Probing non-spherical dark halos in the Galactic dwarf galaxies

TL;DR

This study develops axisymmetric models for dwarf spheroidal galaxies to explore the non-spherical shapes of their dark matter halos, revealing they are often oblate and more flattened than previously predicted by spherical models.

Contribution

It introduces a method to model non-spherical dark halos in dwarf galaxies, considering velocity anisotropy and inclination, providing new insights into their shapes and mass distributions.

Findings

Dark halos are often oblate and flattened.

Mass within 300 pc varies from 10^6 to 10^7 solar masses.

Dark halos may be more flattened than N-body predictions.

Abstract

We construct axisymmetric mass models for dwarf spheroidal (dSph) galaxies in the Milky Way to obtain plausible limits on the non-spherical structure of their dark halos. This is motivated by the fact that the observed luminous parts of the dSphs are actually non-spherical and Cold Dark Matter (CDM) models predict non-spherical virialized dark halos. Our models consider velocity anisotropy of stars $\bar{v^2_R} / \bar{v^2_{\phi}}$, which can vary with the adopted cylindrical coordinates under the assumption $\bar{v^2_z}=\bar{v^2_R}$ for simplicity, and also include an inclination of the system as a fitting parameter to explain the observed line-of-sight velocity dispersion profile. Applying these models to six of the bright dSphs in the Milky Way, we find that the best-fitting cases for most of the dSphs yield oblate and flattened dark halos, irrespective of assumed density profiles in their central parts. We also find that the total mass of the dSphs enclosed within a spheroid with major-axis length of 300 pc varies from $10^6M_{\odot}$ to $10^7M_{\odot}$, contrary to the conclusion from spherical models. This suggests the importance of considering shapes of dark halos in mass models of the dSphs. It is also found that dark halos of the Galactic dSphs may be more flattened than N-body predictions, thereby implying our yet incomplete understanding of baryonic and/or non-baryonic dark matter physics in dwarf galaxy scales.