THE BLACK HOLE MASS IN THE BRIGHTEST CLUSTER GALAXY NGC 6086

We present the first direct measurement of the central black hole mass, M., in NGC 6086, the Brightest Cluster Galaxy (BCG) in A2162. Our investigation demonstrates for the first time that stellar-dynamical measurements of M. 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. and the R-band stellar mass-to-light ratio, M*/LR . We find M. = 3.6+1.7-1.1 x 109 Msun and M*/LR = 4.6+0.3-0.7 Msun Lsun-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. ∼ 3 x 109 Msun and M*/LR ∼ 4 Msun Lsun-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 109 Msun, and the best-fit stellar mass-to-light ratio to increase to 6.7 Msun L -1sun,R. The latter value is at further odds with stellar population studies favoring M*/LR ∼ 2 Msun L -1sun. Biases from dark matter omission could extend to dynamical models of other galaxies with stellar cores, and revised measurements of M. could steepen the empirical scaling relationships between black holes and their host galaxies.