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ABSTRACT We address the formation of giant clumps in violently unstable gas-rich disc galaxies at cosmic noon. While these are commonly thought to originate from gravitational Toomre instability, some cosmological simulations have indicated that clumps can form in Lagrangian proto-clump regions where the Toomre Q parameter is well above unity, which are linearly stable. Examining one of these cosmological simulations, we find that it exhibits an excess in compressive modes of turbulence with converging motions. The energy in converging motions within proto-clumps is $${\sim} 70~{{\ \rm per\ cent}}$$ of the total turbulent energy, compared to $${\sim} 17~{{\ \rm per\ cent}}$$ expected in equipartition. When averaged over the whole disc, $${\sim} 40~{{\ \rm per\ cent}}$$ of the turbulent energy is in compressive modes, mostly in converging motions, with the rest in solenoidal modes, compared to the $(1/3)-(2/3)$ division expected in equipartition. By contrast, we find that in an isolated-disc simulation with similar properties, resembling high-z star-forming galaxies, the different turbulence modes are in equipartition, both in proto-clumps and over the whole disc. We conclude that the origin of excessive converging motions in proto-clumps is external to the disc, and propose several mechanisms that can induce them. This is an additional mechanism for clump formation, complementary to and possibly preceding gravitational instability. 

Abstract Cosmic rays (CRs) are the primary driver of ionization in star-forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic-ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a 2000M_⊙MC and the subsequent star formation including CRT using the STAR FORmation in Gaseous Environments framework implemented in the GIZMO code. We show that when CRT is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way–like environment the median CR ionization rate in the cloud is low (ζ≲ 2 × 10⁻¹⁹s⁻¹) during the main star-forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated (ζ≲ 6 × 10⁻¹⁸), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency by 50% to ∼13%. The initial mass function is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments.