Researchers expected the universe’s earliest galaxy clusters to be relatively cold. However, UBC scientists have discovered hot gas in a 12-billion-year-old cluster, shedding light on galaxy clusters in the early universe — and contradicting current models of their evolution.
The largest galaxies in the universe exist in clusters. Matter between galaxies is heated by gravitational forces from supermassive black holes and radiation from newly formed stars. Simulations of galaxy cluster evolution predicted that intracluster gas would be cooler earlier in cosmic history, since the gas would still be accumulating and heating.
Recently, a team of researchers found a reservoir of extremely hot gas in SPT2349–56, a distant young galaxy cluster about 12 billion light-years from Earth. A light-year represents the distance light travels in one year, meaning researchers were observing the cluster as it appeared 12 billion years ago — when the universe was at only 10 per cent of its current age.
Calculations showed the thermal energy of the gas reservoir in SPT2349–56 was about 10 times greater than what gravitational pull could produce.
“It is very surprising, because this kind of hot gas was thought only [to] exist billions of years later,” said Dazhi Zhou, a PhD student in UBC’s department of physics and astronomy and the first author of the publication.
They detected the gas through leftover radiation from the Big Bang, in the form of the cosmic microwave background (CMB). As they travel, photons from the CMB interact with the highly energetic gas in the intracluster medium (ICM), leading to the thermal Sunyaev–Zeldovich (tSZ) effect. This scattering creates detectable distortions in the CMB.
Instead of relying on light emitted by gases, the tSZ effect allows researchers to observe gases through the “shadows” they create in the CMB. “This way, your signal won’t fade when you go to study a more distant object,” said Zhou. Traditional X-ray observations are restricted by the cosmological surface-brightness dimming, which describes the light from the gas becoming fainter with further distance.
To observe the clusters, Zhou used data from the Atacama Large Millimetre/submillimetre Array (ALMA) telescope a powerful radio telescope located in Chile. The study spanned years. Zhou spent around one of those years filtering out emissions from other galaxies to observe the CMB shadow. The team initially estimated the light from the galaxies to be at least 20 times brighter than the shadow. As a result, filtering out the light was difficult and other team members thought it was impossible. “They even suggested my supervisor ask me to give up the project, because it felt so challenging,” said Zhou. “My supervisor was so worried because I didn’t publish even a single paper after one year of effort.”
“I just kept going.”
The hard work paid off when Zhou spotted the shadow around the galaxy light — a cluster of high-temperature gas, just like what detections through the tSZ effect would predict.
“The first time I saw the shadow, because it was too strong, both my supervisor [and I] were thinking, ‘Oh, it couldn’t be real,’” said Zhou. The team spent months validating the observation, which confirmed they were observing extremely hot gas from early in the universe’s life.
So what could explain this surprising discovery? Within SPT2349–56, there are three supermassive black holes with powerful jets, pumping out a huge amount of energy into the surrounding gas, along with several starburst galaxies with high star-formation rates, making the cluster a good candidate for further examination. These sources could have heavily increased the thermal energy of the gas in the cluster during its early life, leading to much higher temperatures than previously modelled.
According to Zhou, not only does this discovery show that galaxy clusters can evolve faster than previously thought, but it also highlights major observation gaps; this is the only hot gas that scientists have discovered from the first 3 billion years of cosmic history, so there have yet to be any other direct observations of a gas in such an early stage.
This means more work needs to be done to understand the formation of massive clusters and their atmospheres. “We do not know whether this hot gas in this system is an outlier, or is pointing us to a very important and unrecognized phase of cluster evolution,” said Zhou.
For the next steps, Zhou hopes to follow up with more independent observations of the hot gas to fill in knowledge gaps.
“Just like every first discovery, we have to be cautious and test it further, whether [the hot gas] is a very common stage of the cluster evolutions or it is a very strange system,” said Zhou.