Axisymmetric Schwarzschild models of an isothermal axisymmetric mock dwarf spheroidal galaxy

Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
e-mail: hagen@astro.rug.nl

Received: 27 June 2019
Accepted: 7 October 2019

Abstract

Aims. The goal of this work is to test the ability of Schwarzschild’s orbit superposition method to measure the mass content, scale radius, and shape of a flattened dwarf spheroidal galaxy. Until now, most dynamical model efforts have assumed that dwarf spheroidal galaxies and their host halos are spherical.

Methods. We used an Evans model (1993, MNRAS, 260, 191) to construct an isothermal mock galaxy whose properties somewhat resemble those of the Sculptor dwarf spheroidal galaxy. This mock galaxy contains flattened luminous and dark matter components, resulting in a logarithmic profile for the global potential. We tested whether the Schwarzschild method could constrain the characteristic parameters of the system for different sample sizes and whether this was possible without knowledge of the functional form of the potential.

Results. When assuming the true functional form of the potential of the system, the Schwarzschild modelling technique is able to provide an accurate and precise measurement of the characteristic mass parameter of the system and accurately reproduces the light distribution and the stellar kinematics of our mock galaxy. When assuming a different functional form for the potential of the model, such as a flattened Navarro–Frenk–White (NFW) profile, we also constrain the mass and scale radius to their corresponding values. However in both cases, we find that the flattening parameter remains largely unconstrained. This is likely because the information content of the velocity dispersion on the geometric shape of the potential is too small.

Conclusions. Our results using Schwarzschild’s method indicate that the mass enclosed can be derived reliably, even if the flattening parameter is unknown, and already for samples containing 2000 line-of-sight radial velocities, such as those currently available. Further applications of the method to more general distribution functions of flattened systems are needed to establish how well the flattening of dSph dark halos can be determined.