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The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way

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Abstract

Galactic outflows are ubiquitous in galaxies containing active star formation or supermassive black hole activity. The presence of a large-scale outflow from the center of our own Galaxy was confirmed after the discovery of two large (~ 8–10 kpc) \(\gamma \)-ray bubbles using the Fermi-LAT telescope. These bubbles, known as the Fermi Bubbles, are highly symmetric about the Galactic disk as well as around the Galactic rotation axis and appear to emanate from the center of our Galaxy. The sharp edges of these bubbles suggest that they are related to the Galactic outflow. These bubbles are surrounded by two even bigger (~ 12–14 kpc) X-ray structures, known as the eROSITA bubbles. Together, they represent the characteristics of an outflow from the Galaxy into the circumgalactic medium. Multi-wavelength observations such as in radio, microwave, and UV toward the Fermi Bubbles have provided us with much information in the last decade. However, the origin and the nature of these bubbles remain elusive. In this review, I summarize the observations related to the Fermi/eROSITA Bubbles at different scales and wavelengths, and give a brief overview of our current understanding of them.

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Fig. 1
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Image reprinted with permission from Ackermann et al. (2014), copyright by AAS

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Image reprinted with permission from Ackermann et al. (2014), copyright by AAS

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Notes

  1. It is quite possible that the estimated temperature is dominated by the gas within \(\lesssim 50\) kpc where the gas is heated by the SNe feedback from the Galaxy (Faerman et al. 2020).

  2. https://fermi.gsfc.nasa.gov/science/overview.html.

  3. An excess emission within the mentioned central ellipse was noticed in an earlier paper by Dobler et al. (2010) and was named as Fermi Haze since it was not clear if the ‘Haze’ has a sharp edge or not.

  4. Note that the cooling time-scale for the gas to cool from 0.3 to 0.2 keV (and not to zero) would only be 1/3 of the \(\tau _{\rm{cool}}\). The new cooling time (\(\sim\,100\) Myr) is still a factor of a few longer than the propagation time, \(\sim\,20\) Myr.

  5. Although the paper was originally intended to explain the absence of an SPS in the ROSAT data, it nonetheless predicted a fainter southern structure.

  6. Ideally the diffusion coefficient in lower density CGM can be \(\sim\,10^{2{-}3}\) higher than the classical value (Chan et al. 2019; Ji et al. 2020) but simultaneously anisotropic diffusion can reduce the diffusion perpendicular to the field by a factor of \(\sim\,10^{2{-}3}\) (Shalchi et al. 2010).

  7. Considering another factor of 10 to get the diffuse wind power.

  8. Note that the central \(\sim 20\) pc also hosts a \(\sim\,7\) keV diffuse plasma (Koyama et al. 1989; Muno et al. 2004), the nature of which is still unknown. Recent claims suggest that this emission could be due to unresolved stars at the GC that have super Solar Fe abundances (Anastasopoulou et al. 2023; Hua et al. 2023) and may not be a diffuse component of the plasma.

  9. See Yuan and Narayan (2014) for a recent review on the different accretion flow models abound black holes.

  10. The reason for categorizing Sofue’s models in AGN-burst is because of his assumption of an instantaneous energy explosion. The energy from a starburst is released continuously over a period of \(\approx 40\) Myr even if all the stars form instantaneously (Leitherer and Heckman 1995; Leitherer et al. 1999).

  11. Note that Eq. (13) should be modified in case of a weak shock. Therefore, even if the current version of the equation produces subsonic flow for the \(L_{\rm{wind}}\sim 10^{40{-}41}\) erg  \(\rm{s}^{-1}\) case, the corrected equation produces a \({\mathcal {M}}\approx 1.5\) flow, as is seen in simulations.

  12. In the paper, the luminosity to SFR conversion was assumed to be \(L_{\rm{wind}} \approx 3\times 10^{41}\) erg  \(\rm{s}^{-1}\) SFR/(\(M_{\odot } \,\, \hbox {yr}^{-1}\)), a factor of 2 lower than the assumed conversion factor in this review. For the same \(L_{\rm{wind}}\), Eq. (1) produces a SFR of 0.3 \(M_{\odot } \,\, \hbox {yr}^{-1}\).

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Acknowledgements

I thank Andrew Fox, Santanu Mondal, Matteo Pais, Peter Predehl, Prateek Sharma, Amiel Sternberg, and Nicholas Stone for providing helpful comments that improved the content of the article. I also thank the referee, Joss Bland-Hawthorn, and the scientific editor, Joel Bregman, for providing critical comments that helped improve the content of this article. I convey my special thanks to Matteo Pais for providing me with the code to calculate shock propagation using Kompaneets’ approximation, Santanu Mondal for providing simulation data from Mondal et al. (2022), and Roland Crocker for providing data from Crocker and Aharonian (2011). My research in Israel has been supported by the German Science Foundation via DFG/DIP Grant STE/ 1869-2 GE/ 625 17-1.

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  1. School of Physics and Astronomy, Tel Aviv University, 6997801, Tel Aviv-Yafo, Tel Aviv, Israel

    Kartick C. Sarkar

  2. Department of Space, Planetary and Astronomical Sciences and Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, 208016, India

    Kartick C. Sarkar

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Sarkar, K.C. The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way. Astron Astrophys Rev 32, 1 (2024). https://doi.org/10.1007/s00159-024-00152-1

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