Veiled birth of a giant –

This image shows the Spider Galaxy protocluster (MRC 1138-262) and the hot gas it contains (blue). © ESO / Di Mascolo et al. ; HST: H Ford

Galaxy clusters are among the largest structures in the universe. The formations, which consist of galaxies, gas clouds, and dark matter, can include many thousands of galaxies and combine up to a trillion solar masses. However, much of this mass does not come from stars, but from the so-called inner cluster medium (ICM), huge plumes of gas that permeate the cluster’s intergalactic space. How this intergalactic gas is distributed and its composition has only been studied for nearby, fully developed galaxy clusters.

However, astronomers have long suspected that the medium within a cluster plays an important role in the formation and growth of galaxy clusters. Thus, the incoming hot gas promotes the formation of new stars and galaxies. “Cosmological simulations have predicted the presence of hot gas in primary clusters for more than a decade, but they lack observational confirmation,” explains Elena Rascia of the Italian National Institute for Astrophysics (INAF) in Trieste.

To test this, Rasia and her colleagues used the Atacama Large Millimeter/Submillimeter Array (ALMA) to study a number of these proto-clusters of galaxies in the early universe. “The search for such important experimental confirmation led us to select one of the most promising candidate protozoan groups,” explains the astronomer. The chosen candidate is the Spiderweb Protocluster, an irregular galaxy about 10.6 billion light-years away that is poised to become the center of a new group of galaxies.

To visualize the diffuse gas, which is not visible through normal viewing, astronomers have exploited the fact that the presence of a cluster medium within the cluster slightly amplifies the radio emission of the cosmic microwave background behind it. Indeed, this is how the team was able to determine the amount and distribution of the gas in this elemental cluster. It can be seen in this image as a bluish haze.

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Observations show that the spider cluster contains a huge reservoir of hot gas with a temperature of tens of millions of degrees Celsius. “These observations confirm long-standing theoretical predictions about the formation of the largest gravitationally bound objects in the universe,” says co-author Tony Mrozkowski of the European Southern Observatory (ESO). The astronomers conclude from their results that the proto-Spider Cluster will evolve into a massive galaxy cluster over the course of about ten billion years. In doing so, it would increase its mass by at least tenfold.

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