A kinematic study of the Andromeda dwarf spheroidal system

TL;DR

This study conducts a detailed kinematic analysis of 18 Andromeda dwarf spheroidal galaxies, revealing their dark matter properties, metallicities, and the effects of tidal interactions, and compares these with Milky Way counterparts.

Contribution

It provides the first homogeneous kinematic analysis of multiple M31 dSphs, including dark matter halo properties and metallicity trends, highlighting outliers likely affected by tidal forces.

Findings

Three outliers with large radii and low velocity dispersions.

And XXV's mass-to-light ratio suggests minimal dark matter presence.

Metallicity decreases with luminosity, consistent with Milky Way dSphs.

Abstract

We present a homogeneous kinematic analysis of red giant branch stars within 18 of the 28 Andromeda dwarf spheroidal (dSph) galaxies, obtained using the Keck I LRIS and Keck II DEIMOS spectrographs. Based on their g-i colors (taken with the CFHT MegaCam imager), physical positions on the sky, and radial velocities, we assign probabilities of dSph membership to each observed star. Using this information, the velocity dispersions, central masses and central densities of the dark matter halos are calculated for these objects, and compared with the properties of the Milky Way dSph population. We also measure the average metallicity ([Fe/H]) from the co-added spectra of member stars for each M31 dSph and find that they are consistent with the trend of decreasing [Fe/H] with luminosity observed in the Milky Way population. We find that three of our studied M31 dSphs appear as significant outliers in terms of their central velocity dispersion, And XIX, XXI and XXV, all of which have large half-light radii (>700 pc) and low velocity dispersions (sigma_v<5 km/s). In addition, And XXV has a mass-to-light ratio within its half-light radius of just [M/L]_{half}=10.3^{+7.0}_{-6.7}, making it consistent with a simple stellar system with no appreciable dark matter component within its 1 sigma uncertainties. We suggest that the structure of the dark matter halos of these outliers have been significantly altered by tides.