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Article overview
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Unlocking the Full Potential of Extragalactic Lyman-alpha through its Polarization Properties | | Marius Berge Eide
; Max Gronke
; Mark Dijkstra
; Matthew Hayes
; | | Date: |
12 Feb 2018 | | Abstract: | Lyman-$alpha$ (Ly$alpha$) is a powerful astrophysical probe. Not only is it
ubiquitous at high redshifts, it is also a resonant line, which makes
Ly$alpha$ photons scatter. This scattering process depends on the physical
conditions of gas through which Ly$alpha$ propagates, and these are imprinted
on observables such as the Ly$alpha$ spectrum and its surface brightness
profile. In this work, we focus on a less used observable that can probe any
scattering process: polarization. We implement the density matrix formalism of
polarization into the Monte-Carlo radiative transfer code tlac. This allows us
to treat it as a quantum mechanical process where single photons develop and
lose polarization from scatterings in arbitrary gas geometries. We explore
static and expanding ellipsoids, biconical outflows and clumpy, multiphase
media. We find that photons become increasingly polarized as they scatter and
diffuse into the wings of the line profiles, making scattered Ly$alpha$
polarized in general. The degree and orientation Ly$alpha$ polarization
depends on the kinematics and distribution of the scattering HI gas. The degree
of polarization generally increases with the degree of asymmetry of the gas (in
terms of its physical distribution, or distribution in velocity space), and the
orientation generally aligns tangentially to the source of emission. We show
that the spectra, the surface brightness profile and the polarization of
Ly$alpha$ photons leave signatures that are degenerate between different
models when studied separately. However, we show that such degeneracies can be
broken when observables are combined. We conclude by revealing how a joint
analysis of these can help us extract unique physical information on galaxies
and their environments from their strongest, most prominent emission line. | | Source: | arXiv, 1802.4280 | | Services: | Forum | Review | PDF | Favorites | |
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