Can filamentary accretion explain the orbital poles of the Milky Way satellites?

TL;DR

This paper introduces a new method to analyze satellite galaxy orbital poles and finds that filamentary accretion models are unlikely to explain the Milky Way's satellite distribution, whereas galaxy interaction models are more consistent.

Contribution

A novel analysis technique for orbital pole data is presented, demonstrating that tidal dwarf galaxy formation from galaxy interactions better explains the Milky Way's satellite distribution.

Findings

Filamentary accretion models have less than 0.5% likelihood to match observations.

Isotropic angular momentum distributions are more likely than filamentary models to produce observed poles.

Galaxy interaction models can reproduce the observed satellite orbital pole distribution with up to 90% probability.

Abstract

Several scenarios have been suggested to explain the phase-space distribution of the Milky Way (MW) satellite galaxies in a disc of satellites (DoS). To quantitatively compare these different possibilities, a new method analysing angular momentum directions in modelled data is presented. It determines how likely it is to find sets of angular momenta as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles. The method can be easily applied to orbital pole data from different models. The observed distribution of satellite orbital poles is compared to published angular momentum directions of subhalos derived from six cosmological state-of-the-art simulations in the Aquarius project. This tests the possibility that filamentary accretion might be able to naturally explain the satellite orbits within the DoS. For the most likely alignment of main halo and MW disc spin, the probability to reproduce the MW satellite orbital pole properties turns out to be less than 0.5 per cent in Aquarius models. Even an isotropic distribution of angular momenta has a higher likelihood to produce the observed distribution. The two Via Lactea cosmological simulations give results similar to the Aquarius simulations. Comparing instead with numerical models of galaxy-interactions gives a probability of up to 90 per cent for some models to draw the observed distribution of orbital poles from the angular momenta of tidal debris. This indicates that the formation as tidal dwarf galaxies in a single encounter is a viable, if not the only, process to explain the phase-space distribution of the MW satellite galaxies.