P-Cygni Type Lya from Starburst Galaxies

Abstract

P-Cygni type Lya profiles exhibited in nearly half of starburst galaxies, both nearby and high-z, are believed to be formed by an expanding supershell surrounding a star-forming region. We apply the Monte Carlo code which was developed previously for static and plane-parallel medium to calculate the Lya line transfer in a supershell of neutral hydrogen which are expanding radially in a spherical bulk flow. We consider typical cases that the supershell has the Lya line-centre optical depth of $τ_0=10^5-10^7$, a radial expansion velocity of $V_exp = 300 km/s, and the turbulence of b=40 km/s. We find that there appear a few emission peaks at the frequencies corresponding to (2N-1) V_exp, where the order of back scatterings N > 1. As V_exp -> b, the emergent profiles become similar to those for the static medium and the peaks are less prominent. We also investigate the effects of column density of the supershell on the emergent Lya profiles. We find that the number and the flux ratios of emission peaks are determined by interplay of $τ_0$ and V_exp of the supershell. We discuss the effects of dust extinction and the implication of our works in relation to recent spectroscopic observations of starburst galaxies.