Zeldovich pancakes in observational data are cold

The present day universe consists of galaxies, galaxy clusters, one-dimensional filaments and two-dimensional sheets or pancakes, all of which combine to form the cosmic web. The so called Zeldovich pancakes, are very difficult to observe, because their overdensity is only slightly greater than the average density of the universe. Falco et al (2014) presented a method to identify Zeldovich pancakes in observational data, and these were used as a tool for estimating the mass of galaxy clusters. Here we expand and refine that observational detection method. We study two pancakes on scales of 10 Mpc, identified from spectroscopically observed galaxies near the Coma cluster, and compare with twenty numerical pancakes. We find that the observed structures have velocity dispersion about 100 km/sec, which is relatively low compared to typical groups and filaments. These velocity dispersions are consistent with those found for the numerical pancakes. We also confirm that the identified structures are in fact two-dimensional structures. Finally, we estimate the stellar to total mass of the observational pancakes to be $2 times 10^{-4}$, within one order of magnitude, which is smaller than that of clusters of galaxies.

The impact of going beyond the Maxwell distribution in direct dark matter detection rates

We consider direct dark matter detection rates and investigate the difference between a standard Maxwell-Boltzmann velocity distribution and a realistic distribution like the ones extracted from numerical N-body simulations. Sizable differences are observed when such results are compared to the standard Maxwell-Boltzmann distribution. For a light target both the total rate and the annual modulation are reduced by ~25%. For a heavy target the total rate is virtually unchanged, whereas the annual modulation is modified by up to 50%, depending on the WIMP mass and detector energy threshold. We also consider the effect of a possible velocity anisotropy, and the effect is found to be largest for a light target For the realistic velocity distribution the anisotropy may reduce the annual modulation, in contrast to the Maxwell-Boltzmann case.