New Study Sheds Light on Elusive Dark Matter in Galaxies
29 October 2014
Strong gravitational lensing as observed by the Hubble Space Telescope in the galaxy cluster Abell 1689 indicates the presence of dark matter (Image Credit: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA).
The results of an indirect search for dark matter in dwarf spheroidal (dSph) galaxies reported at the 5th International Fermi Symposium held in Nagoya Japan, were anticipated by many scientists trying to understand the mysterious dark matter component in the Universe. The Fermi Large Area Telescope (LAT) “did not confirm a postulated gamma ray signal from annihilating dark matter” according to Brandon Anderson, a postdoctoral researcher at the Oskar Klein Centre at Stockholm University. Brandon is part of a team, also including US colleagues in Stanford and European colleagues in Montpellier, which are behind the Fermi-LAT’s Collaboration dSph study. The search results were particularly anticipated in the light of a recent possible detection of dark matter in the center of Milky Way.
The Fermi Large Area Telescope (LAT) is a NASA-led gamma-ray detector with strong European contributions that has been in orbit since 2008. Its energy range (about 100 MeV to 1 TeV) enables scientists to test a popular theoretical dark matter candidate, the Weakly Interacting Massive Particle (WIMP). In many versions of this model, when two WIMPs come together they can annihilate one another. This means they transform their mass energy into conventional particles, including gamma rays, the most energetic form of radiation.
“WIMPs give with very few assumptions and well known physics the right amount of dark matter, within a factor of few, known from measurements for example of the cosmic microwave background,” says Jan Conrad, professor in Astroparticle Physics at Stockholm University. “We usually refer to this as the WIMP miracle. In addition there are theories, proposed for completely different reasons than the existence of dark matter, (in particular supersymmetry) that predict the existence of WIMPs”.
Brandon Anderson is a member of Jan Conrad’s group in Stockholm University, working in astrophysical searches for particle dark matter. “On Earth, the density of dark matter is so low that WIMPs annihilation almost never happens,” Anderson says. “But there are places where we know DM is very concentrated: locally, the densest regions are in the galactic center and nearby dwarf spheroidal galaxies”. Being an all-sky monitor, the LAT observes them all pretty equally. Gamma rays in excess to the expected background due to known gamma ray emitters coming from the dense regions are the signature that is looked for.
Although the galactic center is nearby and massive, it is astrophysically complex. So much so that interpretation of LAT data from the galactic center has still to be provided by the Fermi-LAT Collaboration. Undaunted, several groups now report that models of known sources do not account for all the gamma-ray emission, leaving what is blandly referred to as the galactic center excess (GCE). Despite the astrophysical uncertainty, all parties agree that the GCE is peaked at around 1 GeV (1 billion electron volts) and extends fairly symmetrically to about ten degrees (5,000 light years) from the centre of the Milky Way. Plausible conventional production mechanisms not requiring dark matter include pulsars and cosmic ray collisions on gas clouds. But the GCE can also be neatly accounted for by the existence of a 30 GeV WIMP which self-annihilates into b quarks. “Without completely ruling out alternatives, this possible dark matter explanation remains just that – possible,” Anderson says.
A large spiral galaxy (M51) colliding with its dwarf companion (NGC 5195) galaxy (Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler)
What made this model so exciting was that it also happened to fit a slight gamma-ray excess seen in a previous dSphs analysis published this spring in the journal Physical Review D. Dwarf galaxies orbiting the Milky Way represent an independent data set, and one that is free of the uncertainties which plague the galactic centre data, because they lack other types of gamma-ray emitters. An excess signal of the same character in both the galactic centre and dSphs would be strong evidence of a dark matter detection. The spring dSph excess was so low that, if it was not just a fluctuation in the gamma-ray background, we would have to wait many years to gather enough photons for it to become significant.
“Fortunately, the LAT’s ground team just gave us a big push,” says Anderson. The LAT relies on extensively calibrated classification algorithms to reconstruct incoming gamma rays from their electronic signatures. These routines have been periodically overhauled throughout the mission as knowledge of the instrument continues to improve. The latest overhaul, known as Pass 8, marks the biggest advance yet, boosting the instrument’s effective area and sharpening its angular resolution. The resulting improved data set warranted a fresh look at the dSphs.
The announcement from Brandon Anderson and colleagues at the Fermi Symposium may have come as a disappointment for some scientists: For dSphs the significance of the GCE model dropped drastically, along with all other WIMP annihilation masses and channels. It dropped so far, in fact, that the limits now exclude the annihilation cross section (essentially the proximity at which they interact), which WIMPs should exhibit would they be the dark matter, up to particle masses of 100 GeV. “These are now the best WIMP model constraints in the world below 1 TeV, but what is more important is that if the simple WIMP paradigm would be correct, our results would require them to be heavier than 100 GeV,” Anderson says. “While these constraints do not conclusively rule out the dark matter interpretation of the GCE, they lend no support”.
Is there any other, unrelated evidence for the existence of WIMPs? “The direct detection experiment DAMA/LIBRA has reported evidence for WIMPs on high significance level, but this result is highly controversial and could not be confirmed by other experiments,” Conrad says. “Other direct detection experiments, e.g. CDMS, have reported hints that do not exceed the required significance to be called “detection” or “evidence”. So the simple answer is: no. ” he adds.
So, what are the alternatives to the standard WIMP picture? Apparently there is a myriad of particle candidates for dark matter. “From my point of view a good candidate has to a) give the right properties (e.g. the abundance) without too many additional assumptions and b) hopefully provide more solutions except the dark matter problem,” Conrad says. The WIMP fulfills both of these requirements. “If the WIMP would be ruled out, I would probably turn to axions, which could constitute dark matter but in addition solve another problem, the so called strong CP problem in Quantum Chromo Dynamics (QCD)”.
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Eleni Chatzichristou