Are the superstructures in the two-degree field galaxy redshift survey a problem for hierarchical models?

• Published in: Monthly notices of the Royal Astronomical Society Vol. 413; no. 2; pp. 1311 - 1317
• Main Authors: Yaryura, C. Yamila, Baugh, C. M., Angulo, R. E.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01-05-2011; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; cosmology: theory; Dark matter; Earth, ocean, space; Exact sciences and technology; large-scale structure of Universe; Mathematical models; methods: numerical; methods: statistical; Red shift; Stars & galaxies; methods: statistical; methods: numerical; large-scale structure of Universe; cosmology: theory; Superclusters; Red shift; Galaxies; Digital simulation; Probability; N body system; Numerical method; Large-scale structure; Statistical method; Dark matter; Cold dark matter; Extreme value; Cosmology; Models
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2011.18233.x

We introduce an objective method to assess the probability of finding extreme events in the distribution of cold dark matter such as voids, overdensities or very high mass haloes. Our approach uses an ensemble of N-body simulations of the hierarchical clustering of dark matter to find extreme structures. The frequency of extreme events, in our case the cell or smoothing volume with the highest count of cluster-mass dark matter haloes, is well described by a Gumbel distribution. This distribution can then be used to forecast the probability of finding even more extreme events, which would otherwise require a much larger ensemble of simulations to quantify. We use our technique to assess the chance of finding concentrations of massive clusters or superclusters, like the two found in the two-degree field galaxy redshift survey (2dFGRS), using a counts-in-cells analysis. The Gumbel distribution gives an excellent description of the distribution of extreme cell counts across two large ensembles of simulations covering different cosmologies, and also when measuring the clustering in both real and redshift space. We find examples of structures like those found in the 2dFGRS in the simulations. The chance of finding such structures in a volume equal to that of the 2dFGRS is around 2 per cent.