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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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.
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.
Molinari, S. et al. A 100 pc elliptical and twisted ring of cold and dense molecular clouds revealed by Herschel around the Galactic center. Astrophys. J. Lett. 735, 33 (2011).
Bland-Hawthorn, J. & Cohen, M. The large-scale bipolar wind in the Galactic center. Astrophys. J. 582, 246–256 (2003).
Kataoka, J. et al. Suzaku observations of the diffuse X-ray emission across the Fermi Bubbles’ edges. Astrophys. J. 779, 57 (2013).
Ponti, G. et al. An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre. Nature 567, 347–350 (2019).
Fox, A. J. et al. Probing the Fermi Bubbles in ultraviolet absorption: a spectroscopic signature of the Milky Way’s biconical nuclear outflow. Astrophys. J. 799, L7 (2015).
Bordoloi, R. et al. Mapping the nuclear outflow of the Milky Way: studying the kinematics and spatial extent of the northern Fermi Bubble. Astrophys. J. 834, 191 (2017).
McClure-Griffiths, N. M. et al. Atomic hydrogen in a Galactic center outflow. Astrophys. J. Lett. 770, 4 (2013); erratum 884, 27 (2019).
Di Teodoro, E. M. et al. Blowing in the Milky Way wind: neutral hydrogen clouds tracing the Galactic nuclear outflow. Astrophys. J. 855, 33 (2018).
Gravity Collaboration. Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole. Astron. Astrophys. 636, L5 (2020).
Su, M., Slatyer, T. R. & Finkbeiner, D. P. Giant gamma-ray bubbles from Fermi-LAT: active galactic nucleus activity or bipolar Galactic wind? Astrophys. J. 724, 1044–1082 (2010).
Miller, M. J. & Bregman, J. N. The interaction of the Fermi Bubbles with the Milky Way’s hot gas halo. Astrophys. J. 829, 9 (2016).
Lockman, F. J., Di Teodoro, E. M. & McClure-Griffiths, N. M. Observation of acceleration of HI clouds within the Fermi Bubbles. Astrophys. J. 888, 51 (2020).
Bolatto, A. D., Wolfire, M. & Leroy, A. K. The CO-to-H2 conversion factor. Annu. Rev. Astron. Astrophys. 51, 207–268 (2013).
Longmore, S. N. et al. Variations in the Galactic star formation rate and density thresholds for star formation. Mon. Not. R. Astron. Soc. 429, 987–1000 (2013).
Bolatto, A. D. et al. Suppression of star formation in the galaxy NGC253 by a starburst-driven molecular wind. Nature 499, 450–453 (2013).
Veilleux, S., Maiolino, R., Bolatto, A. D. & Aalto, S. Cool outflows in galaxies and their implications. Annu. Rev. Astron. Astrophys. 28, 2 (2020).
Scannapieco, E. & Brüggen, M. The launching of cold clouds by Galaxy outflows. I. Hydrodynamic interactions with radiative cooling. Astrophys. J. 805, 158 (2015).
Thompson, T. A., Fabian, A. C., Quataert, E. & Murray, N. Dynamics of dusty radiation- pressure-driven shells and clouds: fast outflows from galaxies, star clusters, massive stars, and AGN. Mon. Not. R. Astron. Soc. 449, 147–161 (2015).
Mukherjee, D., Bicknell, G. V., Sutherland, R. & Wagner, A. Relativistic jet feedback in high-redshift galaxies – I. Dynamics. Mon. Not. R. Astron. Soc. 461, 967–983 (2016).
Richings, A. J. & Faucher-Giguère, C.-A. Radiative cooling of swept-up gas in AGN-driven galactic winds and its implications for molecular outflows. Mon. Not. R. Astron. Soc. 478, 3100–3119 (2018).
Armillotta, L., Krumholz, M. R., Di Teodoro, E. M. & McClure-Griffiths, N. M. The life cycle of the Central Molecular Zone – I. Inflow, star formation, and winds. Mon. Not. R. Astron. Soc. 490, 4401–4418 (2019).
Barnes, A. T. et al. Star formation rates and efficiencies in the Galactic Centre. Mon. Not. R. Astron. Soc. 469, 2263–2285 (2017).
