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

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The cold molecular gas in contemporary galaxies is structured in discrete cloud complexes. These giant molecular clouds (GMCs), with 104 –107 solar masses (Mꙩ) 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 redshift2-4 . 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).1
© 2020 Springer Nature Limited. Artículo firmado por 15 autores. 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 Number 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 [OII] emission of the Cosmic Snake galaxy and C. Georgy.
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