A Faraday Rotation Study of the Stellar Bubble and H ii Region Associated with the W4 Complex

Abstract

We utilized the Very Large Array to make multifrequency polarization measurements of 20 radio sources viewed through the IC 1805 HII region and "Superbubble". The measurements at frequencies between 4.33 and 7.76 GHz yield Faraday rotation measures (RMs) along 27 lines of sight to these sources. The RMs are used to probe the plasma structure of the IC 1805 HII region and to test the degree to which the Galactic magnetic field is heavily modified (amplified) by the dynamics of the HII region. We find that IC 1805 constitutes a "Faraday rotation anomaly", or a region of increased RM relative to the general Galactic background value. The |RM| due to the nebula is commonly 600 -800 rad m −2 . However, the observed RMs are not as large as predicted by simplified analytic models that include substantial amplification of the Galactic magnetic field within the shell. The magnitudes of the observed RMs are consistent with shells in which the Galactic field is unmodified, or increased by a modest factor, such as due to magnetic flux conservation. We also find that with one exception, the sign of the RM is that expected for the polarity of the Galactic field in this direction. Finally, our results show intriguing indications that some of the largest values of |RM| occur for lines of sight that pass outside the fully ionized shell of the IC 1805 HII region but pass through the Photodissociation Region associated with IC 1805.

Strong Excess Faraday Rotation on the Inside of the Sagittarius Spiral Arm

“…• 5 with a contrast ∼ 1 K in the THOR continuum image, but no excess that traces the higher RM s. On the other hand, free-free emission from super bubble walls is clearly detected around W4 (Costa & Spangler 2018) and W47 . The high RM sources of Costa & Spangler (2018), including O10, are enclosed by the contour T B = 5.7 K, while the Galactic background is in the range 4.9 K to 5.1 K. Also, Gray et al (1999) found that depolarization of diffuse emission is closely correlated with total intensity. A stronger mean magnetic field B 15 µG (B 45 µG for RM ≈ 3 × 10 3 rad m −2 ) would reduce the implied emission measure beyond detection in the radio continuum image.…”

Revisiting Rotation Measures from the Canadian Galactic Plane Survey: the Magnetic Field in the Disk of the Outer Galaxy

“…A few sources show conspicuously large deviations between new and old RM measurements. Both of the sources that were also present in the Tabara & Inoue (1980) catalog have very different RMs (>100 rad m −2 difference), as do two of the sources from Costa & Spangler (2018). However, all 4 of these sources show signs of Faraday complexity in the older measurements: significant changes in polarized fraction at different frequencies for the Tabara & Inoue (1980) RMs, and large linear fit residuals for the Costa & Spangler (2018) RMs.…”

Magnetism Science with the Square Kilometre Array

“…Interstellar objects such as supernova remnants, planetary nebulae, H IIregions, globular clusters, and gigantic radio loops are all shaped by the influence of the Galactic magnetic field (e.g., [127][128][129][130]); see also Section 2.3.2. The magnetic content of high velocity clouds (HVCs) such as the Smith Cloud [131] is important for their longevity.…”

“…The SKA will test this through sensitivity to polarized emission from large grains aligned with the magnetic field, which will also probe grain growth in these disks [168,169]. Magnetic fields in the ISM surrounding star-forming regions both influence the evolution of and may also be impacted by H II regions [129,170]. The SKA will be able to probe the detailed physical conditions in and around H II regions [171,172].…”