2025NatAs...9..406L

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 ( E _k ) counterbalances their self-gravitational energy ( E _g ), setting their virial parameter α _vir = 2 E _k / ∣ E _g ∣ ≈ 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 R _gc < R _gc, ⊙ _ ≈ 8 kpc). Here we present sensitive mapping observations of optically thin ¹³CO lines towards molecular clouds in the low-metallicity Galactic outer disk ( R _gc ∼ 9-24 kpc). By combining archival data from the inner Galaxy and four nearby metal-poor dwarf galaxies, we reveal a systematic trend of α _vir , 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.