Astronomers study the birth of galaxy clusters in the young universe Distant Space | Articles

ESO2304 Press Release – March 29, 2023

Galaxy clusters, as the name suggests, are clusters of a significant number of galaxies – sometimes thousands. They also contain an amount of matter known as the “intergalactic medium” (intracluster medium, ICM) – the gas that fills the space between galaxies in the cluster. In fact, this gas significantly exceeds the mass of the galaxies themselves. We think we understand much of the physics that describes galaxy clusters. However, observations of the early stages of cloud formation in the intergalactic medium are rare.

So far, ICM clouds have only been studied in fully developed nearby galaxy clusters. Detecting the ICM in distant protoclusters—early in their evolution—would allow astronomers to trace their formation in the early stages of the universe’s evolution. And just such an observation was made by a scientific group led by Luca Di Mascolo (University of Trieste, Italy).

Clusters of galaxies are so large that they can collect gas, which heats up as it moves towards the center of the cluster. “Cosmological simulations have predicted the presence of hot gas in protoclusters for more than a decade, but there is no observational evidence.“,” notes the study’s co-author, astrophysicist Elena Rasia (Italian National Institute of Astrophysics, INAF, Trieste, Italy). “The quest to obtain such a fundamental observation forced us to carefully select one of the most promising candidates.” When the universe was only 3 billion years old, it became the protocluster of the Spider’s web that we observe. Although it is one of the most intensively studied objects of this type, ICM has not been recorded. The discovery of a large reservoir of hot gas in the Spiderweb protocluster suggests that the system is on its way to becoming a full-fledged permanent cluster instead of disintegrating.

Luca Di Mascolo and his team discovered the intergalactic medium in the Spider Web protocluster through a process known as the Sunyaev-Zeldovich effect. This happens when the cosmic microwave background radiation—relic radiation from the Big Bang—passes through the ICM. The photons of the relic radiation interact with the fast-moving electrons in the hot gas and gain a little extra energy by slightly changing their wavelength. “At appropriate wavelengths, this effect seems to shield the cosmic background radiation from the galaxy cluster.“, explains Luca Di Mascolo.

By measuring the shadow of relic radiation, astronomers can detect the presence of hot gas, estimate its mass, and map its distribution. “Because of its extraordinary resolution and sensitivity, ALMA is currently the only instrument capable of making such measurements for the distant progenitors of massive galaxy clusters.“,” notes Luca Di Mascolo.

Scientists have discovered that the Spider Protocluster contains a huge reservoir of hot gas with a temperature of tens of millions of degrees Celsius. Cold gas has been detected in this protocluster in the past, but the hot gas found in this new study is thousands of times more massive. Based on these results, astronomers predict that in about 10 billion years the Spider Protocluster will indeed have increased in mass at least tenfold, becoming a giant galaxy cluster.

Tony Mroczkowski, ESO scientist and co-author of the paper, adds: “We observe great contrasts in this system. The hot thermal component destroys most of the cold component during further evolution, and we witness this gradual transformation. Our observations confirm the correctness of theoretical predictions describing the formation of the largest objects in the universe due to gravity.”

These results also help lay the groundwork for future synergies between ALMA and ESO’s upcoming new Large Telescope/.ELT (Very large telescope). «The joint work of these two facilities will revolutionize the study of cosmic structures such as the spider’s web.“,” expects researcher and study co-author Mario Nonino (Trieste Astronomical Observatory, Italy). ELT and its advanced tools, HARMONY a MICA, will be able to look inside protoclusters and learn details about the galaxies that make them up. Together with ALMA’s ability to observe the forming clouds of the intergalactic medium, they will provide important information about the evolution of large structures in the early universe.


