Absorption of the Linear Polarization of the Galactic Background Radiation by the $λ$21-cm Line of HI

Absorption lines at $\lambda$ 21-cm are detected in the Stokes Q and U components of the Galactic synchrotron background. The lower limit distance implied for the emission region is 2 kpc in the direction (l,b) = (329.5$^{\circ}$,+1.15$^{\circ}$). The Australia Telescope Compact Array has the capability of mapping this absorption over large areas of the Galactic plane. Observations like these have the potential to reveal the three dimensional structure of the Galactic synchrotron emission throughout the Milky Way disk.