It’s only a few months since nine suspected dwarf galaxies were announced, orbiting the Milky Way. Now one of these Milky Way satellites has been found emitting gamma rays that may help us to solve the mystery of the nature of dark matter.
When the existence of Reticulum 2 was reported it was as part of a survey of the sky near the Magellanic Clouds. This study sought objects where dark matter dominates over ordinary matter even more strongly than it does in the universe as a whole, and found three definite galaxies and six objects in a doubtful zone between dwarf galaxy and globular cluster.
Reticulum 2 was observed to be the brightest of the new finds, a consequence of easily being the closest, at 100,000 light-years away. It also has a distinctively elongated shape, but did not otherwise stand out.
Now, however, Dr Alex Geringer-Sameth of Carnegie Mellon reports in Physical Review Letters that Reticulum 2 is the source of gamma rays in the range of 2–10 billion electron volts, “consistent with the annihilation of dark matter particle masses less than a few [hundred billion electron volts.]”
“Reticulum II has the most significant gamma ray signal of any known dwarf galaxy,” the paper adds. “There's no conventional reason this galaxy should be giving off gamma rays,” Geringer-Sameth said, “So it's potentially a signal for dark matter.”
We know that dark matter exists because we see galaxies turning at rates that only make sense if they contain mass many times greater than the stars and gas we can observe in other ways. These movements, and the effect of gravity on light, give us a reasonable idea of where dark matter is concentrated in the universe, spurring a quest lasting decades to observe it in other ways that might tell us more about its nature.
Where once this debate was an epic battle between MACHOS (Massive Compact Halo Objects) and WIMPS (Weakly Interactive Massive Particles) support has now swung behind the WIMPS, although tiny black holes also have some support. Knowing that dark matter is formed from particles that interact only slightly, if at all, with the three non-gravitational fundamental forces still leaves plenty of room to speculate on what these particles are.
Nevertheless, we have gathered hints of dark matter constituents, and we would expect them to behave like other particles in some ways. One of these is to release gamma rays when they collide and annihilate each other.
Unfortunately, most of the places where dark matter congregates, like the centers of galaxies, are also filled with objects made of ordinary matter that produce plenty of gamma rays themselves. We haven’t worked out how to distinguish between gamma rays from black holes and dark matter.
Reticulum 2 seems like an unlikely place for a large black hole or other major gamma ray source, suggesting that this may be the first time we have seen gamma rays that can be attributed to a known concentration of dark matter.