HINORA II: Testing the Existence of the Council of Giants in {\Lambda}CDM simulations

TL;DR

This study applies a novel method to cosmological simulations to investigate the existence of the Council of Giants, finding it to be a rare or potentially unexplained structure within the standard LCDM model.

Contribution

It extends the HINORA method to analyze LCDM simulations, quantifying the rarity of the observed galaxy ring structure in the Local Universe.

Findings

The observed CoG is more than 2.7 sigma unusual in LCDM simulations.

The existence of CoGs in simulations is a rare phenomenon.

Standard DM-only simulations may miss physical processes influencing CoG formation.

Abstract

The discovery of the galaxy ring known as the Council of Giants (CoG) highlights the need to explain such structures in the Local Universe. In the first paper of this series we presented HINORA - a code to locate (ring-like) structures in 3D point sets - and used it to identify the CoG in the most complete observations of the Local Volume. Here, in Part II, we apply the same method to cosmological simulations to quantify the possible existence of such objects in the LCDM model of structure formation. We analyze DM-only simulations with random and constrained initial conditions, selecting regions that reproduce the properties of the Local Group and Volume, respectively. In order to use the same selection criteria as previsouly done for observations, we relate K-band luminosities to halo masses through semi-empirical relations. After confirming that the selected regions from the simulations match the observed mass function and density of the Local Universe, we use HINORA to search for ring-like structures in them. We find that the existence of CoGs in LCDM simulations is a rather unusual phenomenon. The observed CoG represents an anomaly of more than 2.7 sigma from what is expected in the distribution of massive galaxies in LCDM. These results hint that the CoG could either be a rare chance configuration or the imprint of physical processes at intermediate scales that standard DM-only simulations fail to capture.