Abstract
Abstract 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. 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 − 0.015 + 0.014 and q = c/a = 0.779 ± 0.012 for the intrinsic axis ratios and T = 0.33 ± 0.06 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.
Highlights
Elliptical galaxies exhibit a wide range of isophotal shapes and surface brightness profiles
When stellar kinematics from spectroscopic observations are combined with photometric information, stronger constraints can be placed on the intrinsic 3D shapes of elliptical galaxies (e.g., Binney 1985; Franx et al 1991)
In addition to these code changes, we introduce a new set of shape parameters in this paper (Section 3) that are chosen to improve the efficiency of parameter searches in triaxial galaxy shapes and orientations
Summary
Elliptical galaxies exhibit a wide range of isophotal shapes and surface brightness profiles. All published MBH measurements based on triaxial orbit modeling have been performed using the code initially presented in van den Bosch et al (2008) This code was first applied to determine the intrinsic shapes and MBH of two fast-rotating elliptical galaxies M32 and NGC 3379 (van den Bosch & de Zeeuw 2010). Other major changes include (i) modifying the acceleration table used for orbit integration to gain a significant speedup in runtime, (ii) resolving issues with insufficient orbit sampling that can result in spurious shape preferences, and (iii) using a more uniform mass binning scheme to eliminate frequent problems in satisfying mass constraints The resulting best-fit triaxial model is compared to the best-fit axisymmetric model from Liepold et al (2020)
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