The benchmark black hole in NGC 4258: dynamical models from
  high-resolution two-dimensional stellar kinematics
  NGC 4258 is the galaxy with the most accurate (maser-based) determination for
the mass of the supermassive black hole (SMBH) in its nucleus. In this work we
present a two-dimensional mapping of the stellar kinematics in the inner 3.0 x
3.0 arcsec = 100 x 100 pc of NGC 4258 using adaptative-optics observations
obtained with the Near-Infrared Integral Field Spectrograph of the GEMINI North
telescope at a 0.11 arcsec (4 pc) angular resolution. The observations resolve
the radius of influence of the SMBH, revealing an abrupt increase in the
stellar velocity dispersion within 10 pc from the nucleus, consistent with the
presence of a SMBH there. Assuming that the galaxy nucleus is in a steady state
and that the velocity dispersion ellipsoid is aligned with a cylindrical
coordinate system, we constructed a Jeans anisotropic dynamical model to fit
the observed kinematics distribution. Our dynamical model assumes that the
galaxy has axial symmetry and is constructed using the multi-gaussian expansion
method to parametrize the observed surface brightness distribution. The Jeans
dynamical model has three free parameters: the mass of the central SMBH, the
mass-luminosity ratio of the galaxy and the anisotropy of the velocity
distribution. We test two types of models: one with constant velocity
anisotropy, and another with variable anisotropy. The model that best
reproduces the observed kinematics was obtained considering that the galaxy has
radially varying anisotropy, being the best-fitting parameters with 3$\sigma$
significance $M_\bullet=4.8^{+0.8}_{-0.9}\times 10^7\,{\rm M_\odot}$ and
$\Gamma_k = 4.1^{+0.4}_{-0.5}$. This value for the mass of the SMBH is just 25
per cent larger than that of the maser determination and 50 per cent larger
that a previous stellar dynamical determination obtained via Schwarzschild
models.
