The universe’s ‘most relaxed’ galaxy cluster was shaped by cosmic violence, new study finds

For decades, astronomers have described the galaxy cluster Abell 2029, a vast city of galaxies in the Virgo constellation, as “the most relaxed clusters in the universe.” But beneath that placid exterior, scientists have now found, the cluster is still reverberating from an ancient cosmic collision.

New observations from the Chandra X-ray Observatory suggest giant “sloshing” motions in the cluster’s gas — triggered by a merger roughly 4 billion years ago — may help heat the cluster alongside energy released by the supermassive black hole at its center. That could help explain why the gas in galaxy clusters does not cool as quickly as expected, the researchers say. But of note, Chandra’s new data also revealed massive substructures still visible today, including gigantic spirals, shock fronts and waves of superheated gas rippling through the cluster, according to a statement released last week by Chandra X-ray Observatory.

“Overall, our results suggest that A2029 is still settling from past interactions,” a team of researchers led by astrophysicist Courtney Watson of Boston University write in the new paper, “showing that even the initially most relaxed-looking clusters can be hiding a rich history of dynamical activity.”

Hosting more than a thousand galaxies, Abell 2029 is among the largest known galaxy clusters, which are sprawling collections of galaxies bound together by gravity and immersed in enormous clouds of heated gas. At its center sits IC 1101, a colossal elliptical galaxy estimated to span nearly 6 million light-years across, making it one of the largest galaxies ever discovered.

Astronomers have since the 1990s regarded Abell 2029 as unusually tranquil. Most recently, two studies published in 2025 using the XRISM observatory, which studies the universe in X-ray light, found extremely low levels of turbulence in the superheated gas filling the space between galaxies, suggesting the cluster had not experienced a recent major disruptive interaction such as a merger.

But a third XRISM study, also published in 2025, hinted that all was not entirely calm.

Researchers at that point had detected cooler pockets of gas embedded within the hotter cluster atmosphere — possible leftovers, they said, from ancient “sloshing” motions triggered by a long-ago collision.

The new Chandra X-ray Observatory observations add fresh evidence to that more turbulent history, revealing that — like ripples lingering long after a stone strikes water — the scars etched into Abell 2029’s superheated gas continue to tell the story of a violent cosmic encounter billions of years ago.

Using 21 new X-ray observations collected in 2022 and 2023, along with archival data gathered earlier, Watson and her colleagues traced enormous structures hidden within the cluster’s hot gas, including one of the longest continuous “sloshing spirals” ever observed, the study reports. The spiral stretches nearly 2 million light-years from the cluster’s center.

The observations also revealed a concave “bay”-like depression south of the cluster core, a broad “splash” of cooler gas extending southeastward, and evidence of a possible shock wave propagating through the cluster outskirts, according to the paper.

To uncover the array of hidden substructures, the team used image-processing techniques that removed the cluster’s otherwise smooth and symmetrical X-ray glow. Computer simulations that then traced the cluster’s history suggest the structures likely formed after a smaller galaxy cluster plunged through Abell 2029 billions of years ago, the study notes.

That collision would have displaced the hot gas, causing it to oscillate and swirl through the cluster’s gravitational field, “similar to how wine moves in a wine glass,” the statement read.

The study suggests those giant sloshing motions may help regulate how the cluster cools over time by redistributing heat through the intracluster gas, alongside energy injected by the active supermassive black hole at the center of IC 1101, which is one of the most massive known to date.

That matters because the gas inside galaxy clusters constantly radiates energy away in X-rays and should gradually cool over time, and scientists suspect energy released by supermassive black holes, known as AGN feedback, helps reheat that gas and prevent runaway cooling.

But the new findings suggest black hole activity alone may not fully explain what is happening in Abell 2029, and that the large-scale sloshing caused by the ancient merger may also play an important role in stirring and heating the gas, the new study notes.

Meanwhile, the splash feature of cooler gas may trace a wake of material left behind after the smaller cluster made a second pass through Abell 2029, the study suggests. Simulations indicate the smaller cluster initially swept through the larger one, dragging gas sideways and generating the enormous spiral. Gravity from the larger cluster would have then slowed the smaller object and pulled it back for another encounter, producing shock fronts and additional disturbances in the gas.

The bay structure may represent overlapping gas flows where the outer edge of the spiral intersects with material stripped from the smaller cluster during the collision, according to the study.

Alternatively, the study notes, the bay feature could be associated with the edge of a giant “ghost bubble” — an ancient cavity carved into the gas by activity from the supermassive black hole at the center of IC 1101.

Although Watson and her colleagues found no clear evidence of previously undiscovered radio bubbles in the cluster gas, they note that projection effects could make such structures difficult to detect if they are partially aligned along our line of sight.

This research is described in a paper published December 2025 in The Astrophysical Journal.