Is the Supersonic Turbulence Observed in HII Regions Sustained by the Presence of Magnetic Fields?

Abstract

We describe a set of Hα emission line profiles from populations of H ii regions in nearby spiral galaxies. These are characterized by a strong Gaussian central peak, and lower intensity higher velocity wing features. From the peak we extract a non-thermal velocity component, due to the internal turbulence of the region. The plot of the widths of these non-thermal components against Ha luminosity shows a lower envelope in line width, which we assign to regions in, or close to virial equilibrium, although the region mass derived on this assumption is higher than the mass obtained by summing all known mass components. We speculate that this discrepancy, as well as the supersonicity of the line widths, can be explained by the presence of turbulent magnetic fields within the H ii regions, and make a very rough estimate of the fields implied, of order a few tens of microgauss.