Structural properties of non-spherical dark halos in Milky Way and Andromeda dwarf spheroidal galaxies

TL;DR

This study models the non-spherical dark matter halos of dwarf spheroidal galaxies in the Milky Way and Andromeda, revealing they are generally elongated and highlighting the need for more extensive data to refine these structural estimates.

Contribution

It introduces axisymmetric mass models that incorporate stellar velocity anisotropy, providing new insights into the shapes of dark halos in dwarf spheroidals.

Findings

Dark halos are generally elongated in dSphs.

Shape estimates are sensitive to data coverage and sample size.

More diffuse halos may correlate with prolonged star formation.

Abstract

We investigate the non-spherical density structure of dark halos of the dwarf spheroidal (dSph) galaxies in the Milky Way and Andromeda galaxies based on revised axisymmetric mass models from our previous work. The models we adopt here fully take into account velocity anisotropy of tracer stars confined within a flattened dark halo. Applying our models to the available kinematic data of the 12 bright dSphs, we find that these galaxies associate with, in general, elongated dark halos, even considering the effect of this velocity anisotropy of stars. We also find that the best-fit parameters, especially for the shapes of dark halos and velocity anisotropy, are susceptible to both the availability of velocity data in the outer regions and the effect of the lack of sample stars in each spatial bin. Thus, to obtain more realistic limits on dark halo structures, we require photometric and kinematic data over much larger areas in the dSphs than previously explored. The results obtained from the currently available data suggest that the shapes of dark halos in the dSphs are more elongated than those of $\Lambda$CDM subhalos. This mismatch needs to be solved by theory including baryon components and the associated feedback to dark halos as well as by further observational limits in larger areas of dSphs. It is also found that more diffuse dark halos may have undergone consecutive star formation history, thereby implying that dark-halo structure plays an important role in star formation activity.