Molecular clouds host star formation and thus their lifecycle is essential to understand galactic star formation. However, molecular cloud formation, especially its metallicity dependence, is still poorly understood. Recent JWST and ALMA observations in the Milky way galaxy, the Large Magellanic Cloud, and the Small Magellanic Cloud suggest that bubble structures trigger molecular cloud formation, where supernovae are promising sources that create such bubble structures.
We perform a series of magnetohydrodynamics simulations of supersonic converging flows of atomic hydrogen gas to investigate molecular cloud formation in various metallicity environments. The flow consists of the thermally stable Warm Neutral Medium (WNM) with 1 uG mean magnetic field. The shock destabilizes WNM into the thermally unstable medium. Cold Neutral Medium (CNM) forms subsequently due to thermal instability. We investigate three cases of the metallicity: 1.0, 0.5, 0.2 Zsun, where Zsun represents the Solar metallicity.
Our simulations show that the physical properties of the molecular clouds resemble between metallicities if compared at the same time in the unit of the cooling time. For example, the CNM clump mass function has the power-law index of -1.7, which is expected from the growth of thermal instability with seed Kolmogorov density fluctuation spectrum. The solenoidal mode amounts to 80 percent of the turbulence power, which suggests that galactic-scale converging flow is important to accumulate mass but this does not necessarily enhance star formation efficiency by increasing the compressive turbulence mode in molecular clouds.
Our results also suggest that the typical mass of molecular clouds is smaller in lower metallicity environments. Massive low-metallicity molecular clouds with subsequent active star formation occur only in denser environments and/or in fast flow, such as high-redshift galaxies or galaxies mergers. These findings are consistent with recent radio observations towards clouds in the outer Galaxy and extragalactic sources.