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

TL;DR

This study uses galaxy-scale simulations to generate synthetic Zeeman measurements, revealing that the perceived transition from sub-critical to super-critical magnetic states in clouds is an observational illusion caused by biases.

Contribution

It demonstrates that the apparent magnetic criticality transition is a bias-induced illusion, challenging previous interpretations based on Zeeman effect observations.

Findings

Zeeman measurements suggest a transition from sub-critical to super-critical states.

Simulated observations show most regions are actually super-critical, even in HI clouds.

Biases in chemical and excitation effects create the illusion of a criticality transition.

Abstract

Mass-to-flux ratios measured via the Zeeman effect suggest the existence of a transition from a magnetically sub-critical state in HI clouds to a super-critical 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 magnetised molecular clouds, zooming in from an entire galaxy to sub-pc scales, which we post-process to produce synthetic HI 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 super-critical, even in the HI regime. We conclude that HI clouds in the process of assembling to form molecular clouds are already super-critical even before H_2 forms, and that the apparent transition from sub- to super-criticality between HI and H_2 is primarily an illusion created by chemical and excitation biases affecting the Zeeman measurements.