Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages

Abstract

The formation of molecular clouds out of HI gas is the first step toward star formation. Its metallicity dependence plays a key role to determine star formation through the cosmic history. Previous theoretical studies with detailed chemical networks calculate thermal equilibrium states and/or thermal evolution under one-zone collapsing background. The molecular cloud formation in reality, however, involves supersonic flows, and thus resolving the cloud internal turbulence/density structure in three dimension is still essential. We here perform magnetohydrodynamics simulations of 20 km s^-1 converging flows of Warm Neutral Medium (WNM) with 1 micro Gauss mean magnetic field in the metallicity range from the Solar (1.0 Zsun) to 0.2 Zsun environment. The Cold Neutral Medium (CNM) clumps form faster with higher metallicity due to more efficient cooling. Meanwhile, their mass functions commonly follow dn/dm proportional to m^-1.7 at three cooling times regardless of the metallicity. Their total turbulence power also commonly shows the Kolmogorov spectrum with its 80 percent in the solenoidal mode, while the CNM volume alone indicates the transition towards the Larson's law. These similarities measured at the same time in the unit of the cooling time suggest that the molecular cloud formation directly from the WNM alone requires a longer physical time in a lower metallicity environment in the 1.0--0.2 Zsun range. To explain the rapid formation of molecular clouds and subsequent massive star formation possibly within less than 10 Myr as observed in the Large/Small Magellanic Clouds (LMC/SMC), the HI gas already contains CNM volume instead of pure WNM.

23 pages, 11 figures. Accepted for publication in ApJ

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