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Cold gas in the Milky Way’s nuclear wind

Abstract

The centre of the Milky Way hosts several high-energy processes that have strongly affected the inner regions of our Galaxy. Activity from the super-massive black hole at the Galactic Centre, which is coincident with the radio source Sagittarius A*, and stellar feedback from the inner molecular ring1 expel matter and energy from the disk in the form of a galactic wind2. Multiphase gas has been observed within this outflow, including hot highly ionized3,4 (temperatures of about 106 kelvin), warm ionized5,6 (104 to 105 kelvin) and cool atomic7,8 (103 to 104 kelvin) gas. However, so far there has been no evidence of the cold dense molecular phase (10 to 100 kelvin). Here we report observations of molecular gas outflowing from the centre of our Galaxy. This cold material is associated with atomic hydrogen clouds travelling in the nuclear wind8. The morphology and the kinematics of the molecular gas, resolved on a scale of about one parsec, indicate that these clouds are mixing with the warmer medium and are possibly being disrupted. The data also suggest that the mass of the molecular gas outflow is not negligible and could affect the rate of star formation in the central regions of the Galaxy. The presence of this cold, dense and high-velocity gas is puzzling, because neither Sagittarius A* at its current level of activity nor star formation in the inner Galaxy seems to be a viable source for this material.

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Fig. 1: Atomic hydrogen gas outflowing from the Galactic Centre.
Fig. 2: Atomic hydrogen and molecular gas in two clouds in the Milky Way’s nuclear wind.
Fig. 3: Molecular gas kinematics in MW-C1 and MW-C2.

Data availability

The APEX raw datasets analysed for this study will be available at the end of the proprietary period (September 2020) on the ESO archive, http://archive.eso.org/eso/eso archive main.html. The GBT raw datasets are publicly available at the NRAO archive, https://science.nrao.edu/facilities/gbt/software-and-tools. Fully reduced data are available from the corresponding author on reasonable request.

Code availability

The software used in this work is publicly available. The GILDAS/CLASS packages for submillimetre data reduction can be found at https://www.iram.fr/IRAMFR/GILDAS. The DUCHAMP source finder can be downloaded from https://www.atnf.csiro.au/people/Matthew.Whiting/Duchamp. The DESPOTIC radiative-transfer code is available at https://bitbucket.org/krumholz/despotic.

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Acknowledgements

E.M.D.T. and L.A. thank E. Ostriker, C.-G. Kim and J.-G. Kim for discussions and M. Krumholz for support with the DESPOTIC code. E.M.D.T. was supported by the US National Science Foundation under grant 1616177. E.M.D.T. and N.M.M.-G. acknowledge the support of the Australian Research Council (ARC) through grant DP160100723. N.M.M.-G. acknowledges funding from the ARC via Future Fellowship FT150100024. CO observations were made with APEX under ESO proposal 0104.B-0106A. APEX is a collaboration between Max-Planck-Institut für Radioastronomie, the European Southern Observatory and the Onsala Space Observatory. The Green Bank Observatory is a facility of the US National Science Foundation operated under a cooperative agreement by Associated Universities, Inc. The ATCA is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO.

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Contributions

E.M.D.T., N.M.M.-G. and F.J.L. developed the idea for the project. E.M.D.T. reduced and analysed the APEX data, L.A. ran the radiative-transfer models. E.M.D.T. wrote the paper with direct contributions from N.M.M.-G., F.J.L. and L.A. All authors reviewed the manuscript.

Corresponding author

Correspondence to Enrico M. Di Teodoro.

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The authors declare no competing interests.

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Peer review information Nature thanks Mark Morris and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended Data Table 1 Properties of molecular gas clouds outflowing from the Galactic Centre

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Di Teodoro, E.M., McClure-Griffiths, N.M., Lockman, F.J. et al. Cold gas in the Milky Way’s nuclear wind. Nature 584, 364–367 (2020). https://doi.org/10.1038/s41586-020-2595-z

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