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We measure the black hole mass in the nearby active galaxy Centaurus A (NGC5128) using a new method based on spectroastrometry of a rotating gas disk. Thespectroastrometric approach consists in measuring the photocenter position ofemission lines for different velocity channels. In a previous paper we focusedon the basic methodology and the advantages of the spectroastrometric approachwith a detailed set of simulations demonstrating the possibilities for blackhole mass measurements going below the conventional spatial resolution. In thispaper we apply the spectroastrometric method to multiple longslit and integralfield near infrared spectroscopic observations of Centaurus A. We find that theapplication of the spectroastrometric method provides results perfectlyconsistent with the more complex classical method based on rotation curves: themeasured BH mass is nearly independent of the observational setup and spatialresolution and the spectroastrometric method allows the gas dynamics to beprobed down to spatial scales of ~0.02", i.e. 1/10 of the spatial resolutionand ~1/50 of BH sphere of influence radius. The best estimate for the BH massbased on kinematics of the ionized gas is then log(MBH (sin i)^2/M\odot)=7.5\pm 0.1 which corresponds to MBH = 9.6(+2.5-1.8) \times 10^7 M\odot for anassumed disk inclination of i = 35deg. The complementarity of this method withthe classic rotation curve method will allow us to put constraints on the diskinclination which cannot be otherwise derived from spectroastrometry. With theapplication to Centaurus A, we have shown that spectroastrometry opens up thepossibility of probing spatial scales smaller than the spatial resolution,extending the measured MBH range to new domains which are currently notaccessible: smaller BHs in the local universe and similar BHs in more distantgalaxies.

astronomy and astrophysics

Spectroastrometry of rotating gas disks for the detection of  supermassive black holes in galactic nuclei. II. Application to the galaxy  Centaurus A (NGC 5128)