MAPPING the POLARIZATION of the RADIO-LOUD Lyα NEBULA B3 J2330+3927

Lyα nebulae, or "Lyα blobs," are extended (up to ∼100 kpc), bright (L _Lyα10⁴³ erg s^-1) clouds of Lyα emitting gas that tend to lie in overdense regions at z ∼ 2-5. The origin of the Lyα emission remains unknown, but recent theoretical work suggests that measuring the polarization might discriminate among powering mechanisms. Here we present the first narrowband imaging polarimetry of a radio-loud Lyα nebula, B3 J2330+3927, at z = 3.09, with an embedded active galactic nucleus (AGN). The AGN lies near the blob's Lyα emission peak, and its radio lobes align roughly with the blob's major axis. With the SPOL polarimeter on the 6.5 m MMT telescope, we map the total (Lyα + continuum) polarization in a grid of circular apertures of a radius of 0.″6 (4.4 kpc), detecting a significant (>2σ) polarization fraction P _% in nine apertures and achieving strong upper limits (as low as 2%) elsewhere. P _% increases from <2% at ∼5 kpc from the blob center to 17% at ∼15-25 kpc. The detections are distributed asymmetrically, roughly along the nebula's major axis. The polarization angles θ are mostly perpendicular to this axis. Comparing the Lyα flux to that of the continuum and conservatively assuming that the continuum is highly polarized (20%-100%) and aligned with the total polarization, we place lower limits on the polarization of the Lyα emission P _%,Lyα ranging from no significant polarization at ∼5 kpc from the blob center to 3%-17% at 10-25 kpc. Like the total polarization, the Lyα polarization detections occur more often along the blob's major axis.