Gamma-Ray Beams Show a Recently Active Milky Way
When compared to our knowledge of other galaxies, like the Andromeda Galaxy, researchers know very little about our own Milky Way. This is because we are inside of our galaxy, which makes it very difficult to see the structure of the Milky Way. Still, we do know some things, including that ours is not an active galaxy, but apparently it was not too long ago.
Many of the spiral galaxies we have observed are not just simple disks as two blobs or bubbles of matter spew out above and below the main structure. These bubbles are being blown by the supermassive black hole at the center of galaxy. Even though the common mental image of a black hole is something that just devours everything around it, much of the mass that falls towards a supermassive black hole does not enter it. Instead it gets flung out into space above and below the disk of the home galaxy. Sometimes instead of just matter being shot out from a galaxy's center, gamma rays are ejected. Unlike the matter though, these high energy photons keep in a tight shape and look like a search light peering through the Universe.
Galaxies with a feeding supermassive black hole at their center are called active galaxies and they are continually making these gamma ray beams and bubbles of matter. The Milky Way is not generating these two structures though, which means it is not active, but millions of years ago it was. Researchers at Harvard-Smithsonian Center for Astrophysics have discovered an after-image of two gamma-ray jets coming from the center of our galaxy. These jets are about 27,000 light years long, which matches an old bubble previously discovered, which adds support for the conclusion that our galaxy was active recently.
This is the first time such jets have been found coming from the Milky Way. The researchers estimate it required at least 10,000 solar masses of matter to fall into the black hole to create these jets.