The role of gas fraction and feedback in the stability and evolution of galactic discs: implications for cosmological galaxy formation models

High-redshift star-forming galaxies often have irregular morphologies with giant clumps containing up to 108−9 solar masses of gas and stars. The origin and evolution of giant clumps are debated both theoretically and observationally. In most cosmological simulations, highredshift galaxies have regular spiral structures or short-lived clumps, in contradiction with many idealised high-redshift disc models. Here we test whether this discrepancy can be explained by the low gas fractions of galaxies in cosmological simulations. We present a series of simulations with varying gas fractions, from 25%, typical of galaxies in most cosmological simulations, to 50%, typical of observed galaxies at 1.5 < z < 3. We find that gas-poor models have short-lived clumps, that are unbound and mostly destroyed by galactic shear, even with weak stellar feedback. In contrast, gas-rich models form long-lived clumps even with boosted stellar feedback. This shows that the gasmass fraction is the primary physical parameter driving violent disc instabilities and the evolution of giant clumps on ∼10 yr timescales, with lower impact from the calibration of the stellar feedback. Many cosmological simulations of galaxy formation have relatively gas-poor galactic discs, which could explain why giant clumps are absent or short-lived in such models. Similar baryonic and dark matter mass distribution could produce clumpy galaxies with long-lived clumps at z ∼ 2 if the gas fraction was in better agreement with observations.