Magnetic fields in star-forming environments: how does field strength affect gas on spiral arm and cloud scales?

Abstract

ABSTRACT We investigate star formation from subpc to kpc scales with magnetohydrodynamic models of a cloud structure and a section of galactic spiral arm. We aim to understand how magnetic fields affect star formation and cloud formation, and how feedback couples with magnetic fields on scales of clouds and clumps. We find that magnetic fields overall suppress star formation by ${\sim}$10 per cent with a weak field (5 $\mu$G) and ${\sim} 50$ per cent with a stronger field (50 $\mu$G). Cluster masses are reduced by about 40 per cent with a strong field but show little change with a weak field. We find that clouds tend to be aligned parallel to the field with a weak field and become perpendicularly aligned with a stronger field, whereas on clump scales the alignment is more random. The magnetic fields and densities of clouds and clumps in our models agree with the Zeeman measurements of the Crutcher relation $B\!-\!\rho$ in the weaker field models, while the strongest field models show a relation that is too flat compared to the observations. In all our models, we find that both subcritical and supercritical clouds and clumps are present. We also find that if using a line-of-sight (1D) measure of the magnetic field to determine the critical parameter, the magnetic field, and thereby also criticality, can vary by a factor of 3–4 depending on whether the direction the field is measured along corresponds to the direction of the ordered component of the magnetic field.