The research is presented in the journal Intracluster Gas Formation in a Galactic Protocluster at Redshift 2.16. Nature (doi: 10.1038/s41586-023-05761-x)

Team composition: Luca Di Mascolo (Department of Astronomy, University of Trieste, Italy [UT]; INAF – Trieste Astrophysical Observatory, Italy [INAF Trieste]; IFPU – Institute for Fundamental Physics of the Universe, Italy [IFPU]), Alexandro Saro (UT; INAF Trieste; IFPU; INFN – Trieste, Italy Division [INFN]), Tony Mroczkowski (European Southern Observatory, Germany [ESO]), Stefano Borgani (UT; INAF Trieste; IFPU; INFN), Eugene Churazov (Max-Planck-Institute für Astrophysik, Nemecko; Institute for Space Research, Rusko), Elena Rasia (INAF Trieste; IFPU), Paolo Tozzi (INAF – Observatory Arcetri, an astrophysicist from Italy, Helmut Dannerbauer (Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Spain), Kaustuv Basu (Institute for Astronomy in Argel ander, University of Bonn, Germany), A.Radioilli NN Observatory, USA), Michele Ginolfi (ESO; Department of Physics and Astronomy, University of Florence, Italy), George Miley (Leiden Observatory, Leiden University, Netherlands), Mario Nonino (UT), Maurilio Pannella (UT; INAF Trieste; IFPU ), Laura Pentericci (INAF – Roman Astronomy Observatory, Italy), Francesca Rizzo (Cosmic Dawn Center, Denmark; Niels Bohr Institute, Denmark)

The ALMA astronomical observatory (Atacama Large Millimeter/submillimeter Array) is an international partnership project implemented in Japan in collaboration with ESO, NSF (US National Science Foundation) and NINS (National Institute of Natural Sciences) with the Republic of Chile. ALMA is funded by ESO, NSF on behalf of member states in collaboration with NRC (National Research Council of Canada) and NSC (National Science Council, Taiwan), NSTC (National Science and Technology Council, Taiwan), NINS AS. in Taiwan (Academia Sinica) and KASI (Korea Astronomy and Space Science Institute) in Korea. The construction and operation of the ALMA observatory is managed by ESO on the European side, NRAO (National Radio Astronomy Observatory), AUI (Associated Universities, Inc.) and NAOJ (National Astronomy Observatory) on the North American side. Japan) on the East Asian side. The Joint ALMA Observatory (JAO) provides unified leadership and management for the construction, testing and operation of the ALMA telescope.

The European Southern Observatory (ESO) enables scientists from all over the world to discover the mysteries of the universe for the benefit of all. We design, build and operate world-class ground-based observatories that astronomers use to solve exciting questions and spread the fascination of astronomy. We support international cooperation in the field of astronomy. ESO was founded in 1962 as an intergovernmental organization and today consists of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Ireland, Italy, Netherlands, Portugal, Spain, Sweden, Switzerland, Great Britain and 16 member states. a pair of strategic partners – Chile and Australia, which host all ESO observatories. ESO’s headquarters, the ESO Supernova Visitor Center and Planetarium, are located near Munich, Germany, while Chile’s Atacama Desert, a stunning location with unique conditions for observing the sky, is home to our telescopes. ESO operates three observatories: La Silla, Paranal and Chajnantor. Mount Paranal houses the VLT (Very Large Telescope) and VLTI (Very Large Telescope Interferometer), as well as two survey telescopes – VISTA, which operates in the infrared region, and VST (VLT Survey Telescope) for visible light. At the Paranal Observatory, ESO will also host and operate the CTAS (Cherenkov Telescope Array South) telescope array for the detection of Cherenkov radiation in the atmosphere, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates the APEX and ALMA millimeter and submillimeter telescopes operating in the Çajnantor Plain. At Cerro Armazones near Paranal, we are building a new ELT (Extremely Large Telescope) with a 39 m diameter primary mirror that will be “mankind’s biggest eye on space”. From our offices in Santiago, we manage our operations in Chile and our collaboration with local partners and the community.



Anežka Kabátová; national connection; Astronomy Institute of the Czech Academy of Sciences, Czech Republic; Email:

Jiří Srba; translation; Email:

Luca Di Mascolo; University of Trieste; Trieste, Italy; Email:

Tony Mroczkowski; European Southern Observatory; Garching near Munich, Germany; Phone: +49 89 3200 6174; Email:

Alejandro Saro; University of Trieste; Trieste, Italy; Email:

Juan Carlos Munoz Mateos; ESO Media Specialist; Garching bei München, Germany; Phone: +49 89 3200 6176; Email:

Resources and Recommended Links:
[1] ESO2304 Press Release – March 29, 2023

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