Using dwarf satellite proper motions to determine their origin

TL;DR

This study investigates whether the proper motions of certain Milky Way dwarf satellites support the filamentary infall hypothesis, finding that current data and models challenge this scenario and suggest an earlier infall time.

Contribution

The paper tests the filamentary infall hypothesis using proper motions and orbit modeling, revealing it is unlikely given current measurements and models.

Findings

Fornax's proper motion data is inconsistent with filamentary infall.

Other satellites require very close pericenters to fit the model.

Dwarfs likely fell into the Milky Way over 5 Gyrs ago.

Abstract

The highly organised distribution of satellite galaxies surrounding the Milky Way is a serious challenge to the concordance cosmological model. Perhaps the only remaining solution, in this framework, is that the dwarf satellite galaxies fall into the Milky Way's potential along one or two filaments, which may or may not plausibly reproduce the observed distribution. Here we test this scenario by making use of the proper motions of the Fornax, Sculptor, Ursa Minor and Carina dwarf spheroidals, and trace their orbits back through several variations of the Milky Way's potential and account for dynamical friction. The key parameters are the proper motions and total masses of the dwarf galaxies. Using a simple model we find no tenable set of parameters that can allow Fornax to be consistent with filamentary infall, mainly because the 1 sigma error on its proper motion is relatively small. The other three must walk a tightrope between requiring a small pericentre (less than 20 kpc) to lose enough orbital energy to dynamical friction and avoiding being tidally disrupted. We then employed a more realistic model with host halo mass accretion, and found that the four dwarf galaxies must have fallen in at least 5 Gyrs ago. This time interval is longer than organised distribution is expected to last before being erased by the randomisation of the satellite orbits.