Lyman Alpha Radiative Transfer in a Multi-Phase Medium
Hydrogen Ly-alpha is our primary emission-line window into high redshift galaxies. Surprisingly, despite an extensive literature, Ly-alpha radiative transfer in the most realistic case of a dusty, multi-phase medium has not received detailed theoretical attention. We investigate resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Ly-alpha surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption. We consider two interesting applications: (i) Equivalent widths. Ly-alpha can preferentially escape from a dusty multi-phase ISM if most of the dust lies in cold neutral clouds, possibly explaining anomalously high EWs seen in many high redshift/submm sources. (ii) Multi-phase galactic outflows. We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged. [Abridged]