The team examined two years of LAT-detected gamma rays with energies in the range from 200 million to 100 billion electron volts (GeV) from 10 of the roughly two dozen dwarf galaxies known to orbit the Milky Way. Instead of analyzing the results for each galaxy separately, the scientists developed a statistical technique — they call it a "joint likelihood analysis" — that evaluates all of the galaxies at once without merging the data together. No gamma-ray signal consistent with the annihilations expected from four different types of commonly considered WIMP particles was found.
For the first time, the results show that WIMP candidates within a specific range of masses and interaction rates cannot be dark matter.
"The fact that we look at 10 dwarf galaxies jointly not only increases the statistics, but it also makes the analysis much less sensitive to fluctuations in the gamma-ray background and to uncertainties in the way the dark matter may be distributed around the dwarfs," said Maja Llena Garde, a graduate student at Stockholm University in Sweden and a co-author of the study.
For any given properties of a dark matter particle, the distribution of the particles has a significant impact on the expected gamma-ray signal, a wrinkle that often is handled inadequately, if at all, in previous studies.
The motions of a dwarf galaxy's stars trace out the profile of the massive dark matter halo in which they're embedded, but these tiny galaxies often have very few stars to track. The result is uncertainty in the way dark matter is distributed along the line of sight to the dwarf, which affects the expected flux of gamma rays detected by the LAT. By addressing uncertainties in the dwarfs' dark matter profiles, the LAT team's results are among the most accurate.
"An important element of this work is that we were able to take the statistical uncertainties from an updated study of the dwarf stellar motions and factor it into the LAT data analysis," said Johann Cohen-Tanugi, a physicist at the Laboratory of the Universe and Particles at the University of Montpellier 2 in France and a member of the research team.
"This treatment constitutes a significant step forward, and we hope that future studies will follow our example," noted co-author Jan Conrad, a physics professor at Stockholm University.
The team is in the process of following up the two-year analysis with new ones that will incorporate additional Fermi observing time, improvements made to the LAT's sensitivity, and the inclusion of higher-energy gamma rays. Additionally, sky surveys now ramping up may discover new dwarf galaxies that can be included in future studies.