A Faraday Rotation Study of the Stellar Bubble and HII 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", as well as in the immediate vicinity. The measurements at frequencies between 4.33 and 7.76 GHz yield Faraday rotation measures along 27 lines of sight to these sources (some sources have more than one component). The Faraday rotation measures (RM) 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 similar to the Rosette Nebula (Savage et al. 2013, Costa et al. 2016) and the Cygnus OB1 association (Whiting et al. 2009), IC 1805 constitutes a "Faraday rotation anomaly", or a region of increased RM relative to the general Galactic background value. Although the RM observed on lines of sight through the region vary substantially, the |RM| due to the nebula is commonly 600 -- 800 rad m^-2. In spite of this, the observed RMs are not as large as simple, analytic models of magnetic field amplification in HII regions might indicate. This suggests that the Galactic field is not increased by a substantial factor within the ionized gas in an HII region. We also find that with one exception, the sign of the RM for all sources is that expected for the polarity of the Galactic field in this direction. The same behavior was found for the Rosette Nebula, and qualitatively indicates that turbulent fluctuations in the Galactic field on spatial scales of $\sim 10$ pc are smaller than the mean Galactic field. 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 (PDR) associated with IC 1805.

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