H Ii Region Driven Galactic Bubbles and Their Relationship to the Galactic Magnetic Field

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 H ii 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 H ii regions, based on hydrogen radio recombination linewidths, the subthermal H ii regions showed a marginal deviation from random orientations, but the suprathermal H ii 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 H ii regions were found to preferentially align with the disk magnetic field. If suprathermal linewidths are associated with younger H ii regions, then the evolution of young H ii regions is significantly affected by the Galactic magnetic field. As H ii 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 H ii regions.

The Far-infrared Polarization Spectrum of ρ Ophiuchi A from HAWC+/SOFIA Observations

“…In regions where the magnetic field energy density is subdominant to the kinetic energy of the matter, gas dynamics will influence magnetic field morphology. This effect has been observed for Hi shells (Heiles 1998;Fosalba et al 2002;Santos et al 2011;Frisch & Dwarkadas 2017;Soler et al 2018) as well as at the edges of Hii regions (Pavel & Clemens 2012;Santos et al 2012Santos et al , 2014Planck Collaboration Int. XXXIV 2016).…”

Understanding the Links among the Magnetic Fields, Filament, Bipolar Bubble, and Star Formation in RCW 57A Using NIR Polarimetry

“…they expand anisotropically in the filament, they undergo an additional anisotropic pressure in the presence of strong B-fields (Bisnovatyi-Kogan & Silich 1995). As a result, I-fronts experience accelerated flows along the B-fields (i.e., low pressure) and hindered flows (i.e., high pressure) in the direction perpendicular to the B-fields (Tomisaka 1992;Gaensler 1998;Pavel & Clemens 2012;van Marle et al 2015). Therefore, I-fronts expand a greater extent along the hourglass shaped B-fields, and eventually form the bipolar bubble as shown in schema D of Figure 13.…”

“…A few studies (e.g., toward the IRAS Vela Shell and Wisniewski et al 2007 toward the supershell NGC 2100 in the LMC) suggest the importance of B-fields in the dynamical evolution of the expanding shells driven by O/B-type stars. The interplay between the expansion of an ionized nebula and the influence of B-fields has been the subject of several studies (e.g., Kwon et al 2010Kwon et al , 2011Kwon et al , 2015Pavel & Clemens 2012;Santos et al 2012Santos et al , 2014Planck Collaboration et al 2016c). These studies hint that B-fields permeating through filamentary molecular clouds would also influence the expanding I-fronts or outflowing gas, which are driven from a H II region formed in the filament.…”

Akari Observations of Massive Star-Forming Regions Indicative of Large-Scale Cloud-Cloud Collisions

“…Our sample includes 111 Spitzer bubbles, for which information on the distance is available in several literatures (e.g., Beaumont & Williams, 2009;Churchwell et al, 2006;Deharveng et al, 2010;Pavel & Clemens, 2012;Watson et al, 2010), and the sample consists of 69 closed bubbles and 42 broken bubbles. We also checked that these bubbles did not suffer apparent contamination from nearby objects.…”