Is Dark Matter the Source of a Mysterious X-ray Emission Line?

April 01, 2016

The nature of dark matter is still unknown, but one potential candidate is a theoretical particle known as the “sterile neutrino”. In 2014, two independent groups of astronomers detected an unknown X-ray emission line around an energy of 3.5 keV in stacked X-ray spectra of galaxy clusters and in the centre of the Andromeda galaxy. The properties of this emission line are consistent with many of the expectations for the decay of sterile neutrino dark matter. However, if this hypothesis is correct, all massive objects in the Universe should exhibit this spectral feature. To test this intriguing possibility, scientists at MPA and the University of Michigan examined two large samples of galaxies, finding no evidence for the line in their stacked galaxy spectra. This strongly suggests that the mysterious 3.5 keV emission line does not originate from decaying dark matter. The nature of dark matter, and the origin of this emission line, both remain unknown.

Astronomers have known for decades that about 85% of the matter in the Universe is composed of invisible, non-Baryonic particles known as “dark matter”, which can generally only be studied via gravitational interactions on visible matter. While the nature of this exotic substance is still unclear, a number of potential particles have been proposed. One of the more popular candidates is known as the “sterile neutrino”.

A 2014 study of 73 galaxy clusters, including the Perseus cluster (shown in this image), has revealed a mysterious X-ray signal in the data. Using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton, the stacked spectra of these objects show an excess centered around an energy of 3.57 kiloelectron volts (keV) (see inset).

This theoretical particle could have a mass of several keV (around 1/100 of the mass of the electron), which would potentially make these particles numerous enough and heavy enough to account for the dark matter in the Universe. Sterile neutrinos are occasionally supposed to spontaneously decay into ordinary neutrinos, a process which produces an X-ray photon with half the mass of the sterile neutrino. The best hope to find this line is to look towards very massive objects (galaxies or clusters of galaxies), which have the highest amounts of dark matter particles.

In February 2014, two independent groups of astronomers announced within a few days of each other that they had tentatively detected an unidentified X-ray emission line that could be interpreted as the spontaneous decay of sterile neutrinos. The first group (Bulbul et al. 2014) studied a sample of 73 galaxy clusters, while the second group (Boyarsky et al. 2014) studied the Perseus galaxy cluster as well as the central portion of the Andromeda galaxy. Both groups measured more photons with energies around 3.5 keV than predicted by their models of intracluster gas emission, and the residual emission profiles look similar to what astronomers would expect to see from an emission line.

The immediate question is whether this anomalous line must be due to sterile neutrinos, or whether it may have an astrophysical explanation. If this line indeed comes from decaying dark matter, it should be observed in other, less massive objects as well – and this is what Mike Anderson and Eugene Churazov at MPA, as well as their collaborator Joel Bregman, set out to test.

These figures show the stacked X-ray spectra of 81 galaxies observed with the Chandra X-ray Observatory (upper plot) and 89 galaxies observed with the XMM-Newton telescope (lower plot) alongside two different spectral models. The red curve is a model for the data, which includes the emission line at 3.57 keV from Figure 1 corresponding to the prediction of sterile neutrino dark matter. The black curve is a model for the data which makes no assumptions about the emission at 3.57 keV. In both cases, the latter model is heavily favoured, showing that the unidentified emission line seen in galaxy cluster spectra does not appear in galaxy spectra. If the line were indeed due to sterile neutrino dark matter, we should see it in both galaxies and galaxy clusters, so this is evidence against that interpretation of the line.

Galaxies are much less massive than galaxy clusters, and so their hot gaseous halos are also correspondingly less massive and cooler than in clusters. While galaxy clusters are able to retain enormous halos of hot gas in their gravitational potential which provides the vast majority of the total X-rays, galaxies have almost no diffuse emission at the ~3.5 keV energies corresponding to the location of the new line. The weaker signal from the decaying dark matter in galaxies is therefore balanced by a lower amount of background noise, and galaxies prove to be an excellent complement to galaxy clusters in the study of X-rays from sterile neutrinos.

Anderson and his collaborators assembled very large samples of galaxies for their study: 81 galaxies observed with the Chandra X-ray Observatory and 89 galaxies observed with the XMM-Newton telescope. The total amount of observation time for each sample was about half a year. The team cleaned each image, removed stray X-ray point sources, and added together the X-ray emission from each galaxy, weighting each pixel of every image by the expected dark matter content at that location based on simple models of galactic dark matter halos.

The result is shown in Figure 2, for both the XMM-Newton (top) and Chandra (bottom) datasets. As the analysis shows, in both cases the model including an emission line at 3.57 keV from the decay of sterile neutrinos is very strongly disfavoured by the data compared to no emission line; in fact, the XMM-Newton spectrum prefers to have a line with negative flux at that energy.

Summary of constraints on sterile neutrino dark matter from this work as well as a number of previous studies; the measurements from galaxy clusters are indicated by the green dots (with error bars). The x-axis shows possible neutrino masses, and the y-axis shows possible decay rates for sterile neutrinos (where higher values mean that spontaneous decay is more likely). Sterile neutrino dark matter is only possible in the white region, but the results of this study rule out the portion of the plot above the red and blue curves. It would still be theoretically possible for sterile neutrinos to exist below the red and blue curves (this study did not examine the space to the right or left of these curves), and future X-ray telescopes would be required in order to place constraints on this possibility. 

This study therefore provides very strong statistical evidence against the hypothesis that the unidentified X-ray emission line in the spectra of galaxy clusters originates from sterile neutrino dark matter. Figure 3 illustrates the constraints from this work on the masses and decay probabilities of sterile neutrinos, along with a number of previous constraints from other studies. A large portion of the available parameter space is now ruled out, but there still remains a sizeable region below our constraints where sterile neutrino dark matter might still exist.

If the unidentified X-ray emission line at 3.57 keV is not caused by sterile neutrinos, what is its source? This question is not answered by the current study, and still remains the subject of active debate. One possibility is an atomic interaction such as charge exchange, which may be expected to produce 3.5 keV photons in intracluster plasma but not in the halos of galaxies. There are also several more exotic theories of dark matter, such as axionlike particles which require interactions with magnetic fields to produce X-ray emission and therefore might be likelier to be seen in magnetized intracluster plasma than in the halos of galaxies. New X-ray telescopes such as the Athena observatory will have significantly better spectral resolution, and this will hopefully shed additional light on this question.

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