Using the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner, astronomers have discovered a large reservoir of hot gas in the still-forming cluster of galaxies around the Spider’s Web Galaxy — the most distant detection of such hot gas up to now. Clusters of galaxies are some of the largest objects known in the Universe and this result, published today in Nature, further shows how early these structures begin to form.
Milky Way Clusters, as the name suggests, are home to a large number of galaxies – sometimes even thousands. They also contain a huge “intracluster medium” (ICM) of gas that permeates the space between the galaxies in the cluster. This gas weighs considerably heavier than the galaxies themselves. Much of the physics of galaxy clusters is well understood; however, observations of the earliest stages of the formation of the ICM remain sparse.
Previously, the ICM had only been studied in fully formed nearby galaxy clusters. Detecting the ICM in distant protoclusters — that is, still-forming galaxy clusters — would allow astronomers to catch 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, was eager to detect the ICM in a protocluster from the early stages of the Universe.
Clusters of galaxies are so massive that they can collect gas that heats up as it falls toward the cluster. “Cosmological simulations have predicted the presence of hot gas in protoclusters for more than a decade, but observational confirmations have been lacking,” explains Elena Rasia, a researcher at the Italian National Institute of Astrophysics (INAF) in Trieste, Italy, and co-author of the study. “The pursuit of such an important observation confirmation prompted us to carefully select one of the most promising candidate protoclusters.” That was the Spiderweb Protocluster, set in an epoch when the Universe was only 3 billion years old. Despite being the most intensively studied protocluster, the presence of the ICM has remained elusive. Finding a large reservoir of hot gas in the Spiderweb protocluster would indicate that the system is on its way to becoming a true, long-lasting cluster of galaxies rather than disintegrating.
Di Mascolo’s team discovered the ICM of the Spiderweb protocluster through what is known as the Sunyaev-Zeldovich (SZ) thermal effect. This effect occurs when light from the cosmic microwave background — the remnant 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 little energy and changes its color or wavelength slightly. “So at the right wavelengths, the SZ effect appears as a shadow effect of a cluster of galaxies on the cosmic microwave background,” explains DiMascolo.
By measuring these shadows on the cosmic microwave background, astronomers can deduce the existence of the hot gas, estimate its mass and map its shape. “Thanks to its unparalleled resolution and sensitivity, ALMA is currently the only facility capable of performing such a measurement for the distant progenitors of massive clusters,” says DiMascolo.
They determined that the Spiderweb protocluster contains a huge reservoir of hot gas with a temperature of several tens of millions of degrees Celsius. Cold gas had previously been detected in this protocluster, but the mass of the hot gas found in this new study weighs thousands of times more. This finding shows that the Spiderweb protocluster is indeed expected to transform into a massive cluster of galaxies in about 10 billion years, increasing its mass by at least a factor of ten.
Tony Mroczkowski, co-author of the paper and a researcher at ESO, explains “This system shows huge contrasts. The hot thermal component will destroy much of the cold component as the system evolves, and we are witnessing a delicate transition.” That’s what he concludes “It provides observational confirmation of longstanding theoretical predictions about the formation of the largest gravitationally bound objects in the Universe.”
These results help lay the foundations 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, a co-author of the study and a researcher at the Trieste Astronomical Observatory. The ELT and its state-of-the-art instruments, such as HARMONI and MICADO, will be able to peer into protoclusters and tell us in great detail about the galaxies within. Together with ALMA’s ability to trace the forming ICM, this will provide a crucial glimpse into the composition of some of the largest structures in the early Universe.
- Formation of intracluster gas in a galaxy protocluster with a redshift of 2.16 inches to appear in Nature DOI: 10.1038/s41586-023-05761-x