Origin of compressive turbulence in protoclumps in high redshift disks

Context. The giant star-forming clumps in gas-rich high-redshift disks are commonly assumed to form due to gravitational instabilities, in which protoclumps have a Toomre-Q parameter less than unity. However, some cosmological simulations have shown that clumps can form in regions where Q is significantly greater than unity. In these simulations, there is an energy excess of compressive modes of turbulence that lead to gravitational collapse of regions that were not supposed to collapse under their own self-gravity, according to linear Toomre instability. In contrast, sites of clump formation in isolated simulations do not show this excess, suggesting that the origin of the compressive turbulence may be external. Aims. We explore two external mechanisms that can induce the compressive modes of disk turbulence in protoclumps, namely, compressive tides exerted by the cosmological environment and the direct driving by inflowing streams. Methods. We correlated the local strength of compressive tides and the amount of fresh stream material with protoclump regions in zoom-in cosmological simulations. We derived the local strength of compressive tides from the eigenvalues of the tidal tensor, and the local strength of incoming streams was derived from the fractional presence of the stream compared to the average. Results. We find that the tidal field in protoclumps tends to be over-compressive, while random patches in the disk show substantial diverging tides. In particular, in 25% of the protoclumps, the tidal field is fully compressive, while no random patch resides in regions of fully compressive tides. In addition, the protoclumps tend to reside in regions where the fraction of the incoming stream mass is two to ten times larger than the average at the same galactocentric radius. Conclusions. Both compressive tides and inflowing streams are correlated with the protoclumps and can thus serve as the drivers of the excessive compressive turbulence that can initiate clump formation before self-gravity takes over. This nature of turbulence constitutes a new non-linear mode of violent disk instabilities in high-z galaxies.