Deep X-ray observations of Abell 2029 — sometimes described as the most relaxed galaxy cluster in the Universe — uncovered evidence of an ancient cosmic collision, including a gigantic spiral of superheated gas stretching 2 million light-years across.
This composite image combines both the original X-ray and the subtracted X-ray images of the deep Chandra observations of Abell 2029. Image credit: NASA / CXC / CfA / Watson et al. / PanSTARRS / SAO / N. Wolk / P. Edmonds.
“Galaxy clusters are the largest structures in the Universe held together by gravity,” said Boston University astronomer Courtney Watson and colleagues.
“They are made up of hundreds or even thousands of galaxies, unseen dark matter, and a huge amount of gas that fills in the space between the galaxies.”
“This gas is typically heated to millions of degrees, which makes it glow in X-ray light.”
Using NASA’s Chandra X-ray Observatory, the astronomers obtained the deepest X-ray observation ever made of Abell 2029, a galaxy cluster located approximately one billion light-years away in the constellation of Virgo.
The new data reveal that this galaxy cluster is still settling down after a raucous collision with another smaller cluster about four billion years ago.
“We think the spiral shape in the hot gas formed when gas in the cluster sloshed to the side because of the gravitational effects of the cluster collision — similar to how wine moves in a wine glass,” the researchers said.
“The sloshing spiral in Abell 2029 is one of the longest ever seen, extending about two million light-years from the center of the cluster.”
“There are several other key pieces of evidence for the past bash, never before seen together in a cluster, allowing us to trace the collision history of the cluster in unprecedented detail.”
“For example, we see hints of a wide splash of cooler gas created by the collision.”
“There may also be a shock wave — akin to a sonic boom from a supersonic plane — in the superheated gas left over from the collision.”
“Finally, there is a ‘bay’ feature in the hot gas, which we think might be caused by an overlap between the outer parts of the spiral and gas stripped away from the smaller cluster as it passed through the larger one.”
“Though we think it is a relic from the collision, other explanations for this structure are also possible.”
The team’s computer simulations suggest that the smaller cluster was about ten times less massive than the larger cluster.
“The sloshing spiral formed when the smaller cluster made its first pass through the larger cluster, pulling its gas sideways,” the scientists said.
“The gravity of the larger cluster then caused the other cluster to slow down and get pulled back in for a second collision.”
“This drove a shock front and left behind a wake of material, forming the splash region.”
“To uncover these various features the authors used a special technique that examined how much the cluster’s hot gas deviates from a symmetrical shape.”
“Most of the hot gas is symmetrical and is approximately shaped like an oval.”
The team’s results were published in the Astrophysical Journal.
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Courtney B. Watson et al. 2026. Deep Chandra X-Ray Observations of A2029: The Merger History of a Relaxed, Strong Cool Core Cluster. ApJ 996, 106; doi: 10.3847/1538-4357/ae2026
