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

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 ∼3 × 107 M⊙, 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 ≲ 0.1 pc and the galactic disc 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 ≳104 M⊙ within ∼ 2 Myr. Only the massive cold clouds with ≳ 103 M⊙ can keep collapsing due to gravitational instability, resulting in the formation of star clusters. We find that the clump formation is more efficient in the prograde–prograde merger than the prograde–retrograde case due to the difference in the degree of shear flow. In addition, we investigate the dependence of cloud mass function on metallicity and H2 abundance, and show that the cases with low metallicities (≲10−2 Z⊙) or high H2 abundance (≳10−3) cannot form massive cold clouds with ≳103 M⊙.