Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei

II. Application to the galaxy Centaurus A (NGC 5128)

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Firenze, Italy
e-mail: gnerucci@arcetri.astro.it; marconi@arcetri.astro.it
² INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: capetti@oato.inaf.it
³ Physics Department, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA
e-mail: djasps@rit.edu; axrsps@rit.edu
⁴ School of Mathematical & Physical Sciences, University of Sussex, Falmer, Brighton, BN2 9BH, UK
⁵ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching bei München, Germany
e-mail: nadine.neumayer@universe-cluster.de
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: nneumaye@eso.org

Received: 1 June 2011
Accepted: 30 September 2011

Abstract

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 spectroastrometric method to multiple longslit and integral field near infrared spectroscopic observations of Centaurus A. We find that the application of the 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 BH mass based on kinematics of the ionised gas is then log (M_BH sin i²/M_⊙) ≃ 7.5 ± 0.1 which corresponds to M_BH= 9.6_-1.8^+2.5 × 10⁷ 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 the measured M_BH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.