Galaxy Cluster Abell 2029 Had Violent Past, Chandra Reveals
Extensive X-ray observations of Abell 2029 – sometimes described as the most relaxed galaxy cluster in the Universe – have uncovered evidence of an ancient cosmic collision, including a gigantic spiral of superheated gas spanning 2 million light years.
This composite image combines both the original X-ray images and the subtracted X-ray images from Abell 2029’s deep Chandra observations. Image credit: NASA / CXC / CfA / Watson and others. / 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, invisible dark matter, and a huge amount of gas that fills the space between the galaxies.”
“This gas is typically heated to millions of degrees, which makes it glow under X-ray light.”
Using NASA’s Chandra X-ray Observatory, astronomers have obtained the deepest-ever X-ray observation of Abell 2029, a galaxy cluster about a billion light-years away in the constellation Virgo.
The new data reveals that this galaxy cluster is still stabilizing after a violent collision with another, smaller cluster about four billion years ago.
“We think the spiral shape of the hot gas formed when the gas in the cluster moved sideways due to the gravitational effects of the cluster collision – similar to how wine moves in a wine glass,” the researchers said.
“The tossing spiral of Abell 2029 is one of the longest ever observed, extending about two million light years from the center of the cluster.”
“There are several other key pieces of evidence from the past hit, never before seen together in a cluster, allowing us to trace the cluster’s collision history in unprecedented detail.”
“For example, we see evidence of a large splash of colder gas created by the collision.”
“There may also be a shock wave – similar to a sonic boom from a supersonic aircraft – in the superheated gas left by the collision.”
“Finally, there is a ‘bay’ feature in the hot gas, which we think could be caused by overlap between the outer parts of the spiral and gas away from the smaller cluster as it passes through the larger one.”
“Although we believe this 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 largest.
“The sloshing spiral formed when the smaller cluster made its first pass through the larger cluster, pulling its gas sideways,” the scientists explained.
“The gravity of the larger cluster then caused the other cluster to slow down and retreat for a second collision.”
“This caused a shock front and left behind a wake of material, forming the splash region.”
“To discover these different features, the authors used a special technique that examined the extent to which hot gases in the cluster deviate from a symmetrical shape.”
“Most hot gases are symmetrical and approximately shaped like an oval.”
The team’s results were published in the Astrophysics Journal.
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Courtney B. Watson and others. 2026. Deep Chandra X-ray observations of A2029: the merger history of a cool, relaxed core cluster. ApJ 996, 106; doi: 10.3847/1538-4357/ae2026
