Galactic Tides and the Shape and Orientation of Dwarf Galaxy Satellites

TL;DR

This study uses cosmological simulations to examine how tidal forces from dark matter halos influence the shape and orientation of dwarf galaxy satellites, revealing that tides tend to make them more spherical and radially aligned.

Contribution

It provides new insights into the transient effects of tidal forces on subhalo morphology and orientation, linking these effects to observable properties of dwarf spheroidal galaxies.

Findings

Tidal effects are short-lived, making subhaloes more spherical.

Subhaloes show strong radial alignment toward host halo centers.

Evidence suggests tides influence satellite galaxy evolution, especially in M31.

Abstract

We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a dark matter halo on the shape and orientation of its substructure. Although tides are often assumed to enhance asphericity and to stretch subhaloes tangentially, these effects are short lived: as in earlier work, we find that subhaloes affected by tides become substantially more spherical and show a strong radial alignment toward the centre of the host halo. These results, combined with a semi-analytic model of galaxy formation, may be used to assess the effect of Galactic tides on the observed population of dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda galaxies. If, as the model suggests, the relatively low dark matter content of luminous dSphs such as Fornax and Leo I is due to tidal stripping, then their gravitational potential must be substantially more spherical than that of more heavily dark matter-dominated systems such as Draco or Carina. The model also predicts a tidally-induced statistical excess of satellites whose major axis aligns with the direction to the central galaxy. We find tantalizing evidence of this in the M31 satellite population, which suggests that tides may have played an important role in its evolution.