Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies

TL;DR

This paper challenges recent claims that satellite galaxy planes are common in ΛCDM models, showing they are actually very rare and not well explained by current standard cosmological theories.

Contribution

It identifies methodological issues in previous studies and demonstrates that observed satellite planes are inconsistent with ΛCDM predictions, questioning their claimed commonality.

Findings

Satellite planes are extremely rare in ΛCDM simulations (probability < 10^{-3})

Cold stream accretion does not naturally explain the observed planes

The origin of satellite planes remains unexplained in standard models

Abstract

Both major galaxies in the Local Group host planar distributions of co-orbiting satellite galaxies, the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). The $\Lambda$CDM cosmological model did not predict these features. However, according to three recent studies the properties of the GPoA and the flattening of the VPOS are common features among sub-halo based $\Lambda$CDM satellite systems, and the GPoA can be naturally explained by satellites being acquired along cold gas streams. We point out some methodological issues in these studies: either the selection of model satellites is different from that of the observed ones, or an incomplete set of observational constraints has been considered, or the observed satellite distribution is inconsistent with basic assumptions. Once these issues have been addressed, the conclusions are different: features like the VPOS and GPoA are very rare (each with probability $\lesssim 10^{-3}$, and combined probability $< 10^{-5}$) if satellites are selected from a $\Lambda$CDM simulation combined with semi-analytic modelling, and accretion along cold streams is no natural explanation of the GPoA. The origin of planar dwarf galaxy structures remains unexplained in the standard paradigm of galaxy formation.