ALMA Detects Huge Reservoir of Hot Gas in Very Distant Galaxy Protocluster

The Spiderweb protocluster is clearly visible in this snapshot from a period when the cosmos were just 3 billion years old. the Hubble Space Telescope/NASA/ESA/Di Mascolo et al./H. Ford/ESO.

Numerous galaxies of varying ages, sizes, and shapes can be found within galaxy clusters.

As smaller groups of galaxies gradually come together over billions of years, their mass increases to about a million billion times that of the Sun.

The space in between galaxies in a cluster is filled with a massive gas cloud called the intracluster medium (ICM). In reality, this gas is far more massive than the galaxies themselves.

The mechanics of galaxy clusters are well understood. Fewer and fewer early ICM developmental stages have been observed.

Previously, the ICM had only been investigated in nearby clusters of mature galaxies. Observing the ICM in distant protoclusters would give astronomers a window into the formation of these objects.

Luca Di Mascolo, an astronomer at the University of Trieste, and his colleagues were keen to pinpoint the ICM in a protocluster from the universe’s formative years.

Dr. Elena Rasia of the Italian National Institute for Astrophysics said, “Cosmological simulations have actually forecasted the existence of hot gas in protoclusters for over a years, however observational verifications has actually been missing.”

“Since obtaining this kind of observational confirmation is so important, we took our time picking from among the most promising prospect protoclusters.”

There, at a time when the cosmos were just 3 billion years old, was the Spiderweb protocluster.

The ICM is the protocluster that has been investigated the most, although its existence remains a mystery.

If a large reservoir of hot gas is found inside the Spiderweb protocluster, it indicates that the system is evolving into a proper, long-lived galaxy cluster rather than dispersing.

Authors of the study used the thermal Sunyaev-Zel’dovich finding to determine the ICM of the Spiderweb protocluster.

This happens when radiation from the Big Bang’s remnant, the Cosmic Microwave Background, passes through the intergalactic medium (ICM).

When this light interacts with the fast-moving electrons in the hot gas, it picks up a tiny bit of energy and slightly shifts in colour, or wavelength.

Dr. Di Mascolo said that at the right wavelengths, the Sunyaev-Zel’dovich finding resembles the observational signature of a cluster of galaxies on the Cosmic Microwave Background.

Astronomers may assume the existence of the hot gas, approximating its mass and mapping its form, by calculating these shadows on the Cosmic Microwave Background.

For the distant progenitors of massive clusters, “thanks to its unrivalled resolution and level of sensitivity, ALMA is the only centre currently efficient in carrying out such a measurement,” Dr. Di Mascolo said.

The researchers found a vast reservoir of hot gas in the Spiderweb protocluster, with temperatures of a few tens of thousands of degrees Celsius.

Although cold gas had previously been discovered in this protocluster, the mass of the hot gas discovered in this new study far exceeds that of the cold gas.

This discovery proves that the Spiderweb protocluster will very probably transform into a massive galaxy cluster in around 10 billion years, increasing its mass by at least a factor of 10.

Dr. Tony Mroczkowski, an astronomer at ESO, said, “This system displays substantial contrasts.”

As the system develops, “the hot thermal element will damage much of the cold part,” and this change is precarious.

It provides observable confirmation of long-standing theoretical predictions about the growth of the largest gravitationally bound objects in the universe’s depths.

The findings of the team are published in today’s issue of Nature.

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Authors: L. Di Mascolo et al. Nature 615, 809-812; doi: 10.1038/ s41586-023-05761-x, discusses the formation of intracluster gas in a galaxy protocluster at redshift 2.16.