Like moons orbiting a planet, there are smaller bodies circling the Milky Way. Known as dwarf galaxies, they can be dim enough to escape detection—it’s not known how many there are in total, and new dwarfs are still being detected. One such dwarf galaxy was discovered within the last few weeks using data from the Dark Energy Survey, an experiment that scans the southern sky in order to learn about the accelerating expansion of the Universe (the experiment’s name comes from the mysterious dark energy that causes that acceleration).
The dwarf, known as Reticulum 2, is about 98,000 light-years from Earth, making it one of the Milky Way’s closest discovered satellites. But that’s not its most exciting feature. The mini-galaxy seems to be emitting a strong gamma ray signal, a research team concludes in a paper submitted to the journal Physical Review Letters. That’s surprising for a dwarf, since they tend to be mostly devoid of the objects that typically produce gamma rays. While it’s too early to say for sure what the source of the gamma rays is, the authors have tentatively come to a very intriguing conclusion: dark matter annihilation.
Dwarfs and dark matter
Like their larger counterparts, dwarf galaxies rest within a spherical blob, or halo, of dark matter that accounts for most of the galaxy’s mass. In the case of the Milky Way’s satellites, their halos rest within the Milky Way’s own larger halo, making them subhalos.
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