The galaxy protocluster is question is forming around the recently-discovered Spiderweb galaxy (also known as MRC 1138-262 or PKS 1138-262), an object nearly 10 billion light-years away.
This image shows the Spiderweb protocluster, seen at a time when the Universe was only 3 billion years old. Image credit: ESO / Di Mascolo et al. / NASA / ESA / Hubble / H. Ford.
Galaxy clusters contain thousands of galaxies of all ages, shapes and sizes.
They have a mass of about one million billion times the mass of the Sun and form over billions of years as smaller groups of galaxies slowly come together.
Galaxy clusters also contain a vast intracluster medium (ICM) of gas that permeates the space between the galaxies in the cluster. This gas in fact considerably outweighs the galaxies themselves.
Much of the physics of galaxy clusters is well understood. However, observations of the earliest phases of formation of the ICM remain scarce.
Previously, the ICM had only been studied in fully-formed nearby galaxy clusters. Detecting the ICM in distant protoclusters would allow astronomers to catch these clusters in the early stages of formation.
University of Trieste astronomer Luca Di Mascolo and colleagues were keen to detect the ICM in a protocluster from the early stages of the Universe.
“Cosmological simulations have predicted the presence of hot gas in protoclusters for over a decade, but observational confirmations has been missing,” said Dr. Elena Rasia, a researcher at the Italian National Institute for Astrophysics.
“Pursuing such key observational confirmation led us to carefully select one of the most promising candidate protoclusters.”
That was the Spiderweb protocluster, located at 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 proper, long-lasting galaxy cluster rather than dispersing.
The study authors detected the ICM of the Spiderweb protocluster through what’s known as the thermal Sunyaev-Zel’dovich effect.
This effect happens when light from the Cosmic Microwave Background — 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 color, or wavelength, changes slightly.
“At the right wavelengths, the Sunyaev-Zel’dovich effect thus appears as a shadowing effect of a galaxy cluster on the Cosmic Microwave Background,” Dr. Di Mascolo said.
By measuring these shadows on the Cosmic Microwave Background, astronomers 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,” Dr. Di Mascolo said.
The astronomers determined that the Spiderweb protocluster contains a vast reservoir of hot gas at a temperature of a few tens of millions of degrees Celsius.
Previously, cold gas had been detected in this protocluster, but the mass of the hot gas found in this new study outweighs it by thousands of times.
This finding shows that the Spiderweb protocluster 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.
“This system exhibits huge contrasts,” said Dr. Tony Mroczkowski, an astronomer at ESO.
“The hot thermal component will destroy much of the cold component as the system evolves, and we are witnessing a delicate transition.”
“It provides observational confirmation of long-standing theoretical predictions about the formation of the largest gravitationally bound objects in the Universe.”
The team’s results appear today in the journal Nature.
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L. Di Mascolo et al. 2023. Forming intracluster gas in a galaxy protocluster at a redshift of 2.16. Nature 615, 809-812; doi: 10.1038/s41586-023-05761-x
