Astronomers have discovered a giant protocluster of galaxies, born in a period when the universe was young, only one billion years old. It is characterized by a redshift of 5.7 and once collapses into a galactic cluster with a total mass of 3.6 quadrillion solar masses, which makes it the most massive protocluster known for redshifts above 4. The research is presented in Nature Astronomy.
There are three things worth knowing about this protocluster. First, its galaxies are confirmed by deep spectroscopic observations, which allowed scientists to remove interference and measure their properties. Secondly, it is extremely large and occupies about 35 × 35 × 35 cubic megaparsecs (1 megaparsec is about 3.26 million light years). Third, the density of the protocluster galaxies is very high, about 6.6 times the average density observed with a red shift of 5.7. Because of the huge size and high density, protocling is extremely massive.
The discovery of this giant protocluster was unexpectedly the result of a program that is not directly related to clusters, but is devoted to the evolution of galaxies with the highest redshift and space reionization. The epoch of cosmic reionization is approximately between the redshift of 15 and 6. More than 10 years ago, the co-author of the discovery, Professor N. Kashikawa, proved that cosmic reionization ends at redshift 6, but later studies of different groups gave completely different results.
To solve this important problem, in 2015, scientists began to determine a much larger sample of objects compared to previous studies with a red shift from 5.7 to 6.5 over a much larger area of the sky. The protoclusters have become an associated goal of scientists. They drew attention to five well-studied areas. In one of the fields, called SXDS, scientists identified a large area with a redshift of 5.7 and soon spectroscopically confirmed at least 41 objects. It turned out that the giant protocluster (SXDS_gPC) for the era in which it was born, has an enormous size and an unprecedentedly high density.
Both classical theories and cosmological simulations tell us that SXDS_gPC inevitably collapses into a giant cluster of galaxies. Scientists were faced with the problem of estimating the total mass of this cluster, because such objects are rare, and at the time of finding SXDS_gPC there were no suitable simulations. Thus, they had to extrapolate the mass of the most massive protocluster in existing models in order to estimate the mass of the newly discovered. It is not surprising that the resultant mass turned out to be huge and amounted to 3.6 quadrillion solar masses, which is comparable with the most gigantic clusters known to date.
The discovery of such a protocluster is intriguing. The cold dark matter model predicts that small structures merge to form large structures. Therefore, it is expected that in later cosmic times, larger ones will form. It is surprising that the giant SXDS_gPC already existed when the age of the Universe was only 7% of its total evolution. Another important issue is how rare such protoclusters are. The scientific team hopes that more of these ancient objects will soon be found, and they can be used as an ideal guide to understanding the early formation of the structure of the cosmos.
Giant protocluster of galaxies discovered in young universe
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