Unveiling the largest structures in the nearby Universe: Discovery of the Quipu superstructure

TL;DR

This paper identifies and characterizes the largest known cosmic structures, called Quipu, using galaxy cluster data, and discusses their implications for cosmology, including effects on the CMB and galaxy evolution.

Contribution

First all-sky assessment of superstructures between 130 and 250 Mpc, discovering the largest structure, Quipu, and analyzing its cosmological and astrophysical significance.

Findings

Largest structure, Quipu, exceeds 400 Mpc in length.

Quipu contains about 45% of galaxy clusters and 25% of matter.

Detected a low-significance integrated Sachs-Wolfe effect signal in Planck data.

Abstract

For a precise determination of cosmological parameters we need to understand the effects of the local large-scale structure of the Universe on the measurements. They include modifications of the cosmic microwave background, distortions of sky images by large-scale gravitational lensing, and the influence of large-scale streaming motions on measurements of the Hubble constant. The streaming motions, for example, originate from mass concentrations with distances up to 250 Mpc. In this paper we provide the first all-sky assessment of the largest structures at distances between 130 and 250 Mpc and discuss their observational consequences, using X-ray galaxy clusters to map the matter density distribution. Among the five most prominent superstructures found, the largest has a length longer than 400 Mpc with an estimated mass of about 2 10e17 Msun. This entity, which we named Quipu, is the largest cosmic structure discovered to date. These superstructures contain about 45% of the galaxy clusters, 30% of the galaxies, 25% of the matter, and occupy a volume fraction of 13%, thus constituting a major part of the Universe. The galaxy density is enhanced in the environment of superstructures out to larger distances from the nearest member clusters compared to the outskirts of clusters in the field. We find superstructures with similar properties in simulations based on Lambda-CDM cosmology models. We show that the superstructures should produce a modification on the cosmic microwave background through the integrated Sachs-Wolf effect. Searching for this effect in the Planck data we found a signal of the expected strength, however, with low significance. Characterising these superstructures is also important for astrophysical research, for example the study of the environmental dependence of galaxy evolution as well as for precision tests of cosmological models.