A new study of a cluster of galaxies has offered astronomers a chance to understand the elusive dark matter. X-ray observations from NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton and Hitomi, a Japanese-led X-ray telescope, have meant that astronomers have had to come up with an innovative interpretation of the data. This could tell us something about the strange unseen material that makes up 85 percent of all matter in the universe.
The premise of this work began in 2014, when a team of astronomers led by Esra Bulbul of the Harvard-Smithsonian Centre for Astrophysics, Cambridge, Massachusetts, discovered a noticeable spike in intensity at a very specific energy level. While studying the hot gas within the Perseus galaxy cluster, the Chandra and XMM-Newton observatories revealed an unexpected spike, or emission line, corresponding to an energy of 3.5 kiloelectron volts (keV). This wavelength is very difficult to explain, as it cannot be described by previously observed – or even predicted – astronomical objects. For this reason, a dark matter theory was suggested in order to explain it. Bulbul and her colleagues had also mentioned that the same emission energy existed in a study of 73 other galaxy clusters which were gathered by the XMM-Newton observatory.
“We expect that this result will either be hugely important or a total dud,” says Joseph Conlon of Oxford University. “I don’t think there is a halfway point when you are looking for answers to one of the biggest questions in science.”
A week after Bulbul had submitted their work, a different group of astronomers, led by Alexey Boyarsky of the Leiden University in the Netherlands, had also reported evidence of the 3.5 keV spike. Their work showed that this energy emission was present in the Andromeda Galaxy, also known as M31, and the outskirts of the Perseus cluster. This supported the claims made by Bulbul and her team.
There was much debate over the results, as some astronomers could detect the 3.5 keV emission, while others could not. In 2016, the Hitomi telescope, specifically designed to detect detailed features like the 3.5 keV emission line, failed to detect any emission at this energy level in the Perseus galaxy cluster.
“One might think that when Hitomi didn’t see the 3.5 keV line that we would have just thrown in the towel for this line of investigation,” says Francesca Day, also from Oxford University. “On the contrary, this is where, like in any good story, an interesting plot twist occurred.”
Conlon and the team had pointed out that the Hitomi telescope produced much fuzzier images than Chandra. This meant they couldn’t separate the X-ray signals from the diffuse component of the hot gas encompassing the large galaxy, and the X-ray emission from near the supermassive black hole in the galaxy. As Chandra has a much finer resolving power, Bulbul was able to differentiate between the two and isolate the X-ray emission from the hot gas.
To test the usefulness of isolation, the Oxford University team reanalysed Chandra data close to the black hole at the centre of the Perseus cluster, taken in 2009. A surprising result arose from this: there was a lack of X-ray energy at 3.5 keV, rather than excess. This suggests that something in Perseus is absorbing X-rays at this particular wavelength. The researchers then simulated the Hitomi spectrum by including absorption lines with the emission energies from the hot gas seen with Chandra and XMM-Newton. No evidence was found from this simulation that was consistent with Hitomi data, meaning the team had to try and explain this unusual behaviour.
The detection of absorption lines in X-rays close to the black hole, and emission lines at the same energy from hot gas away from black hole, is similar to how starlight is affected by surrounding clouds of gas. When starlight interacts with a surrounding gas cloud, the gas absorbs the starlight and creates a similar absorption line in the spectrum of the starlight. On the other hand, an atom in the gas cloud can absorb the starlight, thus scattering energy in all directions and at the same wavelength, creating an emission line.
The Oxford team compared these scenarios, and they suggest that the dark matter particles may be the atoms producing the emission and absorption lines at 3.5 keV. “This is not a simple picture to paint, but it’s possible that we’ve found a way to both explain the unusual X-ray signals coming from Perseus and uncover a hint about what dark matter actually is,” says Nicholas Jennings, also of Oxford University.
To confirm these results, much more data is needed. The team of astronomers will continue to observe the Perseus cluster, and other galaxy clusters like it. This will confirm the reality of the situation, ruling out any sort of uncertainty. It could be that these observations are the result of an unexpected instrumental effect. If this discovery were correct, it would be an amazing step forward in understanding the nature of dark matter.
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