Spectra from Forming Region of the First Galaxies : The Effect of Aspherical Deceleration

Abstract

Ly$\alpha$ line emission from the Loeb-Rybicki (LR) halo, which is the expanding HI IGM (intergalactic medium) around the first star clusters and the ionized interstellar medium, is investigated by solving a radiative transfer problem. While the initial scattering optical depth is $\sim 10^5$ for the Ly$\alpha$ photons, most of the Ly$\alpha$ photons can escape when the cumulative frequency-shift due to the expansion of the HI IGM becomes significantly large. The current paper improves upon previous treatments of the scattering processes and the opacity for the Ly$\alpha$ transfer. Confirming the previous results of the LR halo, we investigate the effect of the aspherical expansion of the IGM. The asphericity is hypothesized to follow the initial stage of the gravitational deceleration to form the large scale filamentary structure of the Universe. According to our results, the effect of the asphericity lets the peak wavelength of the line profile shift to longer wavelengths and the FWHM of the profile become wider than those of the spherically expanding model. To detect these features is meaningful if we are interested in the initial evolution of the large scale structure, since they reflect the dynamical properties of the IGM at that time. Furthermore, given the recent discovery of the high redshift cosmological reionization, we briefly comment on the effects of the redshift and the cosmological parameters on the line profile.