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

The infrared spectrograph GNIRS on Gemini South unlocks new possibilities tostudy the central black holes in dusty galaxies that have been inaccessible toprevious black hole studies. We exploit good near-infrared seeing to measurethe central black hole (BH) of Centaurus A (NGC 5128). We measure the stellarkinematics of NGC 5128 using the region around the CO bandheads at 2.3 micronsand determine the black hole mass using orbit-based models. Black holes arebelieved to be essential components of galaxies, and their evolutionary statesappear to be closely linked to those of their hosts. Our current knowledge doesnot go much beyond this; galaxies such as NGC 5128 (an AGN and recent merger)can further develop this knowledge. However, NGC 5128 and galaxies like itcontain large amounts of dust which hamper optical spectroscopy, makingnear-infrared measurements an attractive alternative. We find a BH mass of2.4e8 solar masses for an edge-on model, 1.8e8 solar masses for a model withinclination of 45 degrees, and 1.5e8 solar masses for a model with inclinationof 20 degrees. We adopt the value for the edge-on model, which hassignificantly lower chi^2. These estimates are consistent with a previous gasdynamical estimate and are five to ten times higher than that predicted by thecorrelation between BH mass and velocity dispersion. If NGC 5128 willeventually follow the trend for quiescent galaxies, this result suggests thatits BH assembled first before its host component. NGC 5128 thus provides animportant example for our knowledge of central black holes; this technique canbe applied to other such galaxies to further explore this question.Comment: 12 pages, 11 figures, to appear in August 2005 issue of AJ (130, 406