The eRosita X-ray satellite was launched in July 2019, and just several months later, it revealed a striking structure in the Milky Way Galaxy. The so-called "eRosita bubbles" are gigantic bubbles extending to ~80 degrees above and below the Galactic center (GC). Their morphology resembles the "Fermi bubbles" detected in 2010 by the Fermi Gamma-ray Space Telescope and its counterpart observed by WMAP and Planck, the "microwave haze", suggesting that these fascinating structures likely share the same origin. Because of the symmetry about the Galactic plane, they likely originate from some powerful energy injection from the GC in the past, possibly related to a nuclear starburst, or activity from the central supermassive black hole (SMBH), Sgr A*. The proximity of the bubbles/haze has allowed astronomers to gather unprecedented observational data in multiple wavelengths, which provides valuable information to disentangle the different theories of bubble formation.

Prior to the discovery of the eRosita bubbles, we have been investigating the possibility of forming the Fermi bubbles and microwave haze by invoking past jet activity from the Sgr A*. The results have been very promising -- the black-hole-jet model is shown to be broadly consistent with the multi-wavelength observational data. However, some of the other alternative theoretical models can also reproduce the data. That is why the new X-ray data from eRosita is crucial -- it provides additional constraints that enable us to further tell the different theories apart.

In our new work, which was published in Nature Astronomy in March 2022, we performed numerical simulations and showed that the black-hole-jet scenario passed this test -- by invoking a single episode of jet activity from the central SMBH, the morphology and spectra of the eRosita bubbles, Fermi bubbles, as well as the microwave haze could be simultaneously reproduced! Our simulations tracked the interactions between the thermal gas in the Milky Way halo and the cosmic rays (CRs) injected with the jets, which allowed us to compute the X-ray emission from the gas and the gamma-ray and microwave radiation from the CRs self-consistently. The excellent match with the observational data is a nontrivial result, as it requires very specific 3D spatial distribution as well as energy distribution of both the thermal gas and the CRs. Our results thus suggest that the bubbles/haze are likely generated by past jet activity of the SMBH at the GC!

The new data of the eRosita bubbles have allowed us to put stringent constraints on the age of the bubbles, the duration of the jet activity, and the activity level of the SMBH during the active period. We found that the bubbles are inflated by an episode of jet injection about 2.6 million years ago for a duration of 0.1 million years. The SMBH was accreting material from its surroundings at a very high rate, corresponding to ~1,000-10,000 solar masses within the active phase. The results showed that, although Sgr A* is extremely quiescent at the present time, the SMBH was much more active a few million years ago!

Are there Fermi/eRosita bubble-like structures in other galaxies? How would these energetic events impact the evolution of the host galaxies of SMBHs? How would they shape the growth of SMBHs? Though all these interesting questions remain to be addressed, the gigantic bubbles in the Galaxy certainly have opened up many exciting possibilities for astrophysicists to understand the evolution history of our own Milky Way and how SMBHs interact and co-evolve with their host galaxies.  

[Image credits for poster image: background image of the Milky Way (ESA/Gaia/DPAC, CC BY-SA 3.0 IGO); simulation data (H.-Y. Karen Yang); superposition created by NASA visualization team.]