Estimating Black Hole Masses in Triaxial Galaxies

TL;DR

This study uses advanced triaxial models to reassess black hole masses in galaxies, revealing that galaxy shape assumptions significantly influence mass estimates, especially in triaxial galaxies.

Contribution

It introduces fully general triaxial orbit-based models to evaluate black hole masses, highlighting the impact of galaxy shape assumptions on previous estimates.

Findings

M32's black hole mass remains consistent with previous axisymmetric models.

In NGC 3379, including triaxiality doubles the black hole mass estimate.

Triaxiality can significantly affect black hole demographics in massive galaxies.

Abstract

Most of the super massive black hole mass estimates based on stellar kinematics use the assumption that galaxies are axisymmetric oblate spheroids or spherical. Here we use fully general triaxial orbit-based models to explore the effect of relaxing the axisymmetric assumption on the previously studied galaxies M32 and NGC 3379. We find that M32 can only be modeled accurately using an axisymmetric shape viewed nearly edge-on and our black hole mass estimate is identical to previous studies. When the observed 5 degrees kinematical twist is included in our model of NGC 3379, the best shape is mildly triaxial and we find that our best-fitting black hole mass estimate doubles with respect to the axisymmetric model. This particular black hole mass estimate is still within the errors of that of the axisymmetric model and consistent with the M-sigma relationship. However, this effect may have a pronounced impact on black hole demography, since roughly a third of the most massive galaxies are strongly triaxial.