Too big, too early? Multiple high-redshift galaxy clusters: implications

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 Λ cold dark matter ( Λ CDM ) 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 Λ CDM model and the observed ones. We alleviate the tension, considering non-Gaussian primordial perturbations of the local type, characterized by the parameter f NL , and derive constraints on f NL arising from the mere existence of these clusters. At the 95 % confidence level, f NL > 467 , with cosmological parameters fixed to their most likely WMAP5 values, or f NL ≳ 123 (at 95 % confidence) if we marginalize over prior WMAP5 parameters. In combination with f NL constraints from cosmic microwave background and halo bias, this determination implies a scale dependence of f NL at ≃ 3 σ . 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 f NL > 0 found here. In order to reconcile these massive, high- z clusters with f NL = 0 , their masses would need to be systematically lowered by 1.5 σ , or the σ 8 parameter should be ∼ 3 σ higher than cosmic microwave background (and large-scale structure) constraints. The existence of these objects is a puzzle: it either represents a challenge to the Λ CDM paradigm or it is an indication that the mass estimates of clusters are dramatically more uncertain than we think.