Gas clump formation via thermal instability in high-redshift dwarf galaxy mergers

TL;DR

This study uses high-resolution hydrodynamic simulations to explore how thermal instability leads to cold gas cloud formation in merging dwarf galaxies, shedding light on early star cluster formation in the universe.

Contribution

It demonstrates the formation of cold gas clouds via thermal instability in high-redshift dwarf galaxy mergers and analyzes how metallicity and H2 abundance affect cloud mass distribution.

Findings

Cold gas clouds form in post-shock regions via thermal instability.

Massive clouds (>10^3 M_sun) can collapse to form star clusters.

Low metallicity or high H2 abundance inhibits formation of massive cold clouds.

Abstract

Star formation in high-redshift dwarf galaxies is a key to understand early galaxy evolution in the early Universe. Using the three-dimensional hydrodynamics code GIZMO, we study the formation mechanism of cold, high-density gas clouds in interacting dwarf galaxies with halo masses of $\sim 3 \times 10^{7}~M_{\odot}$, which are likely to be the formation sites of early star clusters. Our simulations can resolve both the structure of interstellar medium on small scales of $\lesssim 0.1$ pc and the galactic disk simultaneously. We find that the cold gas clouds form in the post-shock region via thermal instability due to metal-line cooling, when the cooling time is shorter than the galactic dynamical time. The mass function of cold clouds shows almost a power-law initially with an upper limit of thermally unstable scale. We find that some clouds merge into more massive ones with $\gtrsim 10^{4}~M_{\odot}$ within $\sim 2~{\rm Myr}$. Only the massive cold clouds with $\gtrsim 10^{3}~M_{\odot}$ can keep collapsing due to gravitational instability, resulting in the formation of star clusters. In addition, we investigate the dependence of cloud mass function on metallicity and ${\rm H_{2}}$ abundance, and show that the cases with low metallicities ($\lesssim 10^{-2}~Z_{\odot}$) or high ${\rm H_{2}}$ abundance ($\gtrsim 10^{-3}$) cannot form massive cold clouds with $\gtrsim 10^{3}~M_{\odot}$.