Tuesday, May 19, 2020

At the origin of the Fermi bubbles  

Ten years ago, NASA's Fermi Gamma-ray Large Area Space Telescope, Glast (gamma-ray telescope) discovered a pair of gigantic lobes of gamma radiation, centered in the core of our galaxy, which extended for a total of 50 thousand light years - 25 thousand light years above and 25 thousand light years below the galactic disk. They were called Fermi bubbles and their origin is still unknown.



Recently two researchers from the Shanghai Astronomical Observatory (Shao) of the Chinese Academy of Sciences presented a new model that for the first time simultaneously explains the origin of the Fermi bubbles and the biconical X-ray structure present in the galactic center, discovered in 2003. According to this model, the two structures represent the same phenomenon, originating from shock waves generated by a pair of jets from Sagittarius A * (Sgr A *) - the supermassive black hole that is located in the center of our galaxy - about five million years ago. The study was published in The Astrophysical Journal.

Fermi bubbles are two huge lobes full of very hot gas, cosmic rays and magnetic fields. Although they cannot be seen with the naked eye, they are very bright in gamma rays, where they have very sharp edges that coincide rather well with a biconical structure evident in X-rays. Noting this curious correspondence, the researchers hypothesized that the two structures could share the same origin. In addition, the biconical X-ray structure could naturally be explained by the thin shell of the shock wave of the thermally hot gas, generated by an energy explosion that occurred in the past in the galactic center.


Observations of the galactic center made by gamma rays by the Fermi space telescope, and by X-rays by Rosat, which clearly show how the Fermi bubbles coincide very well with the biconical X-ray structure of the galactic center, at low latitudes. Credits: Bland-Hawthorn, J. et al.

In the previous theoretical models and computer simulations of the Fermi bubbles, two competing energy sources have been proposed: the star formation in the galactic center and Sgr A *.

However, in both models, the Fermi bubbles are explained as expelled bubbles, while the shock wave is always much further away from the edge of the Fermi bubbles. In other words, these models were unable to explain the Fermi bubbles and the biconical X structure of the galactic center simultaneously.

On the contrary, the theoretical model proposed in this study by Guo Fulai and Zhang Ruiyu del Shao, based on computer simulations, is able to demonstrate that the Fermi bubbles and the biconical X-ray structure of the galactic center are actually the same phenomenon. According to this model, the edge of the Fermi bubbles is the wavefront of the pair of jets expelled by Sgr A * about five million years ago.

"A positive aspect of this model is that the energy and age of the Fermi bubbles can be limited quite well by X-ray observations," reports Guo Fulai. The age of the bubbles inferred in this study is also consistent with that derived from recent observations in the ultraviolet of some high-speed clouds, along many lines of sight in the direction of the bubbles. The new model indicates that the total energy injected by the supermassive black hole, during the generation of the event that gave rise to the bubbles, is close to that released by about 20 thousand supernovae. The total matter consumed by Sgr A * during this event is about 100 solar masses.


The simulated X-ray brightness map (at 1.5 keV) in the new model by Zhang & Guo (2020). The dotted line delimits the Fermi bubbles observed in gamma rays. Credits: Zhang and Guo

"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, it is very unlikely that they were produced by star formation or black hole winds," he notes. Guo. Near the center of the galaxy, the biconical X-ray structure has a very narrow base, while a possible wavefront produced by star formation or black hole winds could easily propagate at great distances, leading to the formation of a much more basic broader than that observed. On the contrary, the collimated jets quickly deposit most of the energy over large distances along the direction of the jet, naturally leading to having a shock front close to the very narrow galactic plane.

The supermassive black hole present in the center in our galaxy has been very quiet in recent years and there is no evidence of activity that may have originated jets but, according to Guo, the study strongly suggests that about five million years ago a pair of powerful jets was emitted from the black hole


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