Singly-Peaked P-Cygni type Ly$\alpha$ from starburst galaxies

Abstract

We present results of Monte Carlo calculations for the Lya line transfer in an expanding dusty supershell, where Lya source is a well-localized star cluster in a starburst galaxy.The escape of Lya photons from such system is achieved by a number of back-scattering, and so a series of emission peaks are formed redward of the systemic redshift by back-scattering. However, majority of observed Lya emission from starbursts show singly-peaked asymmetric profiles. We find in this paper that, in order to form a singly-peaked Lya emission, dust should be distributed in the ionized bubble, as well as within the supershell of neutral hydrogen. We also find that the overall escape fraction of Lya photons is determined by the HI column density of the supershell, the expansion velocity of the supershell, and the spatial distribution of dust. However, the kinematic information of the expanding supershell is preserved in the profile of Lya emission even when the supershell is dusty. Our results are potentially useful to fit the P-Cygni type Lya line profiles from starburst galaxies, either nearby galaxies or high-z Lyman break galaxies (LBGs).