The intrinsic ellipticity of dwarf spheroidal galaxies: constraints from the Andromeda system

TL;DR

This study investigates the true shapes of Andromeda's dwarf satellite galaxies, revealing they are generally elongated with an average axis ratio of about 1/2, and finds no significant shape difference between satellites in the thin plane and others.

Contribution

It provides the first detailed deprojection analysis of the intrinsic ellipticity distribution of Andromeda's dwarf satellites assuming prolate shapes.

Findings

Mean intrinsic axis ratio ~ 1/2

Approximately 10% are tidally disrupted remnants

No morphological difference between plane and non-plane satellites

Abstract

We present a study of the intrinsic deprojected ellipticity distribution of the satellite dwarf galaxies of the Andromeda galaxy, assuming that their visible components have a prolate shape, which is a natural outcome of simulations. Different possibilities for the orientation of the major axis of the prolate dwarf galaxies are tested, pointing either as close as possible to the radial direction towards the centre of Andromeda, or tangential to the radial direction, or with a random angle in the plane that contains the major axis and the observer. We find that the mean intrinsic axis ratio is ~ 1/2, with small differences depending on the assumed orientation of the population. Our deprojections also suggest that a significant fraction of the satellites, ~ 10%, are tidally disrupted remnants. We find that there is no evidence of any obvious difference in the morphology and major axis orientation between satellites that belong to the vast thin plane of co-rotating galaxies around Andromeda and those that do not belong to this structure.