Using Polarization to Uncover the Ly<i>α</i> Emission Mechanism in Ly<i>α</i> Nebulae

Abstract “Ly α nebulae” are immense (∼100 kpc) glowing gas clouds located in the distant universe, and the origin of their extended Ly α emission remains a mystery. Mapping their polarization offers critical insights, but imaging polarimetry to date has revealed polarization patterns that are difficult to interpret due to differences in the types and spatial configurations of their embedded sources. In this study, we more than double the sample size of polarization observations by targeting four additional Ly α nebulae: 4C41.17, SSA22-Sb3-LAB1, JACKPOT, and MAMMOTH-1, all of which host active galactic nuclei (AGN), from radio-loud to radio-quiet to type-I to type-II. Significant polarization is detected in three of these four nebulae, with polarization vectors distributed across the extended Ly α -emitting gas. In the two Ly α nebulae hosting radio-loud AGN, the polarization vectors exhibit a preferential alignment perpendicular to the radio jet direction, suggesting a scattering origin related to AGN outflows. In 4C41.17, JACKPOT, and MAMMOTH-1 (3/4 cases), the polarization strength shows a possible trend of increasing with radius. While the spatial patterns are complex and most large-radius measurements are upper limits, our results—together with previous work on three other targets—support the idea that scattering plays a key role in shaping the Ly α emission from these nebulae. Full interpretation requires direct comparison of these observations to radiative transfer models over a wide range of parameter space.