Lyman alpha line formation in an expanding H I superbubble in the primeval galaxy DLA 2233+131

Abstract

Various types of galaxies observed in the cosmological scales show P-Cygni type profiles in the Lyman alpha emission lines. A Monte Carlo code is developed to investigate the Lyman alpha line transfer in an optically thick and moving medium with a careful consideration of the scattering in the damping wing. The main features in emergent line profiles include a primary emission peak in the red part and a much weaker secondary peak in the blue part. The primary peak recedes to the red and the width of the feature increases as $N_{HI}$ increases. The P-Cygni type profile in the Lyman alpha emission line of DLA 2233+131 ($z=3.15$) is noted to be similar to those found in the nearby galaxies and distant ($3<z<3.5$) H II galaxies. Our numerical results are applied to show that the DLA system may possess an expanding H I supershell with bulk flow of $\sim 200 km/s$ and that the H I column density $N_{HI}$ is approximately $10^{20} cm^{-2}$. From the observed Lyman alpha flux and adopting a typical size of the emission region $\sim 1 \kpc$, we estimate the electron density of the H II region to be $\sim 1 \cm^{-3} and the mass of H II region $\sim 10^8 \msun$. We also conclude that it requires $\sim 10^3$ O5 stars for photoionization, which is comparable to first-ranked H II regions found in nearby spiral and irregular galaxies.