Witnesses to the birth of a cluster of galaxies in the early Universe

March 29 () –

Using the Atacama Large Millimeter/submillimeter Array (ALMA)astronomers have discovered a large reservoir of hot gas in the cluster of galaxies still forming around the Spider Web galaxy.

This constitutes the most distant detection of such a hot gas yet.

Galaxy clusters are some of the largest known objects in the Universe and this result, published in the journal ‘Nature’further reveals how early these structures begin to form.

Galaxy clusters, as their name suggests, are home to large numbers of galaxies, sometimes even thousands. They also contain a vast “intracluster medium” (MIC) of gas that permeates the space between the cluster galaxies. In fact, this gas weighs much more than the galaxies themselves. Much of the physics of galaxy clusters is well known, but observations of the early phases of MIC formation remain sparse.

Until now, the MIC has only been studied in fully formed nearby galaxy clusters. Its detection in distant protoclusters –that is, clusters of galaxies still in formation– it would allow astronomers to detect these clusters in the early stages of formation. A team led by Luca Di Mascolo, first author of the study and a researcher at the University of Trieste (Italy), sought to detect the MIC in a protocumulus from the early stages of the Universe.

Galaxy clusters are so massive that they can gather gas that heats up as it falls toward the cluster. “Cosmological simulations have been predicting the presence of hot gas in protocumulus for more than a decade, but observational confirmations were lacking,” he explains. it’s a statement Elena Rasia, a researcher at the Italian National Institute for Astrophysics (INAF) and co-author of the study.

“Pursuing that key observational confirmation led us to carefully select one of the most promising candidate protoclusters,” he adds. It was the Spiderweb protocluster, located at a time when the Universe was only 3 billion years old. Despite being the most intensively studied protocumulus, the presence of the ICM has remained elusive. The discovery of a large reservoir of hot gas in the Cobweb protocumulus would indicate that the system is on its way to becoming a long-lasting, proper galaxy cluster, rather than dispersing.

Di Mascolo’s team detected the MIC of the Cobweb protocumulus through what is known as the Sunyaev-Zeldovich (SZ) thermal effect. This effect occurs when light from the cosmic microwave background — the relict radiation from the Big Bang — passes through the MIC.

When this light interacts with fast-moving electrons in the hot gas, it gains a bit of energy and its color, or wavelength, changes slightly. “At the right wavelengths, the SZ effect appears as a shadow effect of a galaxy cluster on the cosmic microwave background“, explains Di Mascolo.

By measuring these shadows in the cosmic microwave background, astronomers can deduce the existence of the hot gas, estimate its mass, and map its shape. “Thanks to its unprecedented resolution and sensitivity, ALMA is the only facility currently capable of making such a measurement for the distant progenitors of massive clusters,” Di Mascolo highlights.

They determined that the Web protocluster contains a vast reservoir of hot gas at a temperature of a few tens of millions of degrees Celsius. Cold gas had previously been detected in this protocumulus, but the mass of the hot gas found in this new study exceeds it by thousands of times. This finding demonstrates that the Web protocluster is expected to become a massive galaxy cluster in about 10 billion years, with its mass increasing by at least tenfold.

Tony Mroczkowski, co-author of the paper and ESO researcher, explains that “this system exhibits tremendous contrasts. The hot thermal component will destroy much of the cold component as the system evolves, and we are witnessing a delicate transition.” He concludes that it “provides an observational confirmation of long-term theoretical predictions about the formation of the largest gravitationally bound objects in the Universe“.

These results help lay the groundwork for synergies between ALMA and ESO’s forthcoming Extremely Large Telescope (ELT), which “will revolutionize the study of structures such as the Spider’s Web,” says Mario Nonino, co-author of the study and a researcher at the Astronomical Observatory. from Trieste.

The ELT and its state-of-the-art instruments, such as HARMONI and MICADO, will be able to peer into protoclusters and report in great detail on the galaxies they host. Together with ALMA’s capabilities to track the ICM in formation, this will provide crucial insight into the assembly of some of the largest structures in the early Universe, the researchers conclude.