Dwarf galaxies reveal new insights on dark matter

Posted by Kate Taylor

Scientists have narrowed down the range in which dark matter could exist, thanks to a new analysis technique.

As much as 80 percent of the matter in the universe is invisible to telescopes, and no one really knows what this dark matter actually is. Many scientists think the answer will lie in new kinds of subatomic particles.

Scientists working with data from NASA's Fermi Gamma-ray Space Telescope have looked for signals from some of these hypothetical particles by zeroing in on 10 small, faint galaxies that orbit our own. They've found nothing.

"In effect, the Fermi LAT analysis compresses the theoretical box where these particles can hide," says Jennifer Siegal-Gaskins of the California Institute of Technology.

WIMPs, or Weakly Interacting Massive Particles, are the top candidate for dark matter. Some WIMPs may annihilate one anotherwhen pairs of them interact - a process expected to produce gamma rays that could be detected by the LAT.

"One of the best places to look for these faint gamma-ray signals is in dwarf spheroidal galaxies, small satellites of our own Milky Way galaxy that we know possess large amounts of dark matter," Siegal-Gaskins explained.

"From an astrophysical perspective, these are downright boring systems, with little gas or star formation and no objects like pulsars or supernova remnants that emit gamma rays."

In addition, selecting only dwarf galaxies at great distances from the plane of our galaxy helps minimize interference from the Milky Way.

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, dubbed 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, showing that WIMP candidates within a specific range of masses and interaction rates simply can't 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," says Maja Llena Garde, a graduate student at Stockholm University.

The team's now 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.

They also plan to examine any new dwarf galaxies discovered by the sev eral sky surveys now ramping up.