Orbit-based dynamical models of the Sculptor dSph galaxy

TL;DR

This paper develops orbit-based dynamical models for the Sculptor dwarf galaxy, providing robust estimates of its dark matter distribution and orbital structure, and compares different density profiles to understand its core or cusp nature.

Contribution

The study introduces a Schwarzschild orbit superposition method for dwarf spheroidal galaxies, improving mass and orbital structure constraints over traditional Jeans models.

Findings

Mass within 1 kpc is approximately 1.03 x 10^8 solar masses.

Velocity anisotropy is tangentially biased and nearly constant.

Central density slope is consistent with a core or shallow cusp, disfavoring steep cusps.

Abstract

We have developed spherically symmetric dynamical models of dwarf spheroidal galaxies using Schwarzschild's orbit superposition method. This type of modelling yields constraints both on the total mass distribution (e.g. enclosed mass and scale radius) as well as on the orbital structure of the system (e.g. velocity anisotropy). This method is thus less prone to biases introduced by assumptions in comparison to the more commonly used Jeans modelling, and it allows us to derive the dark matter content in a robust way. Here we present our results for the Sculptor dwarf spheroidal galaxy, after testing our methods on mock data sets. We fit both the second and fourth velocity moment profile to break the mass-anisotropy degeneracy. For an NFW dark matter halo profile, we find that the mass of Sculptor within 1 kpc is M_1kpc = 1.03 \pm 0.07 x 10^8 Msol, and that its velocity anisotropy profile is tangentially biased and nearly constant with radius. The preferred concentration (c ~ 15) is low for its dark matter mass but consistent within the scatter found in N-body cosmological simulations. When we let the value of the central logarithmic slope \alpha{} vary, we find that the best-fit model has \alpha{} = 0, although an NFW cusp or shallower is consistent at 1 \sigma{} confidence level. On the other hand, very cuspy density profiles with logarithmic central slopes \alpha{} < -1.5 are strongly disfavoured for Sculptor.