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HINORA II: Testing the Existence of the Council of Giants in ΛCDM simulations

Edward Olex, Alexander Knebe, Noam I. Libeskind, Stefan Gottlöber, Dmitry I. Makarov

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.

HINORA II: Testing the Existence of the Council of Giants in ΛCDM simulations

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.
Paper Structure (13 sections, 8 equations, 3 figures, 4 tables)

This paper contains 13 sections, 8 equations, 3 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Local Volume simulations
  3. Constrained simulations
  4. Random simulations
  5. Methodology
  6. The detection of the Council of Giants
  7. Connecting simulations with observations
  8. Local Volume properties
  9. The Council of Giants in $\rm \Lambda$CDM
  10. Rings in random vs constrained initial conditions
  11. The significance of the Council of Giants detection
  12. Discussion and conclusions
  13. Funding Statement

Figures (3)

  • Figure 1: Cumulative halo mass function for $M_{200}$ in each of the Local Volumes extracted from the HESTIA constrained simulations (blue region) and SMD random simulation (red region). Also shown in green are randomly placed volumes in SMD. The simulations are compared with those obtained for the LVG survey using three different $M_h / L_k$ relations. The vertical lines indicate the main mass cuts at which HINORA was applied in Sec. \ref{['sec:HINORA']}.
  • Figure 2: Probability distribution of the halo density contrast (defined in equation \ref{['eq:deltah']}) for different Local Volumes. The different colors show the LVs obtained in HESTIA following all criteria (blue), SMD with all criteria (red), and SMD without any criteria (green). Value calculated for LVG survey and its associated scatter are shown with the black line + gray region.
  • Figure 3: Fraction of galaxy rings detected in the different sets of simulated Local Volumes. The statistical strength of each detection is classified as weak, moderate, or strong according to the minimum fraction of halos in the volume that belong to the ring, requiring $n_I>0.15$, $n_I>0.20$, and $n_I>0.25$, respectively. The error bars consider the total uncertainty in the $L_{K} \rightarrow M_{200}$ relation.