Radio Polarization from the Galactic Plane in Cygnus

We present 1420 MHz (λ = 21 cm) observations of polarized emission from an area of 117 deg2 in the Galactic plane in Cygnus, covering 82° < l < 95°, -3.°5 < b < +5.°5, a complex region where the line of sight is directed nearly along the Local spiral arm. The angular resolution is ~1', and structures as large as 45' are fully represented in the images. Polarization features bear little resemblance to features detected in total power: while the polarized signal arises in diffuse Galactic synchrotron emission regions, the appearance of the polarized sky is dominated by Faraday rotation occurring in small-scale structure in the intervening warm ionized medium. There is no concentration of polarization structure toward the Galactic plane, indicating that both the emission and Faraday rotation occur nearby. We develop a conceptual framework for interpretation of the observations. We can detect only that polarized emission which has its origin closer than the "polarization horizon," at a distance dph; more distant polarized emission is undetectable because of depth depolarization (differential Faraday rotation) and/or beam depolarization (due to internal and external Faraday dispersion). The value of dph depends on the instrument used (frequency and beamwidth) as well as the direction being studied. In our data we find that dph ≈ 2 kpc, consistent with the polarization features originating in the Local arm. The filling factor of the warm ionized medium is constrained by our data to be considerably less than unity: polarized signals that pass through multiple regions of Faraday rotation experience severe depolarization, but polarized fractions up to ~10% are seen, implying that there are lines of sight that intersect only one Faraday rotation region within the polarization horizon. The rotation measure (RM) of the extended polarized emission has a distribution that peaks at -30 rad m-2 and has a width to half-maximum of 300 rad m-2. The peak and half-width of the distribution of RMs of extragalactic sources in the region are -125 and 600 rad m-2, respectively. This suggests that RM increases monotonically with length of propagation path through the interstellar medium in this direction. Among localized polarization features that we discuss, G83.2+1.8 and G91.8-2.5 stand out for their circular or quasi-circular form and extent of more than 1°; both are probably related to the impact of stellar activity on the surrounding medium, although the stars responsible cannot be identified. Another polarization feature, G91.5+4.5, extends 2.°5 × 1° and coincides with a molecular cloud; it plausibly arises in an ionized skin on the outside of the cloud. Polarized emission seen across the face of the large, dense H II region, W80, must be generated in the 500 pc between the Sun and W80, since W80 must depolarize all extended nonthermal emission generated behind it. Of the supernova remnants G84.2-0.8, G89.0+4.7 (HB 21), G93.7-0.2 (CTB 104A), and G94.0+1.0 (3C 434.1), only HB 21 and CTB 104A show polarized emission; the other two lie beyond the polarization horizon and their emission suffers beam depolarization. Emission from the surrounding medium is depolarized on passage through HB 21.