2018ApJ...865...65C

We utilized the Very Large Array to make multifrequency polarization measurements of 20 radio sources viewed through the IC 1805 H II 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 H II region and to test the degree to which the Galactic magnetic field is heavily modified (amplified) by the dynamics of the H II 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 H II region but pass through the Photodissociation Region associated with IC 1805.