New observations at high latitudes above the H ii region W4 show that the structure formerly identified as a chimney candidate , an opening to the Galactic halo , is instead a superbubble in the process of fragmenting and possibly evolving into a chimney .
Data at high Galactic latitudes ( b > 5 \arcdeg ) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I ( total power ) and 1420 MHz Stokes Q and U ( linear polarization ) reveal an egg-shaped structure with morphological correlations between our data and the H \alpha data of Dennison , Topasna , & Simonetti .
Polarized intensity images show depolarization extending from W4 up the walls of the superbubble , providing strong evidence that the radio continuum is generated by thermal emission coincident with the H \alpha emission regions .
We conclude that the parts of the H ii region hitherto known as W4 and the newly revealed thermal emission are all ionized by the open cluster OCl 352 .
Assuming a distance of 2.35 kpc , the ovoid structure is 164 pc wide and extends 246 pc above the mid-plane of the Galaxy .
The shell ’ s emission decreases in total-intensity and polarized intensity in various locations , appearing to have a break at its top and another on one side .
Using a geometric analysis of the depolarization in the shell ’ s walls , we estimate that a magnetic field line-of-sight component of 3 to 5 \mu G exists in the shell .
We explore the connection between W4 and the Galactic halo , considering whether sufficient radiation can escape from the fragmenting superbubble to ionize the kpc-scale H \alpha loop discovered by Reynolds , Sterling & Haffner .