From giant clumps to clouds I: the impact of gas fraction evolution on
  the stability of galactic discs
  The morphology of gas-rich disc galaxies at redshift ~1-3 is dominated by a
few massive clumps. The process of formation or assembly of these clumps and
their relation to molecular clouds in contemporary spiral galaxies are still
unknown. Using simulations of isolated disc galaxies, we study how the
structure of the interstellar medium and the stability regime of the discs
change when varying the gas fraction. In all galaxies, the stellar component is
the main driver of instabilities. However, the molecular gas plays a
non-negligible role in the inter-clumps medium of gas-rich cases, and thus in
the assembly of the massive clumps. At scales smaller than a few 100 pc, the
Toomre-like disc instabilities are replaced by another regime, specially in the
gas-rich galaxies. We find that galaxies at low gas fraction (10%) stand apart
from discs with more gas, which all share similar properties on virtually all
the aspects we explore. For gas fractions below approximately 20%, the
clump-scale regime of instabilities disappears, only leaving the large-scale
disc-driven regime. When translating the change of gas fraction to the cosmic
evolution of galaxies, this transition marks the end of the clumpy phase of
disc galaxies, and allows for the onset of spiral structures, as commonly found
in the local Universe.
