MAGNETIC FIELDS IN LARGE-DIAMETER H II REGIONS REVEALED BY THE FARADAY ROTATION OF COMPACT EXTRAGALACTIC RADIO SOURCES

We present a study of the line-of-sight magnetic fields in five large-diameter Galactic H II regions. Using the Faraday rotation of background polarized radio sources, as well as dust-corrected Hα surface brightness as a probe of electron density, we estimated the strength and orientation of the magnetic field along 93 individual sight lines through the H II regions. Each of the H II regions displayed a coherent magnetic field. The magnetic field strength (line-of-sight component) in the regions ranges from 2 to 6 μG, which is similar to the typical magnetic field strength in the diffuse interstellar medium. We investigated the relationship between magnetic field strength and electron density in the five H II regions. The slope of magnetic field versus density in the low-density regime (0.8 cm-3 < ne <30 cm-3) is very slightly above zero. We also calculated the ratio of thermal to magnetic pressure, βth, for each data point, which fell in the range 1.01 < βth < 25. Finally, we studied the orientation of the magnetic field in the solar neighborhood (d < 1.1 kpc) using our data from five H II regions along with existing measurements of the line-of-sight magnetic field strength from polarized pulsars whose distances have been determined from their annual parallax. We identify a net direction for the magnetic field in the solar neighborhood, but find no evidence for a preferred vertical direction of the magnetic field above or below the Galactic plane.