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