The polarization of scattered Lyα radiation around high-redshift galaxies

The high-redshift Universe contains luminous Ly alpha emitting sources such as galaxies and quasars. The emitted Ly alpha radiation is often scattered by surrounding neutral hydrogen atoms. We show that the scattered Ly alpha radiation obtains a high level of polarization for a wide range of likely environments of high-redshift galaxies. For example, the backscattered Ly alpha flux observed from galaxies surrounded by a superwind-driven outflow may reach a fractional polarization as high as similar to 40 per cent. Equal levels of polarization may be observed from neutral collapsing protogalaxies. Resonant scattering in the diffuse intergalactic medium typically results in a lower polarization amplitude (less than or similar to 7 per cent), which depends on the flux of the ionizing background. Spectral polarimetry can differentiate between Ly alpha scattering off infalling gas and outflowing gas; for an outflow, the polarization should increase towards longer wavelengths while for infall the opposite is true. Our numerical results suggest that Ly alpha polarimetry is feasible with existing instruments, and may provide a new diagnostic of the distribution and kinematics of neutral hydrogen around high-redshift galaxies. Moreover, polarimetry may help suppress infrared lines originating in the Earth's atmosphere, and thus improve the sensitivity of ground-based observations to high-redshift Ly alpha emitting galaxies outside the currently available redshift windows.