Astronomers who inventory the matter in the local universe keep coming up short. A new result from NASA’s Chandra X-ray Observatory on a system of colliding galaxy clusters may help explain this shortfall.
Although scientists know a lot about the composition of the universe, there has been one annoying problem that they have struggled to explain: There is a significant amount of matter that has not yet been explained.
This missing mass is not the invisible dark matter, which makes up most of the matter in the universe. This is a separate puzzle where about a third of the “normal” matter created in the first billion years or so after the Big Bang has yet to be detected by observations of the local universe, i.e. in regions less than a few billion years ago. light-years from Earth. This matter consists of hydrogen, helium and other elements and forms objects such as stars, planets and people.
Scientists have proposed that at least some of this missing mass could be hidden in giant strands, or filaments, ranging from warm to hot (temperatures from 10,000 to 10,000,000 Kelvin) in the space between galaxies and galaxy clusters. They have called this the “warm-hot intergalactic medium” or WHIM.
A team of astronomers using Chandra to observe a system of colliding galaxy clusters has likely found evidence of this WHIM residing in the space between them.
“It has proven extremely difficult to find these filaments of missing matter, and only a few examples are known,” said Arnab Sarkar of the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts, who led this study. “We’re excited to probably have found another one.”
The researchers used Chandra to study Abell 98, which contains colliding galaxy clusters about 1.4 billion light-years from Earth. The Chandra data reveals a bridge of X-rays between two of the colliding clusters containing gas with a temperature of about 20 million kelvins and cooler gas with a temperature of about 10 million kelvins. The hotter gas in the bridge probably comes from gas in the two clusters that overlap. The temperature and density of the cooler gas match the predictions for the hottest and densest gas in the WHIM.
In addition, the Chandra data shows the presence of a shock wave, which is similar to a sonic boom from a supersonic plane. This shock wave is propelled by and in front of one of the galaxy clusters as it begins to collide with another cluster. This is the first time astronomers have found such a shock wave in the early stages of a galaxy collision, before the centers of the cluster pass each other.
“We think this shock wave is an important discovery because our models have predicted that such features should be there, but we haven’t seen any yet,” said co-author Scott Randall, also of CfA. “They are an important part of the early collision process that will eventually lead to a merger of the clusters.”
This shock wave may be directly related to the discovery of the WHIM in Abell 98 because it heated the gas between the clusters as they collided. This may have raised the temperature of the gas in the WHIM filament — estimated to be about 400 billion times the sun’s mass — high enough to be detected with Chandra data.
Clusters of galaxies — containing thousands of galaxies, vast amounts of hot gas, and huge reservoirs of dark matter — are the largest structures in the universe held together by gravity. Scientists think they can reach their colossal size by merging together over millions or billions of years.
“When galaxy clusters collide, we get the chance to see extreme physics that we rarely see in any other cosmic environment,” said Yuanyuan Su, a co-author from the University of Kentucky.
A paper describing this result by Sarkar et al was published in The astrophysical diary letters
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Arnab Sarkar et al, discovery of a premerger shock in an intercluster filament in Abell 98, The astrophysical diary letters (2022). DOI: 10.3847/2041-8213/ac86d4
Gabriella E. Alvarez et al, Suzaku Observations of the Cluster Outskirts and Intercluster Filament in the Triple Merger Cluster Abell 98. arXiv:2206.08430v1 [astro-ph.CO], arxiv.org/abs/2206.08430
Quote: NASA’s Chandra finds galaxy cluster collision on a ‘WHIM’ (2022, Oct. 14) retrieved Oct. 14, 2022 from https://phys.org/news/2022-10-nasa-chandra-galaxy-cluster-collision.html
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