THE BLACK HOLE MASS IN THE BRIGHTEST CLUSTER GALAXY NGC 6086

We present the first direct measurement of the central black hole mass, M-center dot, in NGC 6086, the Brightest Cluster Galaxy (BCG) in A2162. Our investigation demonstrates for the first time that stellar-dynamical measurements of M-center dot in BCGs are possible beyond the nearest few galaxy clusters. We observed NGC 6086 with laser guide star adaptive optics and the integral-field spectrograph (IFS) OSIRIS at the W. M. Keck Observatory and with the seeing-limited IFS GMOS-N at Gemini Observatory North. We combined the IFS data sets with existing major-axis kinematics and used axisymmetric stellar orbit models to determine M-center dot and the R-band stellar mass-to-light ratio, M-*/LR. We find M-center dot = 3.6(-1.1)(+1.7) x 10(9) M-circle dot and M-*/L-R = 4.6(-0.7)(+0.3) M-circle dot L-circle dot(-1) (68% confidence) from models using the most massive dark matter halo allowed within the gravitational potential of the host cluster. Models fitting only IFS data confirm M-center dot similar to 3x10(9) M circle dot and M-*/L-R similar to 4M(circle dot) L-circle dot(-1), with weak dependence on the assumed dark matter halo structure. When data out to 19 kpc are included, the unrealistic omission of dark matter causes the best-fit black hole mass to decrease dramatically, to 0.6 x 10(9) M-circle dot, and the best-fit stellar mass-to-light ratio to increase to 6.7 M circle dot L-(c),R(-1) . The latter value is at further odds with stellar population studies favoring M-*/L-R similar to 2M(circle dot) L-circle dot(-1) . Biases from dark matter omission could extend to dynamical models of other galaxies with stellar cores, and revised measurements of M-center dot could steepen the empirical scaling relationships between black holes and their host galaxies.