It's Full of Stars

Recently, there’s been a lot of buzz in the astronomy community about the recent discovery of gamma-ray bubbles over the center of our galaxy. Above is an illustration from the press release depicting the bubbles, which were discovered by NASA’s Fermi space telescope. [See the original IFOS post here. See the paper for this discovery here.] The discovery was made by using the highest-resolution gamma-ray detectors ever created, and exposing them to the center of the Milky Way for a total of 1.6 years (done in shifts). Below is the sciencey photo:

There are a few problems, though, that have people scratching their heads. First of all, according to the paper, the “bubbles” are of uniform density. This is a problem because when you look at a sphere, there is more matter in the center and less at the edges, just because of how spheres are shaped.

You’d expect to see a concentration of density near the center, but such a thing is not present in the Fermi bubbles. So what does this mean? Either the gas “bubble” happens to grow more diffuse (the opposite of dense) near the center (which is a convoluted but completely possible theory) or the “bubbles” are in fact flat discs. Why would two flat discs be positioned so perfectly that they are exactly perpendicular to our line of sight? It’s possible that that could happen, but the chances are so unlikely that it’s hard to believe. Those who made the discovery are not sure why the data shows this. [If you’re interested in reading more, check out Section 3 in the paper, specifically pp. 11-13 and Fig. 10.]

There are a few plausible theories as to what these structures could possibly be. When matter falls into an active black hole, relativistic jets shoot out of either one or both of the black hole’s rotational poles.

This happens because when matter is falling into a black hole, it circles the black hole in a thing called an accretion disc, which is not unlike when water circles the drain in your sink. This whole process creates a massive amount of energy due to friction, which escapes in the form of the relativistic jets. Not all black holes have relativistic jets, though. They can enter a state called “quiescence”, where the matter in the accretion disc reaches a kind of uniformity of flow that allows for the energy to stay in the disc instead of escaping.

The black hole supposedly at the center of our galaxy is expected to be quiescent, which is why we wouldn’t expect to find any structures like the Fermi bubbles hovering over the center. It’s been suggested [source] that these bubbles are a remnant of former jets from our black hole, which is quite possible. The structures are very faint; it took 1.6 years worth of exposure to find them.

Another possible explanation is that they’re the remnant of a period of rapid star formation, which releases a lot of energy. The structures have sharp edges [as per the paper and the press release], meaning that the energy was released around the same time. A burst of star formation would explain this well.

At this point, we can only speculate about the Fermi bubbles, but they and their implications will surely be highly studied in years to come.

[By Carly; email me here or drop a line in the ask box.]