MAPPING THE POLARIZATION OF THE RADIO-LOUD Lyα NEBULA B3 J2330+3927*

Abstract

Lyα nebulae, or "Lyα blobs", are extended (up to ∼100 kpc), bright (L Lyα 10 43 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 6.5m MMT telescope, we map the total (Lyα + continuum) polarization in a grid of circular apertures of radius 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.

Unlocking the Full Potential of Extragalactic Lyα through Its Polarization Properties

“…For example, in the bipolar outflow, they emerge from the cones, and the polarization direction is consequently perpendicular to the outflow direction (see § 3.3). This is similar to the observations of the Egg nebula by You et al (2017). When the local radiation fields were isotropized by a high number of core scatterings, we found, as Lee (1994) and Dijkstra & Loeb (2008) that this reduces the emergent polarization.…”

“…This would result in a global (but possibly misleading) polarization signal. Of course, these issues are less important in imaging polarimetry when one can obtain Stokes parameters in a per-pixel basis (as in , Prescott et al 2011and You et al 2017).…”

What Makes Lyα Nebulae Glow? Mapping the Polarization of LABd05

“…Rings of highly polarized Lyα emission (up to 20%) are measured at 4″-8″ (∼45 kpc) from the center of SSA22-LAB1 (Hayes et al 2011), a Lyα blob at z=3.09, suggesting that the Lyα photons are produced centrally and then scattered at large radii. From the previous study of You et al (2017), we detected comparably strong (up to 17%) polarization out to ∼25 kpc from the center in the Lyα nebula surrounding the radio galaxy B3 J2330+3927 at z=3.087. Unlike in SSA22-LAB1, however, the significant polarization is observed only along the blob's major axis, along the radio jet.…”

“…Figure 1 shows the Hubble Space Telescope VJH composite image of LABd05 illustrating its complex structure (Prescott et al 2011), including contours of Lyα surface brightness (blue) and dust (λ obs =1.9 mm) continuum (red) (Yang et al 2014a). Unlike the two previous Lyα blobs with polarization maps, SSA22-LAB1 (Hayes et al 2011) and B3 J2330+3927 (You et al 2017), LABd05 is associated with a bright mid-infrared galaxy at R.A.=14 h 34 m 10 981 and decl.=33°17′32 48 that hosts an obscured radio-quiet AGN (Dey et al 2005;Prescott et al 2011;Yang et al 2014a) and is spatially offset (by ∼2 0, 16 kpc) from the peak of Lyα emission.…”

“…However, that constraint, P=2.6±2.8%, was within a single, large aperture of 8 2 diameter (65.6 kpc) and made with a small telescope (the Bok-2.3 m) under poor seeing. Here we map LABd05 with the MMT-6.5 m telescope 5 and the same, powerful CCD Imaging/ Spectropolarimeter (SPOL) imaging spectrometer used in our previous paper (You et al 2017).…”