Lyman-alpha blobs: polarization arising from cold accretion

TL;DR

This paper demonstrates through simulations that Lyman-alpha blobs' polarization can originate from cooling radiation emitted by gas accreting onto halos, challenging the idea that polarization only indicates central photon production.

Contribution

The study shows that extragalactic gas contributes significantly to Lyman-alpha emission and can produce observed polarization levels, reconciling cooling radiation with polarimetric data.

Findings

Simulations match observed polarization profiles of Lyman-alpha blobs.

Extragalactic gas contributes to Lyman-alpha emission without suppressing polarization.

Pure galactic emission results in polarization profiles inconsistent with observations.

Abstract

Lyman-$\alpha$ nebulae are usually found in massive environments at high redshift ($z > 2$). The origin of their Lyman-$\alpha$ (Lya) emission remains debated. Recent polarimetric observations showed that at least some Lya sources are polarized. This is often interpreted as a proof that the photons are centrally produced, and opposed to the scenario in which the Lya emission is the cooling radiation emitted by gas heated during the accretion onto the halo. We suggest that this scenario is not incompatible with the polarimetric observations. In order to test this idea, we post-process a radiative hydrodynamics simulation of a blob with the MCLya Monte Carlo transfer code. We compute radial profiles for the surface brightness and the degree of polarization and compare them to existing observations. We find that both are consistent with a significant contribution of the extragalactic gas to the Lya emission. Most of the photons are centrally emitted and scattered inside the filament afterwards, producing the observed high level of polarization. We argue that the contribution of the extragalactic gas to the Lya emission does not prevent polarization to arise. On the contrary, we find that pure galactic emission causes the polarization profile to be too steep to be consistent with observations.