The frustrating quest for dark-matter continues. An unidentified X-ray signature recently observed in a nearby galaxy clusters is not due to the decay of dark matter, researchers report. The findings ruled out previously proposed interpretations of dark matter particle physics. But the near future for solving this great mystery of 21st-century astronomy looks bright.
Despite its cosmological abundance and the well-established astrophysical evidence of its existence, little is known about the mysterious, invisible dark matter particles. Some models of dark matter predict that they might slowly decay into ordinary matter. If so, the process of dark matter decay would produce faint photon emissions detectable by X-ray telescopes.
Unidentified X-ray Emission Line
Recent X-ray observations of nearby galaxy clusters have detected an unidentified X-ray emission line at 3.5 kiloelectronvolts (keV), which has been interpreted by some as a signature of dark matter decay. Specifically, the X-ray emission was linked to a hypothetical dark matter particle known as a sterile neutrino – a theoretical particle that is believed to interact only via gravity and not via other fundamental interactions of the Standard Model. If this is correct, dark matter surrounding our Galaxy should decay and produce a similar X-ray emission line, spread faintly across the entire night sky.
Physicists have suggested that dark matter is a closely related cousin of the neutrino, called the sterile neutrino. Neutrinos – subatomic particles with no charge and which rarely interact with matter – are released during nuclear reactions taking place inside the sun. They have a tiny amount of mass, but this mass isn’t explained by the Standard Model of Particle Physics. Physicists suggest that the sterile neutrino, a hypothetical particle, could account for this mass and also be dark matter.
Searching the Milky Way’s Dark-Matter Halo
Christopher Dessert and colleagues searched for the 3.5 keV signal within the ambient halo of the Milky Way using data from the European Space Agency’s XMM-Newton space telescope. “Everywhere we look, there should be some flux of dark matter from the Milky Way halo,” said Nicholas Rodd, currently a particle phenomenologist at CERN, because of our solar system’s location in the galaxy. “We exploited the fact that we live in a halo of dark matter.” Rodd’s research focuses on the search for dark matter in astrophysical datasets, an approach known as indirect detection.
Dessert, Rodd and colleagues analyzed blank-sky observations (parts of the sky away from large X-ray emitting regions) with a total exposure time of roughly a year, finding no evidence for the predicted 3.5 KeV line. According to the authors, the findings rule out the predicted signal strength by over an order of magnitude.
“When searching for X-ray photons coming from dark matter, arguably the two most important parameters of your telescope are: 1. How well it can determine the photon energy; and 2. How many photons it collects. It is very challenging to make strides on the second of these,” Nicholas Rodd wrote in an email to The Daily Galaxy. “We already have instruments like XMM-Newton that have been collecting X-rays for over two decades, giving them a significant head start on any new telescope,” he explained.
“But technological improvements will allow new instruments to determine X-ray energies far more accurately,” Rodd observed in his email. “This matters for a potential dark-matter signal like the 3.5 keV line, because as the name suggests, it is expected to be a very narrow line in X-rays. Future instruments – including the NASA’s X-Ray Imaging and Spectroscopy Mission
(XRISM), due to launch within a year, and in the longer term Advanced Telescope for High-ENergy Astrophysics (Athena) – should start to see a very narrow feature coming from dark matter, if that’s where the anomaly originates, and play a key role in resolving this dark-matter mystery.”
Image credit: Data gathered by the Hubble Space Telescope creates a map of dark matter.