Triaxial Orbit-based Dynamical Modeling of Galaxies with Supermassive
  Black Holes and an Application to Massive Elliptical Galaxy NGC 1453
  Most stellar-dynamical determinations of the masses of nearby supermassive
black holes (SMBHs) have been obtained with the orbit superposition technique
under the assumption of axisymmetry. However, few galaxies -- in particular
massive early-type galaxies -- obey exact axisymmetry. Here we present a
revised orbit superposition code and a new approach for dynamically determining
the intrinsic shapes and mass parameters of triaxial galaxies based on
spatially-resolved stellar kinematic data. The triaxial TriOS code described
here corrects an error in the original van den Bosch et al. (2008) code that
gives rise to incorrect projections for most orbits in triaxial models and can
significantly impact parameter search results. The revised code also contains
significant improvements in orbit sampling, mass constraints, and run time.
Furthermore, we introduce two new parameter searching strategies -- a new set
of triaxial shape parameters and a novel grid-free sampling technique -- that
together lead to a remarkable gain in efficiency in locating the best-fit
model. We apply the updated code and search method to NGC 1453, a fast-rotating
massive elliptical galaxy. A full 6D parameter search finds
$p=b/a=0.933^{+0.014}_{-0.015}$ and $q=c/a=0.779\pm0.012$ for the intrinsic
axis ratios and $T=0.33\pm0.06$ for the triaxiality parameter. Despite the
deviations from axisymmetry, the best-fit SMBH mass, stellar mass-to-light
ratio, and dark matter enclosed mass for NGC 1453 are consistent with the
axisymmetric results. More comparisons between axisymmetric and triaxial
modeling are needed before drawing general conclusions.
