March 20, 2012
Orbiting the Milky Way are nearly two dozen small galaxies, like ducklings crowding around their mama. Although they contain enough stars to make them visible, astrophysicists now think these tiny satellite galaxies are made mostly of dark matter – that mysterious substance, invisible not only to our eyes but to our most sophisticated detectors, which makes up most of the matter in the universe.
If two dozen sounds like a big brood, computer simulations indicate that the Milky Way's flock of satellite galaxies, called dwarf galaxies, should be even bigger. But any additional dwarfs are thought to be made almost entirely of dark matter, and they are simply too dim to detect.
A team of scientists from the SLAC/Stanford Kavli Institute for Particle Astrophysics and Cosmology sought to identify possible dark-matter dwarf satellites using the Fermi Gamma Ray Space Telescope's Large Area Telescope. The LAT can't see the tiny galaxies directly, but some theories about dark matter call for invisible dark-matter particles to sometimes annihilate with each other, resulting in gamma rays the LAT can see.
So the KIPAC team combed the first Fermi-LAT Source Catalog for gamma-ray sources that didn't seem to emit any other type of light, such as optical or radio, and that fit with expectations of what dark matter sources may look like. Out of two promising candidates, one was discovered to be a new pulsar, while the second turned out to be two distinct point sources. Neither was what they were looking for.
However, by combining their lack of findings with predictions from simulations of the growth of dark matter structures, the team was able to determine a limit for how often dark matter can annihilate into gamma rays, which will help future searches. Their research recently appeared in the Astrophysical Journal. KIPAC members who contributed to the research include graduate students Alex Drlica-Wagner and Ping Wang, postdoctoral researcher Louis Strigari and Professor Elliott Bloom.