The Polarization of Scattered Lyman Alpha Radiation Around High-Redshift Galaxies

TL;DR

This paper demonstrates that Lyman Alpha radiation around high-redshift galaxies can be highly polarized due to scattering, offering a new method to study the distribution and movement of neutral hydrogen in the early universe.

Contribution

It shows that polarization levels of Lyman Alpha radiation can reveal the kinematics and environment of high-redshift galaxies, providing a novel observational diagnostic.

Findings

Polarization can reach up to 40% in certain environments.

Spectral polarimetry distinguishes between inflowing and outflowing gas.

Lyman Alpha polarimetry is feasible with current instruments.

Abstract

The high-redshift Universe contains luminous Lyman Alpha (hereafter Lya) emitting sources such as galaxies and quasars. The emitted Lya radiation is often scattered by surrounding neutral hydrogen atoms. We show that the scattered Lya radiation obtains a high level of polarization for a wide range of likely environments of high-redshift galaxies. For example, the back-scattered Lya flux observed from galaxies surrounded by a superwind-driven outflow may reach a fractional polarization as high as ~40%. 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 (<7%), which depends on the flux of the ionizing background. Spectral polarimetry can differentiate between Lya 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 Lya 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 Lya emitting galaxies outside the currently available redshift windows.