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Molecular clouds in the Cosmic Snake normal star-forming galaxy 8 billion years ago


The cold molecular gas in contemporary galaxies is structured in discrete cloud complexes. These giant molecular clouds (GMCs), with 104–107 solar masses (M) and radii of 5–100 parsecs, are the seeds of star formation1. Highlighting the molecular gas structure at such small scales in distant galaxies is observationally challenging. Only a handful of molecular clouds were reported in two extreme submillimetre galaxies at high redshift2,3,4. Here we search for GMCs in a typical Milky Way progenitor at z = 1.036. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we mapped the CO(4–3) emission of this gravitationally lensed galaxy at high resolution, reading down to 30 parsecs, which is comparable to the resolution of CO observations of nearby galaxies5. We identify 17 molecular clouds, characterized by masses, surface densities and supersonic turbulence all of which are 10–100 times higher than present-day analogues. These properties question the universality of GMCs6 and suggest that GMCs inherit their properties from ambient interstellar medium. The measured cloud gas masses are similar to the masses of stellar clumps seen in the galaxy in comparable numbers7. This corroborates the formation of molecular clouds by fragmentation of distant turbulent galactic gas disks8,9, which then turn into stellar clumps ubiquitously observed in galaxies at ‘cosmic noon’ (ref. 10).

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Fig. 1: Molecular gas distribution in the strongly lensed Cosmic Snake galaxy.
Fig. 2: Normalized distributions of molecular gas mass for different GMC populations.
Fig. 3: Larson scaling relations.
Fig. 4: Pressure confinement versus self-gravitating confinement of the Cosmic Snake GMCs.

Data availability

The ALMA raw data of the Cosmic Snake arc are available through the ALMA archive under the project identification 2013.1.01330.S. The HST images of MACS J1206.2–0847 are part of the CLASH, available at The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The reduction of the ALMA data was performed with the CASA pipeline version 4.2.2, available at The PdBI data were reduced using GILDAS software, available at The lens model was obtained using Lenstool, publicly available at The spectral energy distribution fitting was performed with a modified version of the Hyperz code, available in its original form at


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The work of M.D.-Z., D.S., L.M. and A.C. was supported by the STARFORM Sinergia Project funded by the Swiss National Science Foundation. J.R. acknowledges support from the European Research Council starting grant 336736-CALENDS. W.R. is supported by the Thailand Research Fund/Office of the Higher Education Commission grant no. MRG6280259 and Chulalongkorn University’s CUniverse. P.G.P.-G. acknowledges support from the Spanish Government grant AYA2015-63650-P. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.01330.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We also used PdBI observations. PdBI is run by the Institut de Radioastronomie Millimétrique (IRAM, France), a partnership of the French CNRS, the German MPG and the Spanish IGN. Part of the analysis presented herein is also based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). We thank E. Chapillon from the ALMA Regional Center node of IRAM for her help and training on the reduction of the ALMA data, V. Patricio for sharing the kinematic analysis of the [O ii] emission of the Cosmic Snake galaxy and C. Georgy for the presentation of the VisIt 3D visualization tool.

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The data reduction was performed by M.D.-Z. W.R. contributed to the production of the final CO(4–3) data cube. J.R. and A.C. were responsible for the lens model. M.D.-Z. carried out all the data analysis, following advice from J.R., A.C., F.C., W.R. and F.B. F.C. computed the radial dynamical properties and associated figures. Data interpretation was led by M.D.-Z., with feedback from F.C., D.S., L.M., D.P. and R.T. The main text and Methods, with related figures and table, were written by M.D.-Z. All authors commented on the paper, with particular involvement of D.S., L.M., F.C. and T.D.R.

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Correspondence to Miroslava Dessauges-Zavadsky.

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Supplementary Figs. 1–8, Table 1.

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Dessauges-Zavadsky, M., Richard, J., Combes, F. et al. Molecular clouds in the Cosmic Snake normal star-forming galaxy 8 billion years ago. Nat Astron 3, 1115–1121 (2019).

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