Integral-field spectroscopy of Centaurus A nucleus

Abstract

[abridged] We report integral-field spectroscopic observations with 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 beta. 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 arcsec North-East of the nucleus, possibly related to jet-induced 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 Pa beta 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 arcsec (for both Pa beta and [FeII]). The kinematics of the two emission-lines are similar, but the Pa beta 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. We estimate the mass of the central black hole using Pa beta kinematics, which is consistent with being distributed in a circularly rotating disk. We obtain Mbh = 8.25^{+2.25}_{-4.25} * 10^7 Msun, for PA = -3 degr and i=25 degr, which is in good agreement with previous studies.