Free Astronomy Magazine July-August 2023

27 JULY-AUGUST 2023 ASTRO PUBLISHING ICM in a protocluster from the early stages of the Universe. Galaxy clusters are so massive that they can bring together gas that heats up as it falls towards the clus- ter. “Cosmological simulations have predicted the presence of hot gas in protoclusters for over a decade, but observational confirmations has been missing,” explains Elena Rasia, researcher at the Italian National Institute for Astrophysics (INAF) in Trieste, Italy, and co-author of the study. “Pursuing such key observa- tional confirmation led us to care- fully select one of the most promis- ing candidate protoclusters.” That was the Spiderweb protoclus- ter, located at an epoch when the Universe was only 3 billion years old. Despite being the most inten- sively studied protocluster, the pres- ence of the ICM has remained elu- sive. Finding a large reservoir of hot gas in the Spiderweb protocluster would indicate that the system is on its way to becoming a proper, long- lasting galaxy cluster rather than dispersing. Di Mascolo’s team detected the ICM of the Spiderweb protocluster through what’s known as the ther- mal Sunyaev-Zeldovich (SZ) effect. This effect happens when light from the cosmic microwave back- ground — the relic radiation from the Big Bang — passes through the ICM. When this light interacts with the fast-moving electrons in the hot gas it gains a bit of energy and its colour, or wavelength, changes slightly. “At the right wavelengths, the SZ effect thus appears as a shadowing effect of a galaxy clus- ter on the cosmic microwave back- ground,” explains Di Mascolo. By measuring these shadows on the cosmic microwave background, as- tronomers can therefore infer the existence of the hot gas, estimate its mass and map its shape. “Thanks to its unparalleled resolution and sensitivity, ALMA is the only facility currently capable of performing such a measurement for the distant progenitors of massive clusters,” says Di Mascolo. They determined that the Spider- web protocluster contains a vast reservoir of hot gas at a tempera- ture of a few tens of millions of de- grees Celsius. Previously, cold gas had been detected in this protoclus- ter, but the mass of the hot gas found in this new study outweighs it by thousands of times. This find- ing shows that the Spiderweb pro- tocluster is indeed expected to turn into a massive galaxy cluster in around 10 billion years, growing its mass by at least a factor of ten. Tony Mroczkowski, co-author of the paper and researcher at ESO, ex- plains that “this system exhibits huge 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 ob- servational confirmation of long- standing theoretical predictions about the formation of the largest gravitationally bound objects in the Universe.” These results help to set the ground- work for synergies between ALMA and ESO’s upcoming Extremely Large Telescope (ELT), which “will revolutionise the study of structures like the Spiderweb,” says Mario Nonino, a co-author of the study and researcher at the Astronomical Observatory of Trieste. The ELT and its state-of-the-art instruments, such as HARMONI and MICADO, will be able to peer into protoclus- ters and tell us about the galaxies in them in great detail. Together with ALMA’s capabilities to trace the forming ICM, this will provide a crucial glimpse into the assembly of some of the largest structures in the early Universe. ! T his image shows the proto- cluster around the Spider- web galaxy (formally known as MRC 1138-262), seen at a time when the Universe was only 3 billion years old. Most of the mass in the protocluster does not reside in the galaxies that can be seen in the centre of the image, but in the gas known as the intracluster medium (ICM). The hot gas in the ICM is shown as an overlaid blue cloud. [ESO/ Di Mascolo et al.; HST: H. Ford]