Thursday, March 17, 2011

Galactic Blast

The Fermi Gamma-ray Space Telescope (FGST) is a space observatory which observes the universe in gamma-rays from its vantage point in low Earth orbit. One interesting discovery by Fermi are two enormous gamma-ray-emitting bubbles that extend about 30 thousand light years above and below the centre of the Milky Way galaxy. The existence of the two gamma-ray-emitting bubbles was first hinted by previous detections of a localized excess of radio signals. In this article, the two gamma-ray-emitting bubbles will be referred to as the Fermi Bubble. I recently read a paper entitled “Origin of the Fermi Bubble” and this paper suggests that the periodic capture of stars by the supermassive black hole at the centre of the Milky Way galaxy can inject the required amounts of high energy plasma into the galactic halo to form the Fermi Bubble.

A supermassive black hole with a mass of approximately 4 million Suns sits in the heart of the Milky Way galaxy. Stars which happen to come too close to the supermassive black hole can be destroyed by tidal disruption. When a star gets tidally disrupted by the supermassive black hole, about half of its mass becomes tightly bound to the black hole while the other half gets violently ejected. The amount of energy carried by the ejected mass can significantly exceed the amount of energy released by a normal supernova explosion. Approximations have shown that the supermassive black hole at the galactic centre destroys a star by tidal disruption at a rate of roughly one star every ten thousand years or so. This means that tens of stars are expected to get tidally disrupted every one million years.

The ejecta from each tidally disrupted star expand as a spherically symmetric wind of high energy plasma and ‘snowploughs’ its way out of the galactic centre to form a pair of bipolar outflows which contribute to the existence of the Fermi Bubble. The high energy outflows from each tidal disruption event expand hydrodynamically out of the galactic centre and into the galactic halo, forming shock fronts which accelerate electrons to near the speed of light. Interaction of the high energy electrons with background photons via synchrotron radiation and inverse Compton scattering produces the observed radio and gamma-ray emissions respectively. Since the mean interval between each tidal disruption event is smaller than the timescale for energy loss, the gamma-ray emissions produced from each individual shock front can be approximated to be uniformly distributed over the entire Fermi Bubble.

Finally, the existence of the Fermi Bubble cannot be explained by a previous episode of starburst activity in the galactic centre because there is no evidence of an excessive amount of supernova explosions in the past 10 million years or so in the galactic centre. Furthermore, supernova remnants can be traced by the radioactive aluminium-26 they produce and the sparse concentration of aluminium-26 in the galactic centre does not support a previous episode of starburst activity.