Integral-field spectroscopy of Centaurus A nucleus

We report integral-field spectroscopic observations with the Cambridge Infra-Red Panoramic Survey Spectrograph (CIRPASS) mounted on the GEMINI South telescope of the nucleus of the nearby galaxy NGC 5128 (Centaurus A). We detect two-dimensional distributions of the following emission-lines: [PII], [FeII] and Paschen β. We compare our observations with previously published radio observations (VLA) and archival space-based near-infrared imaging (HST/NICMOS) and find similar features, as well as a region of high continuum coinciding with the jet (and its N1 knot) at about 2 North-East of the nucleus, possibly related to jetinduced star formation. We use the [FeII]/[PII] ratio to probe the ionisation mechanism, which suggests that with increasing radius shocks play an increasingly important role. We extract spatially resolved 2D kinematics of Paβ and [FeII] emission-lines. All emission-line regions are part of the same kinematic structure which shows a twist in the zero-velocity curve beyond ∼ 1 ′′ (for both Paβ and [FeII]). The kinematics of the two emission-lines are similar, but the Paβ velocity gradient is steeper in the centre while the velocity dispersion is low everywhere. The velocity dispersion of the [FeII] emission is relatively high featuring a plateau, approximately oriented in the same way as the central part of the warped disk. We use 2D kinematic information to test the hypothesis that the ionised gas is distributed in a circularly rotating disk. Assuming simple disk geometry we estimate the mass of the central black hole using Paβ kinematics, which is consistent with being distributed in a circularly rotating disk. We obtain M• = 8.25 +2.25 −4.25×10 7 M⊙, for PA = −3 and i = 25, excluding the M•−σ relation prediction at a 3σ confidence level, which is in good agreement with previous studies.