Escape of Resonantly Scattered Ly${\hat{I}}^2$ and H$\hat{I}\pm$ from Hot and Optically Thick Media

Abstract

We investigate the escape of Ly${\hat{I}}^2$ from emission nebulae with a significant population of excited hydrogen atoms in the level $n=2$, rendering them optically thick in H$\hat{I}\pm$. The transfer of Ly${\hat{I}}^2$ line photons in these optically thick regions is complicated by the presence of another scattering channel leading to re-emission of H$\hat{I}\pm$, alternating their identities between Ly${\hat{I}}^2$ and H$\hat{I}\pm$. In this work, we develop a Monte Carlo code to simulate the transfer of Ly${\hat{I}}^2$ line photons incorporating the scattering channel into H$\hat{I}\pm$. Both H$\hat{I}\pm$ and Ly${\hat{I}}^2$ lines are formed through diffusion in frequency space, where a line photon enters the wing regime after a fairly large number of resonance scatterings with hydrogen atoms. Various line profiles of H$\hat{I}\pm$ and Ly${\hat{I}}^2$ emergent from our model nebulae are presented. It is argued that the electron temperature is a critical parameter which controls the flux ratio of emergent Ly${\hat{I}}^2$ and H$\hat{I}\pm$. Specifically for $T=3\times 10^4\mathrm{K}$ and H$\hat{I}\pm$ line center optical depth $\ddot{I}{„}_{\hat{I}}\pm =10$, the number flux ratio of emergent Ly${\hat{I}}^2$ and H$\hat{I}\pm$ is $\sim 49$ percent, which is quite significant. We propose that the leaking Ly${\hat{I}}^2$ can be an interesting source for the formation of H$\hat{I}\pm$ wings observed in many symbiotic stars and active galactic nuclei. Similar broad H$\hat{I}\pm$ wings are also expected in Ly$\hat{I}\pm$ emitting halos found in the early universe, which can be potentially probed by the {\it James Webb Telescope} in the future.

13 pages, 12 figures, It will be published on Feb 2018 in JKAS (Journal of Korea Astronomical Society)