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 $10^4$-$10^7$ solar
masses and radii of 5-100 parsecs, are the seeds of star formation.
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 redshift. 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 galaxies. 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 GMCs 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 numbers. This corroborates the
formation of molecular clouds by fragmentation of distant turbulent galactic
gas disks, which then turn into stellar clumps ubiquitously observed in
galaxies at cosmic noon.