Krumholz, M. R., Kruijssen, J. M. D. & Crocker, R. M. A dynamical model for gas flows, star formation and nuclear winds in galactic centres. Mon. Not. R. Astron. Soc. 466, 1213–1233 (2017).
Armillotta, L., Krumholz, M. R. & Di Teodoro, E. M. The life cycle of the Central Molecular Zone – II. Distribution of atomic and molecular gas tracers. Mon. Not. R. Astron. Soc. 493, 5273–5289 (2020).
Girichidis, P., Naab, T., Hanasz, M. & Walch, S. Cooler and smoother – the impact of cosmic rays on the phase structure of galactic outflows. Mon. Not. R. Astron. Soc. 479, 3042–3067 (2018).
Zhang, D., Thompson, T. A., Quataert, E. & Murray, N. Entrainment in trouble: cool cloud acceleration and destruction in hot supernova-driven galactic winds. Mon. Not. R. Astron. Soc. 468, 4801–4814 (2017).
McCourt, M., O’Leary, R. M., Madigan, A.-M. & Quataert, E. Magnetized gas clouds can survive acceleration by a hot wind. Mon. Not. R. Astron. Soc. 449, 2–7 (2015).
Armillotta, L., Fraternali, F., Werk, J. K., Prochaska, J. X. & Marinacci, F. The survival of gas clouds in the circumgalactic medium of Milky Way-like galaxies. Mon. Not. R. Astron. Soc. 470, 114–125 (2017).
Gronke, M. & Oh, S. P. The growth and entrainment of cold gas in a hot wind. Mon. Not. R. Astron. Soc. 480, L111–L115 (2018).
Schneider, E. E., Ostriker, E. C., Robertson, B. E. & Thompson, T. A. The physical nature of starburst-driven Galactic outflows. Astrophys. J. 895, 43 (2020).
Güsten, R. et al. The Atacama Pathfinder EXperiment (APEX) – a new submillimeter facility for southern skies. Astron. Astrophys. 454, L13–L16 (2006).
Klein, B. et al. High-resolution wide-band fast Fourier transform spectrometers. Astron. Astrophys. 542, L3 (2012).
Gildas Team. GILDAS: Grenoble Image and Line Data Analysis Software. Astrophysics Source Code Library http://www.ascl.net/1305.010 (2013).
Whiting, M. T. DUCHAMP: a 3D source finder for spectral-line data. Mon. Not. R. Astron. Soc. 421, 3242–3256 (2012).
Roberts, M. S. Radio observations of neutral hydrogen in galaxies. In Galaxies and the Universe (eds Sandage, A., Sandage, M. & Kristian, J.) 309–358 (Univ. of Chicago Press, 1975).
Heyer, M., Krawczyk, C., Duval, J. & Jackson, J. M. Re-examining Larson’s scaling relationships in galactic molecular clouds. Astrophys. J. 699, 1092–1103 (2009).
Krumholz, M. R. DESPOTIC – a new software library to Derive the Energetics and SPectra of Optically Thick Interstellar Clouds. Mon. Not. R. Astron. Soc. 437, 1662–1680 (2014).
Gong, M., Ostriker, E. C. & Wolfire, M. G. A simple and accurate network for hydrogen and carbon chemistry in the interstellar medium. Astrophys. J. 843, 38 (2017); erratum 866, 163 (2018).
Draine, B. T. Photoelectric heating of interstellar gas. Astrophys. J. Suppl. Ser. 36, 595–619 (1978).
Indriolo, N. & McCall, B. J. Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observations of H3 +. Astrophys. J. 745, 91 (2012).
Oka, T. et al. The central 300 pc of the Galaxy probed by infrared spectra of H3 + and CO. I. Predominance of warm and diffuse gas and high H2 ionization rate. Astrophys. J. 883, 54 (2019).
Bland-Hawthorn, J. & Gerhard, O. The Galaxy in context: structural, kinematic, and integrated properties. Annu. Rev. Astron. Astrophys. 54, 529–596 (2016).
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.
The authors declare no competing interests.
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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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