Resonant Scattering and Ly-alpha Radiation Emergent from Neutral Hydrogen Halos

With a state-of-the-art numerical method used for solving the integral-differential equation of radiative transfer, we investigate the flux of the Ly alpha photon nu(0) emergent from an optically thick halo containing a central light source. Our focus is on the time-dependent effects of the resonant scattering. We first show that the frequency distribution of photons in the halo is quickly approaching a locally thermalized state around the resonant frequency, even when the mean intensity of the radiation is highly time dependent. Since initial conditions are forgotten during the thermalization, some features of the flux, such as the two-peak structure of its profile, are actually independent of the intrinsic width and time behavior of the central source, if the emergent photons are mainly from photons in the thermalized state. In this case, the difference vertical bar nu(+/-) - nu(0)vertical bar, where nu(+/-) are the frequencies of the two peaks of the flux, cannot be less than 2 times the Doppler broadening. We then study the radiative transfer in the case where the light emitted from the central source is a flash. We calculate the light curves of the flux from the halo. It shows that the flux is still a flash. The time duration of the flash for the flux, however, is independent of the original time duration of the light source but depends on the optical depth of the halo. Therefore, the spatial transfer of resonant photons is a diffusion process, even though it is not a purely Brownian diffusion. This property enables an optically thick halo to trap and store thermalized photons around nu(0) for a long time after the cessation of the central source emission. The photons trapped in the halo can yield delayed emission, of which the profile also shows typical two-peak structure as that from locally thermalized photons. Possible applications of these results are addressed.