Astronomers are witnessing the birth of a very distant group of galaxies from the early universe

Using the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner, astronomers have detected a large reservoir of hot gas in the still-forming cluster of galaxies around the Spiderweb galaxy – the furthest detection of such hot gas to date. Galaxy clusters are some of the largest known objects in the universe and this finding was published today in naturefurther reveals when early these structures begin to form.

Galaxy clusters, as the name implies, host a large number of galaxies – sometimes even thousands. They also contain an extensive “inner cluster medium” (ICM) of gas that permeates the space between the galaxies in the cluster. This gas is actually much larger than the galaxies themselves. Much of the physics of galaxy clusters is well understood; However, observations of the early stages of ICM formation are still scarce.

Previously, ICM has only been studied in fully formed nearby galaxy clusters. The detection of ICM in distant protoclusters — that is, galaxy clusters that are still forming — will allow astronomers to capture 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 in Italy, was eager to discover an ICM in a proto-cluster from the very early stages of the universe.

Galaxy clusters are so massive that they can accumulate gas that gets heated up as it falls toward the cluster. explains Elena Rascia, a researcher at the Italian National Institute for Astrophysics (INAF) in Trieste, Italy, and co-author of the study book. “The pursuit of this primary observational confirmation led to the selection of one of the most promising candidate primary cohorts.”

This was the first spider cluster, which was located in an era when the universe was only 3 billion years old. Despite being the most intensively studied protoelement, the existence of ICM has remained elusive. The finding of a large reservoir of hot gas in the Spiderweb protocluster indicates that the system is on its way to becoming a proper, long-lived cluster of galaxies rather than dispersing.

Di Mascolo’s team detected the ICM of the proto-spider cluster through what is known as the thermal Sunyaev-Zeldovich (SZ) effect. This effect occurs when light from the cosmic microwave background — radiation left over from the Big Bang — passes through the ICM. When this light interacts with the fast-moving electrons in the hot gas, it gains a little energy and changes its color or wavelength slightly. “At the right wavelengths, the SZ effect appears as a shadow effect of a galactic cluster on the cosmic microwave background,” Di Mascolo explains.

By measuring these shadows on the cosmic microwave background, astronomers can thus infer the presence of hot gas, estimate its mass, and map its shape. “With unparalleled precision and sensitivity, ALMA is the only facility currently able to make such a measurement of the distant ancestors of megaclusters,” says Di Mascolo.

They determined that the Spiderweb protocluster contained a huge reservoir of hot gas at a temperature of a few tens of millions of degrees Celsius. Previously, cold gas was detected in this protocluster, but the mass of hot gas found in this new study is thousands of times greater. This discovery shows that the proto-cluster of spiders is already expected to transform into a huge cluster of galaxies in about 10 billion years, increasing its mass by at least ten times.

Tony Mrozkowski, co-author of the paper and researcher at ESO, explains, “This system shows huge variances. The hot thermal component will destroy a lot of the cold component as the system evolves, and we are witnessing a microtransformation.” He concludes, “It provides observational confirmation of ancient theoretical predictions about the formation of the largest gravitationally bound objects in the universe.”

These results help lay the foundations for a synergy between ALMA and ESO’s upcoming Extremely Large Telescope (ELT), which will “revolutionize the study of structures like the Spiderweb,” says Mario Nonino, co-author of the study and researcher for the Astronomical Journal. Trieste Observatory. ELT and its latest instruments, such as HARMONI and MICADO, will be able to look at primary clusters and tell us about the galaxies in them in great detail. Combined with ALMA’s capabilities to track the formation of the ICM, this will provide an important glimpse into the assembly of some of the largest structures in the early universe.