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

We measure the black hole mass in the nearby active galaxy Centaurus A (NGC 5128) using a new method based on spectroastrometry of a rotating gas disk. The spectroastrometric approach consists in measuring the photocenter position of emission lines for different velocity channels. In a previous paper we focused on the basic methodology and the advantages of the spectroastrometric approach with a detailed set of simulations demonstrating the possibilities for black hole mass measurements going below the conventional spatial resolution. In this paper we apply the spectroastromet ric method to multiple longslit and integral field near infra red spectroscopic observations of Centaurus A. We find that the application of t he spectroastrometric method provides results perfectly consistent with the more complex classical method based on rotation curves: the measured BH mass is nearly independent of the observational setup and spatial resolution and the spectroastrometric method allows the gas dynamics to be probed down to spatial scales of∼ 0.02 ′′ , i.e. 1/10 of the spatial resolution and∼ 1/50 of BH sphere of influence radius. The best estimate for the B H mass based on kinematics of the ionised gas is then log( MBH sin i 2 /M⊙)≃ 7.5± 0.1 which corresponds to MBH = 9.6 +2.5 −1.8 × 10 7 M⊙ for an assumed disk inclination of i = 35 ◦ . The complementarity of this method with the classic rotation curve method will allow us to put constraints on the disk inclination which cannot be otherwise derived from spectroastrometry. With the application to Centaurus A, we have shown that spectroastrometry opens up the possibility of probing spatial scales smaller than the spatial resolution, extending t he measured MBH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.