Schwarzschild dynamical model of the Fornax dwarf spheroidal galaxy

TL;DR

This paper constructs a detailed dynamical model of the Fornax dwarf galaxy using Schwarzschild orbit superposition, revealing a massive dark matter halo and near-isotropic stellar orbits, based on extensive kinematic data.

Contribution

It applies the Schwarzschild method to Fornax with the largest available data set, providing new insights into its mass distribution and orbital anisotropy without assuming orbital anisotropy.

Findings

Fornax is embedded in a massive dark matter halo.

Total mass within 1 kpc is approximately 1.25 x 10^8 solar masses.

The orbital anisotropy profile is close to isotropic across the galaxy.

Abstract

We present a full dynamical model of the Fornax dwarf spheroidal galaxy obtained with the spherically symmetric Schwarzschild orbit superposition method applied to the largest kinematic data set presently available. We modelled the total mass content of the dwarf with the mass-to-light ratio $\Upsilon$ varying with radius and found that Fornax is with high probability embedded in a massive and extended dark matter halo. We estimated the total mass contained within 1$\,$kpc to be $M(<1\,$kpc$)=1.25^{+0.06}_{-0.13} \times 10^8\,$M$_{\odot}$. The data are consistent with the constant mass-to-light ratio, i.e. the mass-follows-light model, only at 3$\sigma$ level, but still require a high value of $\Upsilon \approx 11.2\,$M$_{\odot}/$L$_{\odot}$. Our results are in general agreement with previous estimates of the dynamical mass profile of Fornax. As the Schwarzschild method does not require any assumptions on the orbital anisotropy of the stars, we obtained a profile of the anisotropy parameter $\beta$ as an output of our modelling. The derived anisotropy is close to zero in the centre of the galaxy and decreases with radius, but remains consistent with isotropic orbits at all radii at 1$\sigma$ confidence level.