_(bh) estimate when using higher resolution kinematics was also found for many other black hole mass measurements as HST data became available. With our revised M_(bh) estimate, Cen A's offset from the M_(bh)-σ relation is significantly reduced.