Mysterious Emission from Galaxy Clusters: Is it Dark Matter?

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Mysterious signal seen in X-ray data from over 70 galaxy clusters with Chandra and XMM-Newton. Perseus cluster shown here: Image courtesy of NASA/CXC/SAO/E. Bulbul, et.al.

When I first saw it I didn’t believe it,” Dr. Esra Bulbul told Decoded Science, “We could not explain the data with anything we thought.” She and her associates report a mysterious X-ray emission in the Perseus galaxy cluster — raising the “intriguing possibility” it may be photons from the decay of dark matter. The authors published their findings in the July 1 issue of The Astrophysical Journal.

X-Ray Emissions: An Unexpected Discovery

Recently, I had the pleasure of talking to Dr. Esra Bulbul of the Harvard-Smithsonian Center for Astrophysics (CfA). She and her associates have discovered a faint, never-before-seen X-ray emission line in the central region of the Perseus galaxy cluster. The mysterious signal was visible using both NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton.

Additional studies with XMM-Newton showed this unexplained signal in 73 other galaxy clusters.

Galaxy clusters are concentrations of some tens to hundreds of galaxies held together by mutual gravity. The space between galaxies is not empty. It is filled with superheated gas from the remnants of exploding stars. This hot ionized gas, or plasma, is made up of mostly hydrogen with traces of other elements. The plasma’s temperature exceeds 10 million degrees Celsius — so hot it emits X-rays.

Atoms and molecules in the plasma give off photons of X-ray light at specific colors or frequencies (and corresponding energies). Astronomers examine these spectral emission lines to determine what elements and compounds are in the gases.

Both Chandra and XMM-Newton data showed a faint emission line at an energy of about 3.55 kilo-electron-volts. Astrophysicists know the energy transitions of interstellar gases very well. “The energy of this line corresponds to none of these,” Dr. Bulbul told me.

I asked why no one had discovered this line before. Because, she said, “nobody had looked in this depth before.” In her study, she stacked up a lot of long-exposure data on top of each other, which served to reduce the noise and bring out the faint signal.

Could This Space Signal Be Dark Matter?

Astrophysicist Esra Bulbul: Image courtesy of Chandra X-ray Observatory, NASA.

In her paper, Bulbul proposed sterile neutrinos — a candidate dark matter particle — as a possible source of the mysterious emission line. This hypothetical particle has an energy of 7.1 kilo-electron-volts, and can transform (decay) into two X-ray photons at 3.55 kilo-electron-volts — just the energy seen in the galaxy cluster data.

Per the Chandra press release:

The discovery has “ignited a flurry of activity, with 55 new papers . . . mostly discussing the emission line as possible evidence for dark matter. Some . . . explore the sterile neutrino interpretation, but others suggest different . . .  candidate dark matter particles, such as the axion . . .

Some have even dubbed the source of the mysterious emission line a “bulbula.”

There are other, more mundane possibilities. The emission is very weak and near several known faint lines — the most likely being the element argon. “But this would require thirty times the argon in a cluster than expected,” Bulbul told me. “This is almost impossible.” Nonetheless, she is planning experiments at Lawrence Livermore National Laboratory to resolve this issue.

What about instrument errors? They are virtually ruled out due to the stacking of data from different galaxy clusters. The emission line data from each cluster has to be corrected for red-shift (the shift in light from galaxy clusters to lower frequency due to the expansion of the universe). The further away a galaxy cluster is the more the red-shift. Once red-shift corrected data is overlaid, instrument errors tend to smear out while true signals are enhanced.


I asked whether the signal could be from black holes, as gases falling into a black hole also radiate in the X-ray region. “We are sure (the emission line) is not caused by black holes (or neutron stars),” Bulbul said. “These are considered point sources. They look like dots in the image and are removed from the data. Even if some few remain, there is so much bulky gas that the black hole signals would wash out.”

There are other concerns. The line observed in the Perseus cluster is five times brighter than expected in the model of a sterile neutrino at 7.1 kilo-electron-volts. “The error bars are huge,” Bulbul said. “Uncertainties in temperature and distance may contribute to this discrepancy. This (issue) needs to be checked.”

Some astrophysicists questioned why the Virgo galaxy cluster and our own Milky Way seem to show no such line. Per Bulbul, there was not enough data regarding Virgo, and Alexi Boyarsky’s study saw the same line on the outskirts of Virgo. Nonetheless, per Bulbul, “this is problematical.” As to the Milky Way, she indicated there need to be a better statistical analysis here. She thinks in the end the data may agree with hers.

X-Ray Signal in Space: Future Investigations

Illustration of Astro-H satellite observatory. SXS is soft X-ray Spectrometer: Image courtesy of NASA Goddard Space Flight Center.

A number of observations are planned to confirm the strange emission line and hopefully determine its cause. Per the Chandra press release: “Our next step is to combine data from Chandra and JAXA’s Suzaku mission for a large number of galaxy clusters to see if we find the same X-ray signal,” said paper co-author Adam Foster, also of CfA.

JAXA’s ASTRO-H X-ray astronomy satellite — scheduled for launch in 2015 — could provide a definitive answer. Onboard is NASA’s High-Resolution Soft X-Ray Spectrometer (SXS). According to Bulbul, it has much better resolution and some twenty times more sensitivity than current instruments.

Is dark matter the solution to this puzzle?

Bulbul admits this is a long shot. “Even if it is not sterile neutrinos or some other form of dark matter, it could be a new astrophysical discovery (involving known matter).” I could hear the excitement in her voice.

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