Simulations suggest that our Milky Way galaxy has many dark-matter-dominated satellites swarming around it, but without large numbers of stars they are too dim to be seen as satellite galaxies. However, KIPAC astrophysicists using data from the Fermi Gamma-ray Space Telescope can learn about dark matter by fishing for these dark satellites with gamma rays.
Visualization of the Via Lactea II simulation of the growth of dark matter halos, showing a Milky Way size halo with its accompanying satellite sub-halos. Brightness corresponds to density. Some of the sub-halos would host stars and be visible.
Orbiting the Milky Way are nearly two dozen small satellite galaxies. These 'dwarf' galaxies have been found to be dominated by dark matter, the component that makes up most of the total matter in our Universe but thus far has evaded direct detection by particle physics experiments. The proximity of these dwarf galaxies, their relative lack of standard high-energy astrophysical sources such as pulsars, and their relative abundance of dark matter makes them ideal targets for the indirect detection of dark matter via the hypothesized annihilation of dark matter particles into gamma rays, which we could then see. In fact, these dwarf galaxies have been one of the prime dark matter targets for study with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, the orbiting gamma-ray observatory in which KIPAC is the major science institution. Several months ago a team of KIPAC and other scientists used data from Fermi-LAT observations of known dwarf satellite galaxies to achieve the most stringent limits on crucial dark matter parameters to date.
However, there are very likely to be many more satellite dark matter halos surrounding our Galaxy that we do not know about yet, because we haven't seen them as dwarf galaxies chock full of stars. Simulations of dark matter and the growth of structure in the Universe predict that our Milky Way should possess many more dark-matter-dominated satellites than are observed. Some of these dark matter satellites may house dwarf galaxies with stars that are yet to be discovered, while others may consist entirely of dark matter. While most of these dark matter satellites are predicted to be smaller than the dwarfs we have already observed, they are expected to exist in closer proximity to Earth and may be even better dark matter targets than the known dwarfs. Because of this, it is just as important to search for the gamma-ray annihilation signatures in the satellites that are not yet known as it is to search in the ones that are already known to us as dwarf galaxies.
To address that challenge, a team of KIPAC scientists consisting of graduate students Alex Drlica-Wagner and Ping Wang, postdoc Louie Strigari, and professor Elliott Bloom, sought to identify possible dark matter satellites as gamma-ray sources detected by the Fermi-LAT. Dark matter satellites may be distinguished from other astrophysical gamma-ray sources since they would produce proportionally more high-energy gamma rays, they would be unassociated with counterpart sources in other kinds of light such as optical and radio, and, if nearby, they could be resolved as spatially extended sources rather than just point sources of gamma rays for the Fermi-LAT.
The KIPAC team combed the first Fermi-LAT Source Catalog for gamma-ray sources satisfying all of the afore mentioned criteria. Initially, two promising candidates appeared. However, on further examination, one was discovered to be a new pulsar, and the other was resolved into two distinct point sources. The team then combined the current absence of any observed dark matter satellite candidates with the predictions from simulations of the growth of dark matter structure such as Via Lactea II to limit the frequency with which dark matter can annihilate into gamma rays. As the sensitivity of the Fermi-LAT continues to increase, the exciting possibility remains that a faint dark matter satellite is waiting to be discovered relatively close by around our Milky Way.
This work is described in a Fermi-LAT collaboration paper submitted to The Astrophysical Journal and available from astro-ph at arXiv:1201.2691.
Tidbit Author: Jack Singal and Alex Drlica-Wagner