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Detection of large-scale X-ray bubbles in the Milky Way halo

Abstract

The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way1,2,3,4; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova5 but part of a vast Galaxy-scale structure closely related to features seen in γ-rays6. Large energy injections from the Galactic centre7 are the most likely cause of both the γ-ray and X-ray bubbles. The latter have an estimated energy of around 1056 erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way.

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Fig. 1: The Spektr-RG–eROSITA all-sky map.
Fig. 2: The soft-X-ray eROSITA bubbles.
Fig. 3: Comparison of the morphology of the γ-ray and X-ray bubbles.

Data availability

The datasets analysed during this study are not yet publicly available. Their proprietary scientific exploitation rights were granted by the project funding agencies (Roscosmos and DLR) to two consortia led by MPE (Germany) and IKI (Russia), respectively. The SRG–eROSITA all-sky survey data will be released publicly after a minimum period of 2 years.

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Acknowledgements

This work is based on data from eROSITA, the primary instrument aboard SRG, a joint Russian–German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences, represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from IKI and the Max Planck Institute for Extraterrestrial Physics (MPE). The development and construction of the eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl Remeis Observatory Bamberg and ECAP (FAU Erlangen-Nuernberg), the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig Maximilians Universität Munich also participated in the science preparation for eROSITA. The eROSITA data shown here were processed using the eSASS/NRTA software system developed by the German eROSITA consortium. We thank the entire eROSITA collaboration team, in Germany and Russia, who, over many years, have given fundamental contributions to the development of the mission, the instrument and the science exploitation of the eROSITA data. SRG–eROSITA data processing and calibration and data analyses were performed by a large number of collaboration members in both the German and Russian teams, who also discussed and approved the scientific results presented here. This research made use of Montage. It is funded by the National Science Foundation under grant number ACI-1440620, and was previously funded by NASA’s Earth Science Technology Office, Computation Technologies Project, under cooperative agreement number NCC5-626 between NASA and the California Institute of Technology. G.P. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 865637).

Author information

Authors and Affiliations

Authors

Contributions

H.B., M.F., C.M. and J.S.S. developed software to process the eROSITA data and processed the German proprietary data that resulted in the all-sky maps. E.C., M.G., I.K., and P.M. processed the Russian proprietary data. H.B., E.C., M.G., C.M. and J.S.S. performed the analysis that resulted in Fig. 1. V.D., I.K. and J.S.S. performed the image processing that resulted in Figs. 2, 3 and Extended Data Figs. 1, 2. The majority of the text was written by P.P., W.B., M.F., M.G., E.C., G.P., A.W.S., M.S., H.B. and V.D. V.D. and E.C. worked on Fig. 2; Fig. 3 was created by I.K. and V.D. Extended Data Fig. 1 was prepared by A.M. with the support of an MPE graphic design expert. K.N., A.M. and R.A.S. contributed to writing and editing the manuscript. The above-named authors all contributed to the discussion and interpretation of the results and their implications. The remaining co-authors made important contributions to SRG mission planning and operations, eROSITA data acquisition and analysis, and software development for SRG–eROSITA.

Corresponding authors

Correspondence to P. Predehl, R. A. Sunyaev, E. Churazov, M. Gilfanov, A. Merloni or K. Nandra.

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The authors declare no competing interests.

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Peer review information Nature thanks Jun Kataoka and Roland Crocker for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Schematic of the eROSITA and Fermi bubbles.

Schematic of the geometry of the eROSITA bubbles (EBs; yellow) and Fermi bubbles (FBs; purple) with respect to the Galaxy and the Solar System. The approximate sizes of these structures, as derived from our analysis, are also marked (green and purple arrows).

Extended Data Fig. 2 Soft-X-ray data compared to a thick-shell model for the eROSITA bubbles.

Comparison between the thick-shell model (cyan line in Fig. 2) and eROSITA data (0.6–1.0-keV band) in a Lambert zenithal equal-area projection. The model is in red; the data are in cyan. The northern bubble is shown on the left (N); the southern bubble is shown on the right (S). The northern bubble is spherical, with an outer radius of 7 kpc and an inner radius of 5 kpc. It is slightly offset from the vertical above the Galactic centre. The southern shell is instead an ellipse, slightly elongated in the north–south direction (semi-major axis is 7 kpc; semi-minor axis 4.9 kpc).

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Predehl, P., Sunyaev, R.A., Becker, W. et al. Detection of large-scale X-ray bubbles in the Milky Way halo. Nature 588, 227–231 (2020). https://doi.org/10.1038/s41586-020-2979-0

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