Radio polarimetric imaging of the interstellar medium: Magnetic field and diffuse ionized gas structure near the W3/W4/W5/HB 3 complex

We have used polarimetric imaging to study the magneto-ionic medium of the Galaxy, obtaining 1420 MHz images with an angular resolution of 1' over mor e than 40 deg(2) of sky around the W3/W4/W5/HB3 H II region/SNR complex in the Perseus Arm. Features detected in polarization angle are imposed on the linearly polarized Galactic synchrotron background emission by Faraday rot ation arising in foreground ionized gas having an emission measure as low a s 1 cm(-6) pc. Several new remarkable phenomena have been identified, inclu ding: mottled polarization arising from random fluctuations in a magneto-io nic screen that we identify with a medium in the Perseus Arm, probably in t he vicinity of the H II, regions themselves; depolarization arising from ve ry high rotation measures (several times 10(3) rad m(-2)) and rotation meas ure gradients due to the dense, turbulent environs of the H II regions; hig hly ordered features spanning up to several degrees; and an extended influe nce of the H II regions beyond the boundaries defined by earlier observatio ns. In particular, the effects of an extended, low-density ionized halo aro und the H II region W4 are evident, probably an example of the extended H I I envelopes postulated as the origin of weak recombination-line emission de tected from the Galactic ridge. Our polarization observations can be unders tood if the uniform magnetic field component in this envelope scales with t he square-root of electron density and is 20 mu G at the edge of the depola rized region around W4, although this is probably an overestimate since the random field component will have a significant effect.