A 40-billion solar mass black hole in the extreme core of Holm 15A, the
  central galaxy of Abell 85
  Holm 15A, the brightest cluster galaxy (BCG) of the galaxy cluster Abell 85,
has an ultra-diffuse central region, 2 mag fainter than the faintest depleted
core of any early-type galaxy (ETG) that has been dynamically modelled in
detail. We use orbit-based, axisymmetric Schwarzschild models to analyse the
stellar kinematics of Holm 15A from new high-resolution, wide-field spectral
observations obtained with MUSE at the VLT. We find a supermassive black hole
(SMBH) with a mass of (4.0 +- 0.80) x 10^10 solar masses at the center of Holm
15A. This is the most massive black hole with a direct dynamical detection in
the local universe. We find that the distribution of stellar orbits is
increasingly biased towards tangential motions inside the core. However, the
tangential bias is less than in other cored elliptical galaxies. We compare
Holm 15A with N-body simulations of mergers between galaxies with black holes
and find that the observed amount of tangential anisotropy and the shape of the
light profile are consistent with a formation scenario where Holm 15A is the
remnant of a merger between two ETGs with pre-existing depleted cores. We find
that black hole masses in cored galaxies, including Holm 15A, scale inversely
with the central stellar surface brightness and mass density, respectively.
These correlation are independent of a specific parameterization of the light
profile.
