A quartet of black holes and a missing duo: probing the low end of the MBH-σ relation with the adaptive optics assisted integral-field spectroscopy

We present mass estimates of supermassive black holes in six nearby fast rotating early-type galaxies (NGC4339, NGC4434, NGC4474, NGC4551, NGC4578, and NGC4762) with effective stellar velocity dispersion around 100kms(-1). We use near-infrared laser-guide adaptive optics observations with the GEMINI/NIFS to derive stellar kinematics in the galactic nuclei, and SAURON observations from the ATLAS(3D) Survey for large-scale kinematics. We build axisymmetric Jeans anisotropic models and axisymmetric Schwarzschild dynamical models. Both modelling approaches recover consistent orbital anisotropies and black hole masses within 1 sigma-2 sigma confidence level, except for one galaxy for which the difference is just above the 3 sigma level. Two black holes (NGC4339 and NGC4434) are amongst the largest outliers from the current black hole massvelocity dispersion relation, with masses of (4.3(-2.3)(+4.8))x10(7) and (7.0(-2.8)(+2.0))x10(7) M-circle dot, respectively (3 sigma confidence level). The black holes in NGC4578 and NGC4762 lie on the scaling relation with masses of (1.9(-1.4)(+0.6))x10(7) and (2.3(-0.6)(+0.9))x10(7) M-circle dot, respectively (3 sigma confidence level). For two galaxies (NGC4474 and NGC4551), we are able to place upper limits on their black holes masses (<7 x 10(6) and <5 x 10(6) M-circle dot, respectively, 3 sigma confidence level). The kinematics for these galaxies clearly indicate central velocity dispersion drops within a radius of 35 and 80pc, respectively. These drops cannot be associated with cold stellar structures and our data do not have the resolution to exclude black holes with masses an order of magnitude smaller than the predictions. Parametrizing the orbital distribution in spherical coordinates, the vicinity of the black holes is characterized by isotropic or mildly tangential anisotropy.