Formation of giant clumps in high-<i>z</i> disc galaxies by compressive turbulence

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.