In the span of a human lifetime, much of the Universe seems unchanging. But that’s an illusion; things are always changing, and that fact can make galaxies and the clusters they reside in very unruly places due to mergers and collisions.
However, some galaxy clusters seem much calmer than others.
Galaxy clusters are staggering structures that can contain hundreds of individual galaxies, along with vast clouds of gas and huge quantities of dark matter. Our Milky Way galaxy is part of the Virgo Supercluster, which contains more than 100 clusters and galaxy groups. And the Virgo Supercluster itself is part of the Laniakea Supercluster. These nested structures are inevitably compared to Matryoshka Dolls.
Galaxy clusters grow over time through mergers of individual galaxies, groups, and even other clusters. This is a chaotic process that leaves its mark on the resulting clusters.
But some clusters are considered “relaxed,” meaning they show no signs of mergers. Scientists recently found the most distant relaxed cluster yet, named SPT2215. It’s about 8.4 billion light years away, and astronomers are seeing it as it was when the Universe was only 5.3 billion years old. Astronomers have found relaxed galaxy clusters before, but never this far away and never this ancient.
“Up until now, we have not seen a relaxed galaxy cluster as distant as SPT2215,” said Michael Calzadilla of the Massachusetts Institute of Technology (MIT), lead author of a paper on the cluster. That paper is the most recent of three papers covering the discovery and different key properties of the cluster and other clusters.
The three papers are:
It’s surprising to find a relaxed cluster from this epoch of the Universe. That’s because clusters grow through mergers and collisions, and clusters should be going through that messy process on their way to growing so large. This cluster is massive and contains 700 trillion solar masses of material, meaning it had to have experienced mergers.
Several things about SPT2215 indicate that it’s calm, and has been calm for quite some time.
Relaxed clusters are also called ‘cool core clusters.’ In this case, a massive galaxy lies at the center of SPT2215. There’s evidence of prodigious star formation going on at the center of the galaxy, near the galaxy’s supermassive black hole. For all this star formation to take place, the gas near the center has to be cool enough. If it’s too hot, no stars can form. Typically, black hole outbursts can keep the gas near the center too hot for star formation to occur. But for some reason, this black hole is calm.
“The extreme cooling and SFR <star formation rate> of SPT2215 is rare among known cool core clusters, and it is even more remarkable that we observe these at such high redshift when most clusters are still dynamically disturbed,” the authors explain in the first paper.
“It seems like the black hole in SPT2215 is quiet enough to let star formation flourish,” said Michael McDonald, also of MIT, who is a co-author of all three papers.
Another one of SPT2215’s unusual features is that only a single galaxy lies at its center. There are no other galaxies within about 600,000 light-years that are bright or extended. This is more evidence that the cluster hasn’t undergone any mergers recently, at least within the last billion years.
“The fact that this cluster is so massive, so early in the universe, suggests a really exciting, fast formation history.”
The isolated galaxy at the heart of SPT2215 is a highly evolved giant elliptical galaxy. It’s also known as a brightest cluster galaxy (BCG.) They’re the brightest single galaxy in a cluster of galaxies. BCGs are among the most massive and also the most luminous galaxies in our current epoch. Its presence and characteristics also indicate that the cluster is relaxed and that it experienced most of its growth in the Universe’s earlier stages of evolution.
“The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early universe,” the authors explain in their paper.
This figure from the research helps illustrate the galaxy cluster’s unusual properties. It shows successive zoomed-in images of SPT2215 with different telescopes and at different wavelengths. The upper left shows an ASKAP (Australian Square Kilometre Array Pathfinder) image in radio waves with the AGN in the center. The upper right is a Chandra X-Ray image showing “a very relaxed ICM <intracluster medium> morphology and a strongly peaked central surface brightness profile.” The bottom right shows a Hubble composite image of the BCG <bright cluster galaxy>, and the bottom left shows two different Hubble images. Filaments are visible that indicate strong star formation. Image Credit: Calzadilla et al. 2023.Putting aside the cluster’s mysterious side, it can tell astrophysicists something about an ongoing issue in astrophysics: the role black hole feedback plays in star formation.
When a supermassive black hole actively accretes material and emits powerful jets of energy, it’s called an active galactic nucleus. Astrophysicists aren’t certain how the accretion is turned on or off, nor do they clearly understand the jets of energy. Simulations from Illustris TNG show that much of the heated material can cool relatively quickly and allow stars to form, but it’s not conclusive.
In the Hubble image in the above panel, filaments of gas extend out from the BCG to about 20 kiloparsecs. They may extend even further than that, and deeper observations will reveal their full extent. These filaments are basically long nebulae, where young stars are lighting up the gas with their powerful radiation. They’re strong evidence of high star formation rates. “Filamentary nebulae like these typically signal regions of ionization by young stars and can be seen in many other strong cooling clusters,” the authors explain in their paper.
But the main highlight in this discovery concerns the size of the cluster itself. It must have formed very rapidly to be so massive and to be so relaxed.
“The fact that this cluster is so massive, so early in the universe, suggests a really exciting, fast formation history,” said Lindsey Bleem of the U.S. Department of Energy’s Argonne National Laboratory. Bleem and colleagues first reported spotting the cluster in 2020, in the first of the three new papers. “Yet the fact that it is relaxed suggests the opposite. It would be like finding a tidy kitchen right after the dinner rush.”
These new observations dovetail with observations from the James Webb Space Telescope. The JWST looked back in time to the Universe’s early galaxies and found that they’re much more fully formed much earlier than thought. These JWST observations led one researcher to suggest that the Universe is actually much older than thought: twice as old, in fact, though the paper that suggested it hasn’t gained much traction.
Somehow, we’ve misunderstood how quickly galaxies formed and evolved and how they and the clusters they’re part of grew so massive so quickly. The observations of SPT2215 only make us question our understanding even further.
Relaxed clusters play another role in our understanding of the cosmos. They’re signposts used to mark and measure the expansion of the Universe and its cause, dark energy.
“Relaxed clusters like SPT2215 are one of the signposts that have been used to measure the expansion of the universe,” said Adam Mantz of Stanford University, who first reported SPT2215’s relaxed status using Chandra data in 2022, in the second paper. “Adding distant objects like this to our sample of relaxed clusters allows us to better constrain the acceleration of the cosmic expansion, and the properties of the dark energy that drives it.”
