2015MNRAS.452.2500L

The most accurate measurements of magnetic fields in star-forming gas are based on the Zeeman observations analysed by Crutcher et al. We show that their finding that the 3D magnetic field scales approximately as density0.65 can also be obtained from analysis of the observed line-of-sight fields. We present two large-scale adaptive-mesh-refinement magnetohydrodynamic simulations of several thousand M_☉ of turbulent, isothermal, self-gravitating gas, one with a strong initial magnetic field (Alfven Mach number M_A,0=1) and one with a weak initial field (M_A,0=10). We construct samples of the 100 most massive clumps in each simulation and show that they exhibit a power-law relation between field strength and density ({bar}n_H) in excellent agreement with the observed one. Our results imply that the average field in molecular clumps in the interstellar medium (ISM) is <B_tot({bar}n_H) >~42 {bar}n_H,  4^0.65  µG. Furthermore, the median value of the ratio of the line-of-sight field to density0.65 in the simulations is within a factor of about (1.3, 1.7) of the observed value for the strong- and weak-field cases, respectively. The median value of the mass-to-flux ratio, normalized to the critical value, is 70 per cent of the line-of-sight value. This is larger than the 50 per cent usually cited for spherical clouds because the actual mass-to-flux ratio depends on the volume-weighted field, whereas the observed one depends on the mass-weighted field. Our results indicate that the typical molecular clump in the ISM is significantly supercritical ( ∼ factor of 3). The results of our strong-field model are in very good quantitative agreement with the observations of Li et al., which show a strong correlation in field orientation between small and large scales. Because there is a negligible correlation in the weak-field model, we conclude that molecular clouds form from strongly magnetized (although magnetically supercritical) gas, in agreement with the conclusion of Li et al.