The supermassive black hole of Fornax A

Abstract

The radio galaxy Fornax A (NGC 1316) is a prominent merger remnant in the outskirts of the Fornax cluster. Its giant radio lobes suggest the presence of a powerful active galactic nuclei (AGN) and thus a central supermassive black hole (SMBH). Fornax A now seems to be in a transition state between active black hole growth and quiescence, as indicated by the strongly declined activity of the nucleus. Studying objects in this evolutionary phase is particularly important in order to understand the link between bulge formation and black hole growth, which is manifested in the M•-σ relation between black hole mass and bulge velocity dispersion. So far, a measurement of the SMBH mass has not been possible in Fornax A, as it is enshrouded in dust which makes optical measurements impossible. We present high-resolution adaptive optics assisted integral-field data of Fornax A, taken with SINFONI at the Very Large Telescope in the K band, where the influence of dust is negligible. The achieved spatial resolution is 0.085 arcsec, which is about a fifth of the diameter of the expected sphere of influence of the black hole. The stellar kinematics was measured using the region around the CO bandheads at 2.3 μm. Fornax A does not rotate inside the inner ∼3 arcsec. The velocity dispersion increases towards the centre. The weak AGN emission affects the stellar kinematics in the inner ∼0.06 arcsec only. Beyond this radius, the stellar kinematics appears relaxed in the central regions. We use axisymmetric orbit models to determine the mass of the SMBH in the centre of Fornax A. The three-dimensional nature of our data provides the possibility to directly test the consistency of the data with axisymmetry by modelling each of the four quadrants separately. According to our dynamical models, consistent SMBH masses M• and dynamical Ks-band mass-to-light ratios Υ are obtained for all quadrants, with 〈M•〉 = 1.3 × 108 M ⊙ [rms(M•) = 0.4 × 108 M ⊙] and 〈Υ〉 = 0.68 [rms(Υ) = 0.03], confirming the assumption of axisymmetry. For the folded and averaged data, we find M• = 1.5+0.75-0.8 × 10 8 M⊙ and Υ = 0.65+0.075-0.05 (3σ errors). Thus, the black hole mass of Fornax A is consistent within the error with the Tremaine et al. M•-σ relation, but is a factor of ∼4 smaller than expected from its bulge mass and the Marconi & Hunt relation. © 2008 RAS.