Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation

TL;DR

This study detects polarized Lyman-alpha radiation in a high-redshift nebula, revealing that its luminosity is powered by photons from embedded galaxies that are scattered by surrounding neutral hydrogen, rather than local gas emission.

Contribution

First detection of polarized Ly-alpha emission in a high-redshift nebula, providing new insights into the powering mechanisms of Lyman-alpha blobs.

Findings

Polarized Ly-alpha forms an almost complete ring at 45 kpc radius.

Polarization detection rules out in situ gas emission as the primary source.

Luminosity is powered by photons from embedded galaxies, scattered by neutral hydrogen.

Abstract

High-redshift Lyman-alpha blobs are extended, luminous, but rare structures that appear to be associated with the highest peaks in the matter density of the Universe. Their energy output and morphology are similar to powerful radio galaxies, but the source of the luminosity is unclear. Some blobs are associated with ultraviolet or infrared bright galaxies, suggesting an extreme starburst event or accretion onto a central black hole. Another possibility is gas that is shock excited by supernovae. However some blobs are not associated with galaxies, and may instead be heated by gas falling into a dark matter halo. The polarization of the Ly-alpha emission can in principle distinguish between these options, but a previous attempt to detect this signature returned a null detection. Here we report on the detection of polarized Ly-alpha from the blob LAB1. Although the central region shows no measurable polarization, the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc, forming an almost complete polarized ring. The detection of polarized radiation is inconsistent with the in situ production of Ly-alpha photons, and we conclude that they must have been produced in the galaxies hosted within the nebula, and re-scattered by neutral hydrogen.