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Magnetohydrodynamics simulation reveals molecular cloud formation and its metallicity dependence

Top left: density and magnetic field structure in our simulation in the 1 Zsun environment. Bottom left: second-order velocity structure function of the CNM volume alone. Top right: the schematic image of molecular cloud turbulent structure. The shock-heated WNM stirs molecular clouds. Bottom right: expected metallicity-dependence of molecular cloud mass formed in supersonic shock compression.

Masato Kobayashi

The formation of molecular clouds out of atomic hydrogen gas is the first step toward star formation because molecular clouds are the formation site of stars while galaxies host plenty of atomic hydrogen gas. The Universe gradually enrich metals in gas (so-called gas-phase “metallicity”), which controls the thermal balance of the interstellar medium. Therefore, the metallicity dependence of molecular cloud formation plays a key role to determine star formation through the cosmic history. In particular, the warm phase of atomic hydrogen gas (Warm Neutral Medium (WNM)) dominates the mass and volume of the interstellar medium over its cold phase (Cold Neutral Medium (CNM)), the latter of which is the progenitor of molecular clouds. The big question is the conversion mechanism from the WNM to the CNM and if it is the same throughout the Universe even when the gas-phase metallicity changes.

In galaxies, supersonic shocks are ubiquitous due to supernova remnant expansion, galactic spirals, galaxy mergers etc. Previous studies suggest that these shock events compress the WNM and the post-shock compressed WNM experiences run-away condensation due to cooling (so-called “thermal instability"), which evolves into the CNM and molecular clouds. The cooling is driven by the metal line coolings (mainly CII and OI in the gas). Low-metallicity environments have less metal abundance and therefore the thermal instability growth is expected to slow. Previous analytic works suggest that the thermal instability grows slowly in low-metallicity environments, but this is never tested in realistic magnetohydrodynamics simulations with supersonic shocks.

We perform magnetohydrodynamics simulations of 20 km s-1 converging flows of the WNM with 1 μG mean magnetic field in the metallicity range from the Solar (1.0 Zsun) to 0.2 Zsun environment. Our simulations show that the CNM formation indeed occurs after supersonic shocks compress the WNM. The spatial correlation of the CNM velocity structure (velocity structure function, or equivalently velocity power spectrum) shows the Larson’s law, which is observationally found commonly in most of molecular clouds. This result suggests that the turbulent energy in molecular clouds is originated in supersonic shock compressions and therefore shock compression events determine not only molecular cloud formation sites but also its star formation efficiency regulated by the turbulent strength.
We also find that the interface of shock and molecular clouds induces turbulence with less compressive motions to reduce the formation of high-density star formation regions in molecular clouds. These suggest that, even in significant compressive events such as galaxy mergers, shock compression accelerates molecular cloud formation but cannot enhance star formation efficiency in individual molecular clouds. To explain active star formation especially in merging galaxies, we suggest that supersonic shock compression must last long enough so that the molecular clouds themselves become massive and collapse to form star clusters.
We also derive expected mass of molecular clouds formed out of supersonic shock compression, which explains the latest observations toward external galaxies.

These contents are published in Kobayashi et al., 2023, which will appear in the Astrophysical Journal.
arXiv number: 2307.01278