THE BLACK HOLE MASS IN M87 FROM GEMINI/NIFS ADAPTIVE OPTICS OBSERVATIONS

Abstract

We present the stellar kinematics in the central 2" of the luminous\nelliptical galaxy M87 (NGC 4486), using laser adaptive optics to feed the\nGemini telescope integral-field spectrograph, NIFS. The velocity dispersion\nrises to 480 km/s at 0.2". We combine these data with extensive stellar\nkinematics out to large radii to derive a black-hole mass equal to\n(6.6+-0.4)x10^9 Msun, using orbit-based axisymmetric models and including only\nthe NIFS data in the central region. Including previously-reported ground-based\ndata in the central region drops the uncertainty to 0.25x10^9 Msun with no\nchange in the best-fit mass; however, we rely on the values derived from the\nNIFS-only data in the central region in order to limit systematic differences.\nThe best-fit model shows a significant increase in the tangential velocity\nanisotropy of stars orbiting in the central region with decreasing radius;\nsimilar to that seen in the centers of other core galaxies. The black-hole mass\nis insensitive to the inclusion of a dark halo in the models --- the high\nangular-resolution provided by the adaptive optics breaks the degeneracy\nbetween black-hole mass and stellar mass-to-light ratio. The present black-hole\nmass is in excellent agreement with the Gebhardt & Thomas value, implying that\nthe dark halo must be included when the kinematic influence of the black hole\nis poorly resolved. This degeneracy implies that the black-hole masses of\nluminous core galaxies, where this effect is important, may need to be\nre-evaluated. The present value exceeds the prediction of the black\nhole-dispersion and black hole-luminosity relations, both of which predict\nabout 1x10^9 Msun for M87, by close to twice the intrinsic scatter in the\nrelations. The high-end of the black hole correlations may be poorly determined\nat present.\n