The newly launched X-ray satellite, eRosita, has recently revealed two gigantic bubbles extending to ~80 degrees above and below the center of our Milky Way Galaxy. The morphology of these “eRosita bubbles” bears a remarkable resemblance to the Fermi bubbles previously discovered in gamma rays. Before the eRosita bubbles were discovered, the physical origin of the Fermi bubbles has been intensely debated. Generally speaking, because of their enormous energies and symmetry about the Galactic center, they must originate from some energetic outburst from the Galactic center sometime in the past. However, whether they are associated with a starburst event or activity of the central supermassive black hole remains a mystery. The new eRosita data has provided critical information that allowed one to further constrain the formation mechanisms of these fascinating bubbles.
In our recent work led by NTHU, we performed cutting-edge numerical simulations and showed that the eRosita and Fermi bubbles could be simultaneously explained by a single event of jet activity from the central supermassive black hole a few million years ago. Our simulations included modeling of the high-energy particles carried by the black hole jets, the thermal gas within the Milky Way, as well as the interactions between them. As a result, we could predict the gamma-ray emission generated by these high-energy particles as they interact with photons in the Galaxy. We could also compute the X-ray emission produced as the gas is compressed by the shock wave driven by the jet activity. We showed that our model successfully reproduced the morphology and multi-wavelength spectra of the observed Fermi and eRosita bubbles. We have also obtained important constraints on this activity, including when it occurred, how long it lasted, and how much energy is required, etc. We found that the central supermassive black hole needs to be active about 2.6 million years ago for 0.1 million years. During its active period, the black hole needs to consume ~1,000 to 10,000 solar masses of materials in order to convert a fraction of the accreted energy into a pair of relativistic jets. The energetics and timescales of such an activity are also consistent with an independent measurement of ionization features in the Magellanic Stream, giving further support of an elevated activity of the Galactic center black hole. This study serves as an important step forward in our understanding of the past Galactic center activity of our Milky Way Galaxy, and may bring valuable insights into the broader picture of supermassive black hole-galaxy co-evolution in the context of galaxy formation. 
Prof. Hsiang-Yi Yang at NTHU Institute of Astronomy revealed the mysterious nature of the giant bubbles in the Milky Way Galaxy. |
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