Implications of multiple high-redshift galaxy clusters

TL;DR

This paper examines the rarity of high-redshift galaxy clusters within the standard LCDM model, finding that their existence suggests possible non-Gaussian primordial fluctuations or uncertainties in mass estimates, challenging current cosmological paradigms.

Contribution

It provides the first quantitative analysis of the tension between observed high-redshift clusters and LCDM predictions, constraining non-Gaussianity parameter fnl from cluster data.

Findings

High-redshift clusters are rarer than LCDM predicts under Gaussian initial conditions.

Constraints on non-Gaussianity parameter fnl suggest significant primordial non-Gaussianity.

Reconciling observations with LCDM requires either mass adjustments or higher sigma8 values.

Abstract

To date, 14 high-redshift (z>1.0) galaxy clusters with mass measurements have been observed, spectroscopically confirmed and are reported in the literature. These objects should be exceedingly rare in the standard LCDM model. We conservatively approximate the selection functions of these clusters' parent surveys, and quantify the tension between the abundances of massive clusters as predicted by the standard LCDM model and the observed ones. We alleviate the tension considering non-Gaussian primordial perturbations of the local type, characterized by the parameter fnl and derive constraints on fnl arising from the mere existence of these clusters. At the 95% confidence level, fnl>467 with cosmological parameters fixed to their most likely WMAP5 values, or fnl > 123 (at 95% confidence) if we marginalize over WMAP5 parameters priors. In combination with fnl constraints from Cosmic Microwave Background and halo bias, this determination implies a scale-dependence of fnl at approx. 3 sigma. Given the assumptions made in the analysis, we expect any future improvements to the modeling of the non-Gaussian mass function, survey volumes, or selection functions to increase the significance of fnl>0 found here. In order to reconcile these massive, high-z clusters with an fnl=0, their masses would need to be systematically lowered by 1.5 sigma or the sigma8 parameter should be approx. 3 sigma higher than CMB (and large-scale structure) constraints. The existence of these objects is a puzzle: it either represents a challenge to the LCDM paradigme or it is an indication that the mass estimates of clusters is dramatically more uncertain than we think.