X-ray observations shed new light on dark matter

New analyses of the x-ray and gamma-ray emissions from the center of the Milky Way galaxy, the Andromeda galaxy, and the Perseus galaxy cluster have detected significant signs of two possible dark matter particles. One is likely a 7.1 keV sterile neutrino, and the other appears to be a 35 GeV WIMP (Weakly Interacting Massive Particle).

Dark matter was suggested in 1932 by Jan Oort to explain the anomalous orbital velocity of stars in our galaxy, and independently by Fritz Zwicky in 1933 to explain the anomalous orbital velocity of galaxies in clusters of galaxies. The orbital velocities are too large to be explained by the mass that can be seen, suggesting that additional mass, or dark matter, must be present.

With a certainty admittedly short of absolute, astrophysicists know from observational data that there is about six times more dark matter than "ordinary" or bosonic matter. To a good approximation, dark matter only interacts with itself and ordinary matter through gravity. Finally, dark matter is able to form concentrated regions that guide the formation of galaxies, galactic clusters, and the large-scale filamentary structures found in the Universe. Unfortunately, this is not enough information to pin down the nature of dark matter.

A 7 keV sterile neutrino?

Two independent research groups have used different data from the European Space Agency's (ESA's) XMM-Newton and NASA's Chandra x-ray observatories to detect and independently confirm a new x-ray emission line associated with galaxies at an energy of just over 3.5 keV.

A NASA/Harvard-Smithsonian group based its study, led by Esra Bulbul, on x-ray data taken by XMM-Newton's MOS-CCD and PN-CCD cameras. These cameras can discriminate x-ray energies from 0.15 to 15 keV, and provide an angular resolution of about 6 arcseconds. They chose to combine the observed signals from 73 bright and relatively nearby (<1.2 gigaparsec) clusters of galaxies, rescaling the wavelengths to correct for cosmological redshift. The x-ray background from known sources was also subtracted prior to analysis.

The data from both cameras showed unexplained emission lines at an energy of about 3.55 keV, as did analyses based on all or various parts of the x-ray cluster data. No x-ray line associated with conventional physics appears at that energy. They then performed a similar analysis of Chandra x-ray observations of the Perseus and Virgo galactic clusters. The unknown line was seen as the same energy and strength.

A second study was carried out by A. Boyarski of Leyden University and collaborators, using x-ray data from the XMM-Newton observatory study of the Perseus galaxy cluster and the Andromeda galaxy. Neither group was aware of the other's efforts, which remarkably enough were reported just one week apart. Their conclusions were essentially identical, seeing an unexplained emission line at an energy of 3.52 keV.

The energy and location of the signal is consistent with the decay of a form of dark matter – a 7.1 keV sterile neutrino turning into a photon and an ordinary neutrino. The photon receives almost exactly half the energy of the original particle because of the tiny mass of an ordinary neutrino.

Sterile neutrinos are neutrinos not included in the Standard Model of Particle Physics that do not undergo weak interactions – they do not interact with W- and Z-bosons, which are the carriers of the weak force. There are many reasons to consider sterile neutrinos as possible members of the particle zoo. In particular, the simplest well-behaved models that supply ordinary neutrinos with their masses require that at least two sterile neutrinos exist.

The lack of interaction of sterile neutrinos makes them difficult to create. To be produced in the early Universe, sterile neutrinos must mix slightly with ordinary neutrinos through neutrino oscillations, so that the active neutrinos generated as the Universe cools after the Big Bang can be partially converted into the sterile variety. The same process in reverse results in the decay of sterile neutrinos driven by neutrino oscillations.

35 GeV WIMP

A new study of gamma-ray emissions from the central regions of the Milky Way has recently appeared. Astrophysicists at Harvard, the University of Chicago, MIT, Fermilab, and Princeton have taken a deeper look at data from the Fermi gamma-ray space observatory, and find strong evidence that the observations include a signature of dark matter in the form of WIMPs.

They find that after accounting for known gamma-ray background sources affecting the observed gamma-ray emission from the Milky Way's central regions, such as point sources of gamma rays, galactic and extragalactic diffuse gamma-ray emission, and gamma-ray emission from the Fermi Bubbles (two large gamma-ray emitting structures above and below the galactic center), an additional source of GeV gamma-rays is required to match observations. The presence of this excess is claimed to have a statistical significance of 40 sigma (5 sigma is usually considered experimental proof in particle physics.)

This signal (figure above) was first seen in 2009, and has variously interpreted as synchrotron radiation, unresolvable millisecond pulsars, and dark matter annihilation or decay.

The new study reveals that the gamma-ray excess is roughly spherically symmetric, is centered on our central supermassive black hole Sagittarius A* to within about 3 minutes of arc, and the flux density of the excess falls smoothly for perhaps 2 kiloparsecs as one moves away from the center of the excess flux.

The spectrum of the gamma-ray excess matches closely that expected from a massive dark matter WIMP.

The study concludes that the best present interpretation is that the gamma-ray excess from the galaxy's center are a signature of 35 GeV WIMPs annihilating with a cross-section of a fraction of a picobarn (10^-10 square femtometers) into a bottom quark-antiquark pair, which themselves eventually decay into components including gamma rays with an energy of several GeV.

Even though the study described above is strongly suggestive of the annihilation of 35 GeV WIMP dark matter, by itself it is not sufficiently convincing to ease a number of reasonable concerns. Better proof would be if similar signals were seen in dwarf galaxies, which are among the celestial objects most dominated by dark matter. Unfortunately, the signal from a WIMP halo would also be much smaller.

Despite this, a search has been made by the Fermi-Large Area Telescope collaboration for excess gamma-ray emission associated with all 25 of the known dwarf spheroidal galaxies that orbit the Milky Way. While the results of this study were formally negative (to a cross-section of about 3 picobarns), there does appear to be a small excess of gamma rays corresponding to decay of a 10-25 GeV WIMP into bottom quark-antiquark pairs. With a statistical significance or only about 2.5 sigma, this excess is only a hint that dwarf galaxies may share the same type of gamma-ray excess as does the Milky Way galaxy. Observations with increased sensitivity may decide the reality of this signal in upcoming years.

Signs that dark matter particles have been detected continually grow stronger. While the current level of knowledge is short of "smoking-gun" evidence, it seems likely that dark matter may include several kinds of new particles, consistent with the suggestion of Harvard physicist Lisa Randall that the forces and particles of the dark sector may be as complex and interesting as are those we can perceive directly.

Source: E. Bulbul et al. / arvix.org [PDF]