Effect of Dust on Lyα Photon Transfer in an Optically Thick Halo

We investigate the effects of dust on Ly alpha photons emergent from an optically thick medium by solving the integro-differential equation of radiative transfer of resonant photons. To solve the differential equations numerically, we use the weighted essentially non-oscillatory method. Although the effects of dust on radiative transfer are well known, the resonant scattering of Ly alpha photons makes the problem non-trivial. For instance, if the medium has an optical depth of dust absorption and scattering of tau(a) >> 1, tau >> 1, and tau >> tau(a), the effective absorption optical depth in a random walk scenario would be equal to root tau(a) (tau(a) + tau). We show, however, that for a resonant scattering at frequency nu(0), the effective absorption optical depth would be even larger than tau(nu(0)). If the cross section of dust scattering and absorption is frequency-independent, the double-peaked structure of the frequency profile given by the resonant scattering is basically dust-independent. That is, dust causes neither narrowing nor widening of the width of the double-peaked profile. One more result is that the timescales of the Ly alpha photon transfer in an optically thick halo are also basically independent of the dust scattering, even when the scattering is anisotropic. This is because those timescales are mainly determined by the transfer in the frequency space, while dust scattering, either isotropic or anisotropic, does not affect the behavior of the transfer in the frequency space when the cross section of scattering is wavelength-independent. This result does not support the speculation that dust will lead to the smoothing of the brightness distribution of a Ly alpha photon source with an optically thick halo.