Polarization of Rayleigh scattered Lyα in active galactic nuclei

Abstract

The unification scheme of active galactic nuclei (AGNs) invokes an optically thick molecular torus component hiding the broad emission line region. Assuming the presence of a thick neutral component in the molecular torus characterized by a H i column density > 10 22 cm −2 , we propose that far UV radiation around Lyα can be significantly polarized through Rayleigh scattering. Adopting a Monte Carlo technique we compute polarization of Rayleigh scattered radiation near Lyα in a thick neutral region in the shape of a slab and a cylindrical shell. It is found that radiation near Lyα Rayleigh reflected from a very thick slab can be significantly polarized in a fairly large range of wavelength ∆λ ∼ 50Å exhibiting a flux profile similar to the incident one. Rayleigh transmitted radiation in a slab is characterized by the central dip with a complicated polarization behavior. The optically thick part near Lyα center is polarized in the direction perpendicular to the slab normal, which is in contrast to weakly polarized wing parts in the direction parallel to the slab normal. A similar polarization flip phenomenon is also found in the case of a tall cylindrical shell, in which the spatial diffusion along the vertical direction near the inner cylinder wall for core photons leads to a tendency of the electric field aligned to the direction perpendicular to the vertical axis. Observational implications are briefly discussed including spectropolarimetry of the quasar PG 1630+377 by Koratkar et al. in 1990 where Lyα is strongly polarized with no other emission lines polarized.

What Makes Lyα Nebulae Glow? Mapping the Polarization of LABd05

“…The asymmetric polarization pattern in LABd05 suggests a more complex geometry than assumed by theoretical models of Lyα nebulae to date (Dijkstra & Loeb 2008;Chang et al 2017;Eide et al 2018). As discussed above, those models assume that the scattering H I gas and Lyα-emitting region are spherically or cylindrically symmetric, which generally produces symmetric polarization.…”

Lyman-αpolarization intensity mapping

“…The exact value of the degree of polarization depends strongly on the physical properties of the scattering medium (i.e., its bulk and turbulent velocity, as well as its Hi density). The Lyα polarization pattern (i.e., the angle of polarization) around the sources depends strongly on the isotropy and homogeneity of the emission and gas distribution (Lee & Ahn 1998;Ahn et al 2002;Chang et al 2016;Eide et al 2018). These dependences make the polarization signal very sensitive to the specific conditions of the medium, and, as we will show, this boosts the amount of information on galaxies and their environment retrievable from polarization, compared to that from emission alone.…”

“…This definition implies that the values of Q and U depend on the choice of the coordinate system that defines them, while I is simply a scalar quantity invariant under a change of coordinates (we address this coordinate system dependence in § 2.2). We ignore the parameter describing circular polarization, V , because the scattering of Lyα radiation yields linear polarization alone when the incoming radiation is non-circularly polarized, which we assume to be the case here (Chandrasekhar 1960). Derived quantities also useful for our work are the polarized intensity, P = Q 2 + U 2 , and the degree of polarization (or polarization fraction), Π = P/I.…”

“…In this paper, we focus on the polarization of Lyα radiation around galaxies, which is another effect arising from the scattering of Lyα photons, and that has not been previously considered in the intensity mapping formalism 1 . Because in the scattering event the Lyα photons become polarized (e.g., Chandrasekhar 1960), and because scattering contributes to the extended Lyα emission around sources, a net polarization fraction can appear in the diffuse Lyα emission in galaxy halos. Indeed, first noted that high degrees of Lyα polarization, up to ∼ 40−60 %, could occur around pre-reionization sources due to Lyα radiation scattered by the neutral intergalactic gas.…”

“…The expressions above show that wing scattering (equivalent to Rayleigh scattering; Stenflo 1980) can introduce a degree of polarization about three times larger than that in the core (described by the superposition of Rayleigh and isotropic scattering; Brandt & Chamberlain 1959;Brasken & Kyrola 1998), and as high as 100 % for a scattering angle of β = π/2 (Chandrasekhar 1960;Lee & Ahn 1998). Therefore, because wing scattering also implies a high escape probability, the observed scattered Lyα radiation can carry a large (detectable) degree of polarization.…”