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

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 $\sim 2\mathrm{Myr}$. 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 ${\mathrm{H}}_2$ abundance, and show that the cases with low metallicities ($\lesssim 10^{-2}{Z}_{\odot }$) or high ${\mathrm{H}}_2$ abundance ($\gtrsim 10^{-3}$) cannot form massive cold clouds with $\gtrsim 10^3{M}_{\odot }$.

arXiv comment11 pages, 11 figures

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