Inadequate turbulent support in low-metallicity molecular clouds

Abstract

The dynamic properties of molecular clouds are set by the interplay of their self-gravity, turbulence, external pressure and magnetic fields. Extended surveys of Galactic molecular clouds typically find that their kinetic energy (Ek) counterbalances their self-gravitational energy (Eg), setting their virial parameter αvir = 2Ek/∣Eg∣ ≈ 1. However, past studies either have been biased by the use of optically thick lines or have been limited within the solar neighbourhood and the inner Galaxy (Galactocentric radius Rgc < Rgc,⊙ ≈ 8 kpc). Here we present sensitive mapping observations of optically thin 13CO lines towards molecular clouds in the low-metallicity Galactic outer disk (Rgc ~ 9–24 kpc). By combining archival data from the inner Galaxy and four nearby metal-poor dwarf galaxies, we reveal a systematic trend of αvir, which declines from supervirial dynamic states in metal-rich clouds to extremely subvirial dynamic states in metal-poor clouds. In these metal-poor environments, turbulence alone is insufficient to counterbalance the self-gravity of a cloud. A cloud-volumetric magnetic field may replace turbulence as the dominant cloud-supporting mechanism in low-metallicity conditions, for example, the outermost galactic disks, dwarf galaxies and galaxies in the early Universe, which would then inevitably impact the initial conditions for star formation in such environments. Low-metallicity molecular clouds in the Milky Way and nearby galaxies exhibit a strong deficiency in turbulent support against their self-gravity, suggesting that the magnetic field may play a dominant role in supporting clouds under such conditions.