Scientists have known for more than a decade that galaxy clusters emit an unusually large amount of “soft” or low-energy X-rays. A possible explanation is that the centres of the clusters contain vast regions of warm, thermally-radiating gas. If this is the case then spectra of soft X-rays from clusters should contain emission lines — peaks of intensity at certain wavelengths — corresponding to the composition of atoms in the gas. Trouble is, the spectra are generally smooth.

An alternative explanation is that electrons in galaxy clusters are colliding with photons in the cosmic microwave background, which is radiation left over from the Big Bang. After these collisions — known as the inverse Compton effect — some photons would have energies sufficient to be soft X-rays. Now, Max Bonamente at the University of Alabama in Huntsville and colleagues at the same institution and in Finland have calculated how much such a process could account for the abundance of soft X-rays (Astrophys. J. 668 796).

Losing weight

The researchers modelled the range of frequencies that would be produced in electron-photon collisions as a “power law”, and found that by adjusting the size of the power and normalization they could make the distribution fit the spectrum of Abell 3112 — a galaxy cluster for which the Chandra X-ray Observatory has particularly good data on soft X-ray emission. This showed that up to 50% of the soft X-rays coming from the cluster could be coming from electron-photon collisions rather than from atoms in warm gas. Because electrons are much lighter than atoms, this would mean that galaxy clusters, and hence the universe itself, could have up to 20% less mass than we currently estimate.

Extraordinary claims need extraordinary evidence

If this bold claim is true, it might be that galaxies and galaxy clusters do not need as much mass to hold themselves together as our accepted theories of gravity, given by Newton and Einstein, suggest. Scientists would therefore have to look into alternative theories, such as modified Newtonian dynamics, or MOND, that make mass more gravitationally attractive. On the other hand, the “missing” mass could be hidden in dark matter, which is believed to make up some 95% of gravitating mass in the universe.

Other options

The researchers, however, are careful to point out there is much work to be done before their conclusion can be verified. “We want to be sure this is a sound analysis by looking at other clusters,” Bonamente told “Extraordinary claims need extraordinary evidence.”

Bonamente added that there could be other explanations for the lack of emission lines besides electron-photon collisions. Unlike other forms of spectroscopy, he said, X-ray spectroscopy has a very poor resolution, so the emission lines from heavy atoms could be indistinguishable. It is also possible that the gas clouds are there, but are “primordial” in the sense that they only contain the light elements hydrogen and helium, which do not produce emission lines in the soft X-ray part of the spectrum. “But most astrophysical plasmas [gases] have been enriched [with heavier elements], so this would be an exception,” he said.

Andrew Fabian, an astrophysicist at the University of Cambridge, UK is not convinced by Bonamente and colleague’s study, however. He points out that the researchers have used outdated absorption values for our own galaxy’s interstellar gas, which affect the level of soft X-rays received from Abell 3112. “Results from a newer survey could make the abundance of soft X-rays disappear in that cluster,” he said.