HII Region Driven Galactic Bubbles and Their Relationship to the Galactic Magnetic Field

TL;DR

This study investigates the alignment of Galactic bubbles driven by HII regions with the Galactic magnetic field, revealing that younger, suprathermal HII regions tend to align with the magnetic field, unlike older or subthermal ones.

Contribution

It provides new observational evidence linking the orientation of HII region driven bubbles to the Galactic magnetic field, especially for younger regions with suprathermal linewidths.

Findings

Suprathermal HII regions align with the Galactic magnetic field.

Older HII regions show random orientations.

Magnetic-to-ram pressure ratios were estimated for young HII regions.

Abstract

The relative alignments of mid-infrared traced Galactic bubbles are compared to the orientation of the mean Galactic magnetic field in the disk. The orientations of bubbles in the northern Galactic plane were measured and are consistent with random orientations - no preferential alignment with respect to the Galactic disk was found. A subsample of HII region driven Galactic bubbles was identified, and as a single population they show random orientations. When this subsample was further divided into subthermal and suprathermal HII regions, based on hydrogren radio recombination linewidths, the subthermal HII regions showed a marginal deviation from random orientations, but the suprathermal HII regions showed significant alignment with the Galactic plane. The mean orientation of the Galactic disk magnetic field was characterized using new near-infrared starlight polarimetry and the suprathermal HII regions were found to preferentially align with the disk magnetic field. If suprathermal linewidths are associated with younger HII regions, then the evolution of young HII regions is significantly affected by the Galactic magnetic field. As HII regions age, they cease to be strongly linked to the Galactic magnetic field, as surrounding density variations come to dominate their morphological evolution. From the new observations, the ratios of magnetic-to-ram pressures in the expanding ionization fronts were estimated for younger HII regions.