Galaxy Clusters -“Dark Skeletons of the Cosmos”

Galaxy Cluster


Ancient galaxy clusters have been described as “the dark skeletons of the cosmos.” Astronomers at the Instituto de Astrofísica de Canarias (IAC) have found the most densely populated galaxy cluster in formation in the primitive universe. The researchers predict that this structure, among the largest astronomical objects in the Universe, which is at a distance of 12.5 billion light years from us, will have evolved becoming a cluster similar to the 1,300 galaxies of the Virgo Cluster, a neighbor of the Local Group of galaxies to which harbors our home galaxy, the Milky Way.

The Dark Skeletons

Clusters of galaxies –“dark skeletons”– shaped by the underlying distribution of dark matter” –are groups of galaxies which remain together because of the action of gravity. To understand the evolution of these “cities of galaxies” scientists look for structures in formation, the so-called galaxy protoclusters, in the early universe. A protocluster is a structure of galaxies still in the process of forming a cluster.

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In October of 2019, researchers with The Cosmic Dawn Center (DAWN) at the Niels Bohr Institute, University of Copenhagen discovered a protocluster at the dawn of the cosmos, the most distant ever found at 13.0 billion light years away, suggesting that a large structure already existed at a time when the universe was only about 800 million years old or 6 percent of its present age.

Discovery of Protogalaxies at Dawn of Cosmic History

With an earlier discovery in 2012, an international team of astronomers made an accurate determination of the distance of the galaxy HDF850.1, located at the deepest reaches of space known as the ‘Hubble Deep Field’, is one of the galaxies with the highest rate of star formation in the observable universe, forming as many stars as all the other galaxies in the Hubble Deep Field combined, producing the equivalent of 1,000 suns per year, while  an ordinary galaxy like the Milky Way forms only a few solar masses per year.. To their surprise, the scientists also discovered that this galaxy, which is one of the most studied regions on the sky, known as the Hubble Deep Field/GOODS-North, is part of a group of around a dozen protogalaxies which had formed during the first billion years of cosmic history. Before its discovery only one other similar primordial group was known.

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Ancient  City of Galaxies

New research with the OSIRIS instrument on the Gran Telescopio Canarias (GTC, or GRANTECAN), the team has shown that it is one of the most densely populated regions populated with galaxies in the primitive Universe, and have for the first time carried out a detailed study of the physical properties of this system.

“Surprisingly we have discovered that all the members of the cluster studied up to now, around two dozen, are galaxies with normal star formation, and that the central galaxy appears to dominate the production of stars in this structure” explains Rosa Calvi, formerly a postdoctoral researcher at the IAC and first author of the study.

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 “Under Construction”

This recent study shows that this cluster of galaxies in formation is made up of various components, or “zones” with differences in their evolution. The astronomers predict that this structure will change gradually until it becomes a galaxy cluster similar to Virgo, the central region of the supercluster of the same name in which is situated the Local Group of galaxies to which the Milky Way belongs.

“We see this city in construction just as it was 12,500 million years ago, when the Universe had less than 10% of its present age, so we are seeing the childhood of a cluster of galaxies like those which are typical in the local Universe” notes co-author Helmut Dannerbauer, an IAC researcher.

The distance measured to these studied sources agrees perfectly with the predictions based on photometric observations taken previously on GRANTECAN, largest optical and infrared telescope in the world, by Pablo Arrabal Haro, an IAC researcher and Assistant General Secretary of the International Astronomical Union (IAU), and Casiana Muñoz-Tuñón, a researcher and Deputy Director of the IAC. Arrabal developed a method for selecting galaxies with normal star formation rates, based on the photometric survey SHARDS (Survey for High-z Absorption Red and Dead Sources).

“Measuring exactly how these structures are forming, especially at the beginning of the Universe, is not easy, and we need exceptional data such as those we are taking with the GTC telescope as part of the SHARDS and SHARDS Frontier Fields projects, which allow us to determine distances to galaxies and between galaxies at the edge of the Universe with a precision never achieved before,” says Pérez-González,

The Gran Telescopio Canarias and the Observatories of the Instituto de Astrofísica de Canarias (IAC) are part of the network of Singular Scientific and Technical Infrastructures of Spain. The study is published in the specialized journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research, via Chandra X-Ray Observatory and  IAC

Image credit: interacting and merging galaxies in the early universe. ESO/M. Kornmesser

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