The sub-critical illusion: synthetic Zeeman effect observations from galactic zoom-in simulations

Mass-to-flux ratios measured via the Zeeman effect suggest the existence of a transition from a magnetically sub-critical state in H i clouds to a supercritical state in molecular clouds. However, due to projection, chemical, and excitation effects, Zeeman measurements are subject to a number of biases, and may not reflect the true relations between gravitational and magnetic energies. In this paper, we carry out simulations of the formation of magnetized molecular clouds, zooming in from an entire galaxy to sub-pc scales, which we post-process to produce synthetic H i and OH Zeeman measurements. The mass-to-flux ratios we recover from the simulated observations show a transition in magnetic criticality that closely matches observations, but we find that the gravitational-magnetic energy ratios on corresponding scales are mostly supercritical, even in the H i regime. We conclude that H i clouds in the process of assembling to form molecular clouds are already supercritical even before H2 forms, and that the apparent transition from sub- to supercriticality between H i and H2 is primarily an illusion created by chemical and excitation biases affecting the Zeeman measurements.