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G. A. Bower (NOAO/KPNO), A. S. Wilson (U. Maryland), T. M. Heckman (JHU), J. Magorrian (Durham, UK), K. Gebhardt (U. Texas), D. O. Richstone (U. Michigan), B. M. Peterson (Ohio State Univ.), R. F. Green (NOAO/KPNO)
M81 and NGC 3998 are nearby galaxies with mildly active nuclei, which exhibit LINER-type optical spectra including broad permitted lines. Their ionization structure suggests that the nuclear gas is photoionized by a nonstellar nuclear source, presumably a supermassive black hole. For both galaxies, we have obtained ground-based and HST/STIS long-slit spectroscopy, from which we are analyzing the nuclear stellar dynamics to search for dynamical evidence of supermassive black holes.
Both galaxies show very high central velocity dispersions (\sigma of 550 km~s-1 for M81 and 670 km~s-1 for NGC 3998). After measuring the stellar kinematics from the data, we have constructed axisymmetric dynamical models assuming two integrals of motion (orbital energy and angular momentum). The models for each galaxy point to the presence of a central dark compact object (presumably a supermassive black hole) with > 99.5% confidence. The best-fit masses are 6 \times 107 \ M\odot for M81 and 6 \times 108 \ M\odot for NGC~3998, with internal error of 20% and 10%, respectively. We are refining these measurements by constructing axisymmetric three integral models utilizing an orbit-based technique.
Support for this work was provided by NASA through grant GO-7350 from STScI. Ground-based observations were obtained from the Michigan Dartmouth MIT Observatory.