Fig 1: 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). Credits: X-ray: NASA/CXC/SAO/E.Bulbul, et al.
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”.
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.
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.
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.
References:
Non-detection of X-ray emission from sterile neutrinos in stacked galaxy spectra
2015, MNRAS, 452, 3905
Unidentified Line in X-Ray Spectra of the Andromeda Galaxy and Perseus Galaxy Cluster
2014, Physical Review Letters, 113, 251301
Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters
2014, ApJ, 789, 13
