An international investigation, led by the Institute of Astrophysics of the Canary Islands (IAC) and the University of La Laguna (ULL), together with a group of Italian universities, confirms a new method to find galaxy protoclusters, the largest structures of the primordial universe. These progenitors of the current galactic clusters are essential for understanding the evolution of the Universe, but very difficult to detect. The study shows that a particular type of galaxies, called submillimeter galaxies, are excellent indicators of the presence of distant protoclusters. The results are published in the journal Astronomy & Astrophysics.
Protoclusters of galaxies are the largest structures that populated the primordial cosmos just 1 billion years after the Big Bang. The scientific community is especially interested in these galactic populations, progenitors of the current galaxy clusters, since being so old they can help understand the processes of formation and evolution of large-scale structures in the Universe. However, identifying protoclusters is not easy and very few are known.
To solve this problem, an international scientific team has proposed a new method focused on a particular type of object: submillimeter galaxies. Discovered in the late 1990s, they owe their name to their intense emission in the submillimeter band, that is, the region of the electromagnetic spectrum between infrared and microwaves. They are among the most massive and dusty galaxies in the Universe, and have a high rate of star formation, which can exceed that of the Milky Way by more than a hundred times.
“Several previous studies had shown evidence that submillimeter galaxies reside in the center of protoclusters of galaxies, but there was great controversy,” explains the lead author of the article. Rosa Calvi, researcher at the University of Ferrara and previously postdoctoral researcher at the IAC. “Our work represents the first systematic study of the large-scale environment of a sample of spectroscopically confirmed submillimeter galaxies,” she clarifies.
Thanks to the study carried out, the scientific team has unequivocally demonstrated that submillimeter galaxies are excellent indicators of distant protoclusters. To achieve this result, they searched for these primitive structures around twelve submillimeter galaxies and discovered that eleven of them are housed in eight protoclusters. Of these eight, the new study independently confirmed three that were already known and found evidence for the presence of five new structures. One of them, around the galaxy GN10, is among the most distant protoclusters ever observed: its light took more than 12.5 billion years in reaching Earth.
The study also sheds new light on the physical connection between submillimeter galaxies and their surroundings, showing a hitherto unobserved correlation between the amount of molecular gas (the fuel from which stars form) in submillimeter galaxies and the overdensities of galaxies and protoclusters. “To explain this correlation, we have proposed the hypothesis that interactions between galaxies in the densest environments facilitate the collapse of gas and the consequent high rate of star formation that characterizes the brightest submillimeter galaxies,” he says. Helmut Dannerbauerresearcher at the IAC and the ULL who also participated in the study.
In the coming years, the number of confirmed protoclusters is expected to increase considerably thanks to the use of new generation scientific facilities such as the Euclid satellite, one of the main ongoing missions of the European Space Agency (ESA) and in which the IAC actively participates. “With the Euclid satellite, a revolutionary tool for studying large-scale structures, we hope to discover and characterize thousands of distant protoclusters, which will represent an unprecedented boost in the study of the evolution of galaxies,” he concludes. Gianluca Castignaniresearcher at the University of Bologna and co-author of the article.