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[1009.3884] Too big, too early? Multiple High-Redshift Galaxy Clusters: implications
Authors:Ben Hoyle, Raul Jimenez, Licia Verde
Abstract:To date, 15 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>475 with cosmological parameters fixed to their most likely WMAP5 values, or fnl>370 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 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 sigma_8 parameter should be ~4 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 paradigm or it is an indication that the mass estimates of clusters is dramatically more uncertain than we think.
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Syksy Rasanen

Joined: 02 Mar 2005
Posts: 128
Affiliation: University of Helsinki

PostPosted: September 22 2010  Reply with quote

The authors consider 15 massive clusters observed at redshifts >1. In the vanilla ΛCDM model, you don't expect a lot of massive clusters early on. The authors report a probability of 3×10 − 4 for having so many so massive clusters in the observed region.

The authors focus on the influence of non-Gaussianity: the probability of massive clusters rises with fNL, and the authors find that for fNL = 550 the probability rises to one. Personally, I am not that much into in large non-Gaussianity, but the observation of the discrepancy is interesting. I would imagine that for example slower expansion at high redshifts would increase the number of clusters; one could check whether the required change is consistent with the supernova data.

As a caveat, the authors note that the observations would be consistent with the standard picture with no non-Gaussianity if the cluster masses are over-estimated by 1.5σ, which doesn't seem like a lot to me. I am not too familiar with this, but I recall there has been controversy over the cluster abundance and the X-ray mass determinations before ( ; see also
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Florian Pacaud

Joined: 02 Nov 2007
Posts: 1
Affiliation: AIfA, Bonn

PostPosted: September 23 2010  Reply with quote

Hi Syksy,

although the idea behind this article has received a growing interest in the past months/year, I would be careful on the interpretation of these results. Cluster mass measurements at z>1 are extremely difficult and, as you say, a 1.5σ deviation is not at all unexpected.

In addition, the mass definition itself needs some carefull handling for these kind of studies, and I am not sure they really did it properly. More exactly, they dont give us the necessary information to be confident that it was done correctly.

If you look at the caption of table 1, they mention the "mass (converted to M200)" without ever saying how this conversion was performed. This conversion is nothing but straightforward since you need to assume a cluster mass profile and be very careful that you properly understand what convention people have used in the past articles.

All these cluster masses were obtained from spherical mass overdendity considerations, while the Jenkins mass function that they use in their paper uses Friend-of-Friend halo masses. Although many people don't care much about that, I think the translation from one to the other is also not straightforward (and the reason why the galaxy cluster community likes to use the recent mass functions from Tinker et al. 2008).

A frequent assumption is that FOF halo finder give you a mass that is roughly M200. Even neglecting the possible inaccuracy of this assumption, one has to be careful that the overdensity of 200 here is with respect to the mean matter density, while cluster papers often give results with respect to the critical density.

All in all, I'm not saying that the paper is not good and the results may well be valid and rigorous. But I am just desapointed that many important caveats of the analysis are not even mentionned.

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