Gemini Near Infrared Spectrograph Observations of the Central Supermassive Black Hole in Centaurus A

The Gemini Near Infrared Spectrograph on Gemini South unlocks new possibilities to study the central black holes (BHs) in dusty galaxies that have been inaccessible to previous BH studies. We exploit good near-infrared seeing to measure the central BH of Centaurus A (NGC 5128). We measure the stellar kinematics of NGC 5128 using the region around the CO band heads at 2.3 μm and determine the BH mass using axisymmetric orbit-based models. BHs are believed to be essential components of galaxies, and their evolutionary states appear to be closely linked to those of their hosts. Our current knowledge does not go much beyond this; galaxies such as NGC 5128 (a recent merger with an active galactic nucleus) can further develop this knowledge. However, NGC 5128 and galaxies like it contain large amounts of dust, which hamper optical spectroscopy, making near-infrared measurements an attractive alternative. We find a BH mass of 2.4^+0.3_-0.2×10⁸ M_solar for an edge-on model, 1.8^+0.4_-0.4×10⁸ M_solar for a model with an inclination of 45°, and 1.5^+0.3_-0.2×10⁸ M_solar for a model with an inclination of 20°. We adopt the value for the edge-on model, since it has significantly lower χ² however, it is unlikely that we can constrain the actual inclination of moderately triaxial NGC 5128 using an axisymmetric modeling procedure. These estimates, based on adjusting the asymmetric kinematics of NGC 5128 so that we can use an axisymmetric code, are consistent with the range of acceptable BH masses implied by a previous gas dynamical study. However, these estimates are 5-10 times higher than that predicted by the correlation between BH mass and velocity dispersion. If NGC 5128 will eventually follow the trend for quiescent galaxies, this result suggests that its BH assembled first before its host component. NGC 5128 thus provides an important example for our knowledge of central BHs; this technique can be applied to other such galaxies to further explore this question.