It’s a fairly typical large spiral system. This structure can be viewed as consisting of six separate parts: a nucleus, a central bulge, a disk (both a thin and a thick disk), spiral arms, a spherical component, and a massive halo. Some of these components blend into each other.
The Fermi Large Area Telescope first painted a picture of the gamma-ray sky. Several teams of scientists analyzing Fermi data subtracted all known sources from this map, such as pulsars, stellar-mass black holes, and spread-out emission coming from interstellar gas. But even after every source we know about had been removed, our galactic center still appeared as a bright gamma-ray glow. (The Andromeda Galaxy was recently found to have a similar central glow.)
Some groups suggested that the gamma rays could be produced via dark matter particle collisions. Unlike ordinary matter, dark matter particles are their own antimatter particles. If ever the twain shall meet, they annihilate each other, producing gamma rays and perhaps other secondary subatomic particles.
Still, given that dark matter particles have yet to be detected in the lab (where they are spotted not by collisions with each other but by rare interactions with ordinary matter), many scientists have been hesitant to accept that explanation. Pulsars, which also produce gamma rays, are the preferred alternative. But pulsars have proven intransigent: attempts to detect them at radio wavelengths in large enough numbers to explain the excess have failed.
Now, Mattia di Mauro, Eric Charles, and Matthew Wood (SLAC National Accelerator Lab), as well as the rest of the Fermi-LAT Collaboration, have published a new study picking out gamma-ray pulsar candidates from 7½ years of Fermi observations analyzed using the most recent data pipeline, known as “Pass 8.” The paper has been submitted to Astrophysical Journal (preprint available here).
Looking within a box centered on the galaxy’s center, 40° on a side, the team carefully picked out about 100 point sources that are likely to be gamma-ray pulsars. These pulsars represent the tip of an iceberg — there are probably many more point sources that Fermi can’t resolve. But based on how these pulsar candidates are spread out on the sky, as well as their brightnesses, the team concludes that they can easily explain the gamma-ray excess.