Vol. 618
In section 3. Cosmology

The progeny of a cosmic titan: a massive multi-component proto-supercluster in formation at z=2.45 in VUDS

by O. Cucciati, B.C. Lemaux, G. Zamorani, O. Le Fevre, et al. A&A 618, A49

One of the main predictions of the most popular cosmological model, the so-called Lambda-Cold Dark Matter (LCDM), is that galaxies tend to cluster and slowly merge into bigger units. The presence of these large galaxy agglomerations has been known in the present Universe for many decades now. But when did these galaxy clusters, containing several thousand galaxies, form? Answering this question is not only important for its own sake. Accurately measuring the number density of such systems with cosmic time (or, equivalently, redshift) enables us to independently and precisely constrain many of the parameters of the LCDM model. This paper identifies a gigantic protocluster located at redshift z=2.45, when the Universe was only 20% of its current age. By developing a crafted and novel technique to assess cluster membership of each galaxy, the authors were able to unveil the very complex structure of the protocluster, which is made of several sub-components. This system may well hold the key to a full understanding of cosmic structure assembly on large scales.