Spectro-polarimetry confirms central powering of a Ly$\alpha$ nebula at z=3.09

TL;DR

Spectro-polarimetry of the Ly$ ext{a}$ nebula LAB1 at z=3.09 reveals polarization patterns consistent with a weak outflowing shell, confirming the central galaxy as the powering source and providing insights into its kinematic and structural properties.

Contribution

This study provides the first spatially resolved spectro-polarimetric analysis of LAB1, revealing spectral polarization dependence and challenging symmetric outflow models.

Findings

Detected 9-13% polarization at 15 kpc from the center.

Polarization vectors are tangential to the central source.

Spectral dependence of polarization supports a shell model.

Abstract

We present a follow-up study to the imaging polarimetry performed by Hayes et. al. 2011 on LAB1 in the SSA22 protocluster region. Arguably the most well-known Lyman-$\alpha$ "blob", this radio-quiet emission-line nebula likely hosts a galaxy which is either undergoing significant star formation or hosts an AGN, or both. We obtain deep, spatially resolved spectro-polarimetry of the Ly$\alpha$ emission and detect integrated linear polarization of $9$-$13\%\pm2$-$3\%$ at a distance of approximately 15 kpc north and south of the peak of the Lyman-$\alpha$ surface brightness with polarization vectors lying tangential to the galactic central source. In these same regions, we also detect a wavelength dependence in the polarization which is low at the center of the Ly$\alpha$ line profile and rises substantially in the wings of the profile. These polarization signatures are easily explained by a weak out-flowing shell model. The spectral dependence of the polarization presented here provide a framework for future observations and interpretations of the southern portion of LAB1 in that any model for this system must be able to reproduce this particular spectral dependence. However, questions still remain for the northern-most spur of LAB1. In this region we detect total linear polarization of between $3$ and $20\%$ at the $5\%$ significance level. Simulations predict that polarization should increase with radius for a symmetric geometry. That the northern spur does not suggests either that this region is not symmetric (which is likely) and exhibits variations in columns density, or that it is kinematically distinct from the rest of LAB1 and powered by another mechanism altogether.