Quantifying the Rarity of the Local Super-Volume

TL;DR

This paper assesses how the observed number of massive galaxy clusters in the local universe aligns with the standard cosmological model, highlighting the impact of measurement uncertainties on the interpretation of these observations.

Contribution

It provides a quantitative analysis of the rarity of local super-volume clusters within the bcCD model, emphasizing the influence of mass estimation systematics.

Findings

Observed cluster counts vary from 0 to 5 depending on mass estimates.

Likelihood of bcCD model fits ranges from very low to high, depending on observed counts.

Systematic uncertainties significantly affect the interpretation of cluster abundance.

Abstract

We investigate the extent to which the number of clusters of mass exceeding $10^{15}\,M_{\odot}\,h^{-1}$ within the local super-volume ($<135\mathrm{\,Mpc}h^{-1}$) is compatible with the standard $\Lambda$CDM cosmological model. Depending on the mass estimator used, we find that the observed number $N$ of such massive structures can vary between $0$ and $5$. Adopting $N=5$ yields $\Lambda$CDM likelihoods as low as $2.4\times 10^{-3}$ (with $\sigma_8=0.81$) or $3.8\times 10^{-5}$ (with $\sigma_8=0.74$). However, at the other extreme ($N=0$), the likelihood is of order unity. Thus, while potentially very powerful, this method is currently limited by systematic uncertainties in cluster mass estimates. This motivates efforts to reduce these systematics with additional observations and improved modelling.