The effects of metallicity and cooling physics on fragmentation: implications on direct-collapse black hole formation

Abstract

A promising supermassive black hole seed formation channel is that of direct collapse from primordial gas clouds. We perform a suite of 3D hydrodynamics simulations of an isolated turbulent gas cloud to investigate conditions conducive to forming massive black hole seeds via direct collapse, probing the impact of cloud metallicity, gas temperature floor and cooling physics on cloud fragmentation. We find there is no threshold in metallicity which produces a sharp drop in fragmentation. When molecular cooling is not present, metallicity has little effect on fragmentation. When molecular cooling is present, fragmentation is suppressed by at most ∼25%, with the greatest suppression seen at metallicities below 2% solar. A gas temperature floor ∼10^4 K produces the largest drop in fragmentation of any parameter choice, reducing fragmentation by ∼60%. At metallicities below 2% solar or at temperatures ∼10^3 K we see a reduction in fragmentation ∼20−25%. For a cloud of metallicity 2% solar above and a temperature below 10^3 K, the detailed choices of temperature floor, metallicity, and cooling physics have little impact on fragmentation.

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