Is the vast polar structure of dwarf galaxies a serious problem for lambda cold dark matter?

TL;DR

This study investigates whether the observed vast polar structure of dwarf galaxies around the Milky Way challenges the Lambda CDM model, concluding that the structure disperses quickly and is not dynamically stable, thus posing no serious problem.

Contribution

The paper provides a dynamical analysis of the VPOS using proper motion data, demonstrating its transient nature and assessing its implications for Lambda CDM.

Findings

VPOS disperses before a dynamical time

Proper motions of 8 classical dwarfs are reliable

VPOS is not a stable, long-lived structure

Abstract

The dwarf galaxies around the Milky Way are distributed in a so-called vast polar structure (VPOS) that may be in conflict with Lambda CDM simulations. Here, we seek to determine if the VPOS poses a serious challenge to the Lambda cold dark matter paradigm on galactic scales. Specifically, we investigate if the VPOS remains coherent as a function of time. Using the measured Hubble Space Telescope (HST) proper motions and associated uncertainties, we integrate the orbits of the classical Milky Way satellites backwards in time and find that the structure disperses well before a dynamical time. We also examine in particular Leo I and Leo II using their most recent proper motion data, both of which have extreme kinematic properties, but these satellites do not appear to drive the polar fit that is seen at the present day. We have studied the effect of the uncertainties on the HST proper motions on the coherence of the VPOS as a function of time. We find that 8 of the 11 classical dwarfs have reliable proper motions; for these 8, the VPOS also loses significance in less than a dynamical time, indicating that the VPOS is not a dynamically stable structure. Obtaining more accurate proper motion measurements of Ursa Minor, Sculptor, and Carina would bolster these conclusions.