The effect of non-sphericity on mass and anisotropy measurements in dSph galaxies with Schwarzschild method

TL;DR

This study examines how the non-spherical shape of dwarf spheroidal galaxies affects mass and anisotropy measurements using the Schwarzschild method, highlighting biases introduced by viewing angles and assumptions about density profiles.

Contribution

It demonstrates the impact of galaxy shape and viewing angle on mass and anisotropy estimates when applying the Schwarzschild method to non-spherical dwarf galaxies.

Findings

Mass profiles are accurately recovered along the longest axis if density is known.

Anisotropy is underestimated when the density profile is derived from the data.

Viewing angle significantly biases mass and anisotropy measurements.

Abstract

In our previous work we confirmed the reliability of the spherically symmetric Schwarzschild orbit-superposition method to recover the mass and velocity anisotropy profiles of spherical dwarf galaxies. Here we investigate the effect of its application to intrinsically non-spherical objects. For this purpose we use a model of a dwarf spheroidal galaxy formed in a numerical simulation of a major merger of two disky dwarfs. The shape of the stellar component of the merger remnant is axisymmetric and prolate which allows us to identify and measure the bias caused by observing the spheroidal galaxy along different directions, especially the longest and shortest principal axis. The modelling is based on mock data generated from the remnant that are observationally available for dwarfs: projected positions and line-of-sight velocities of the stars. In order to obtain a reliable tool while keeping the number of parameters low we parametrize the total mass distribution as a radius-dependent mass-to-light ratio with just two free parameters we aim to constrain. Our study shows that if the total density profile is known, the true, radially increasing anisotropy profile can be well recovered for the observations along the longest axis whereas the data along the shortest axis lead to the inference an incorrect, isotropic model. On the other hand, if the density profile is derived from the method as well, the anisotropy is always underestimated but the total mass profile is well recovered for the data along the shortest axis whereas for the longest axis the mass content is overestimated.