Photo worth seeing: Veiled Birth of a Giant

Photo worth seeing: Veiled Birth of a Giant
This image shows the protocluster around the Spiderweb Galaxy (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 cosmos. The formations, consisting of galaxies, gas clouds and dark matter, can encompass several thousand galaxies and unite up to a trillion solar masses. However, a large part of this mass does not come from stars, but from the so-called intracluster medium (ICM), enormous plumes of gas that permeate the space between the galaxies in the cluster. How this intergalactic gas is distributed and structured has only been studied for nearby, fully developed galaxy clusters.

However, astronomers have long suspected that the intracluster medium plays a crucial role in the formation and growth of galaxy clusters. The hot, incoming gas therefore promotes the formation of new stars and galaxies. “Cosmological simulations have predicted the presence of hot gas in protoclusters for more than a decade, but lacked observational confirmation,” explains Elena Rasia 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 such proto-clusters of galaxies in the early cosmos. “The search for such an important experimental confirmation has led us to carefully select one of the most promising protocluster candidates,” explains the astronomer. The chosen candidate is the Spiderweb Protocluster, an irregular galaxy about 10.6 billion light-years distant that is poised to become the center of a new cluster of galaxies.

To visualize the diffuse gas, invisible by normal viewing, the astronomers took advantage of the fact that the presence of intracluster medium amplifies the radio emission of the cosmic microwave background behind it a tiny bit. In fact, this is how the team managed to map the amount and distribution of the gas in this proto-cluster. It can be seen in this image as a bluish haze.

The observations show that the Spiderweb protocluster 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 Mroczkowski from the European Southern Observatory ESO. The astronomers conclude from their results that the Spiderweb protocluster will develop into a massive galaxy cluster over the course of around ten billion years. In doing so, it will increase its mass by at least tenfold.

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