Formation of Giant Clumps in High-$z$ Disc Galaxies by Compressive Turbulence

TL;DR

This paper investigates how compressive turbulence, driven by external mechanisms, contributes to the formation of giant clumps in high-redshift disc galaxies, beyond traditional gravitational instability explanations.

Contribution

It demonstrates that external compressive turbulence plays a significant role in clump formation, challenging the sole reliance on Toomre instability in galaxy evolution models.

Findings

Approximately 40% of turbulent energy in proto-clump regions is in compressive modes.

External mechanisms induce excess converging motions, promoting clump formation.

Equipartition of turbulence modes is observed in isolated disc simulations, unlike in cosmological ones.

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, cosmological simulations have indicated that clumps form even in regions where the Toomre $Q$ parameter is well above unity, which should be stable according to linear Toomre theory (Inoue et al., 2016). 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-clump regions is $\sim 70\%$ of the total turbulent energy, compared to $\sim 17\%$ expected in equipartition. When averaged over the whole disc, $\sim 32\%$ of the turbulent energy is in converging motions, with a further $\sim 8\%$ in diverging motions. Thus, a total of $\sim 40\%$ of the turbulent energy is in compressive modes, 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 energy in the different turbulence modes are in equipartition, both in proto-clump regions and over the whole disc. We conclude that the origin of the excessive converging motions in proto-clump regions 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.