A pair of gigantic gamma-ray bubbles centered on the core of the Milky Way galaxy were discovered by the Fermi Gamma-ray Space Telescope 10 years ago. The question of how they were formed still remains as a mystery.

  Recently, researchers at Shanghai Astronomical Observatory presented a new model, explaining for the first time the origins of both the Fermi bubbles and the Galactic center biconical X-ray structure discovered in 2003 simultaneously. The two structures are essentially the same phenomenon caused by the forward shock driven by a pair of jets emanating from Sgr A*, the supermassive black hole lurking at the Galactic center, about 5 million years ago. This work has been accepted for publication on the Astrophysical Journal.

  Fermi Bubbles are two colossal blobs filled with very hot gas, cosmic rays, and magnetic fields. Although they can not be seen with our naked eyes, they are very bright in diffuse gamma ray emissions. In gamma rays, the Fermi bubbles have very sharp edges and the edges coincides well with an X-ray structure called the Galactic center biconical X-ray structure. 

  This may not be a coincidence, and instead it may suggest that these two structures share the same origin. The biconical X-ray structure can be naturally explained by the shock-compressed thin shell of hot thermal gas driven by a past energy outburst from the Galactic center.

  In previous theoretical models and computer simulations of the Fermi bubbles, there are two major competing energy sources proposed, i.e., star formation at the Galactic center and the Sgr A*. However, in both models, the Fermi bubbles are explained as the ejecta bubbles, while the forward shock is always located much further away from the edge of the Fermi bubbles. They could not explain the Fermi bubbles and the Galactic center biconical X-ray structure simultaneously.

  The theoretical model, proposed in the new study by GUO Fulai and his graduate student  ZHANG Ruiyu from Shanghai Astronomical Observatory, for the first time, demonstrates with computer simulations that the Fermi bubbles and the Galactic center biconical X-ray structure are the same phenomenon.

  In this model, the edge of the Fermi bubbles is the forward shock driven by a pair of jets emanating from Sgr A* about 5 million years ago. “One good thing about this model is that the energy and age of the Fermi bubbles can be constrained by the X-ray observations quite well”, says the corresponding author GUO Fulai. The age of the bubbles inferred in this study is also consistent with that derived from recent ultraviolet observations of some high velocity clouds along many sightlines towards the bubble region.

The new model indicates that the total energy injected during the Fermi bubble event by the supermassive black hole is close to that released by about 20,000 supernovae. The total matter consumed by Sagittarius A* during this event is about 100 solar masses.

“Another very interesting thing that we found in our study is that if the bubbles and the biconical X-ray structure share the same origin, they are very unlikely to be produced by star formation or black hole winds.” says GUO Fulai. Near the Galactic center, the biconical X-ray structure has a very narrow base, while the forward shock produced by star formation or black hole winds can easily propagate to large distances, leading to a base much wider than observed.

 In contrast, collimated jets deposit most of the energy quickly to large distances along the jet direction, naturally leading to a narrow base of the shock front near the Galactic plane. The supermassive black hole in our own Galaxy has been very quiescent in recent years without any evidence of current jet activities, but “our study strongly suggests that a pair of powerful jets emanated from it about 5 million years ago, lasted for about 1 million years, and produced the gigantic Fermi bubbles still seen today.” says GUO Fulai.