Lyα Line Spectra of the First Galaxies: Dependence on Observed Direction to the Underlying Cold Dark Matter Filament

Abstract

The first galaxies in the universe are built up where cold dark matter (CDM) forms large-scale filamentary structure. Although the galaxies are expected to emit numerous Ly? photons, they are surrounded by plentiful neutral hydrogen with a typical optical depth for Ly? of ~105 (H I halos) before the era of cosmological reionization. The H I halo almost follows the cosmological Hubble expansion with some anisotropic corrections around the galaxy because of gravitational attraction by the underlying CDM filament. In this paper, we investigate the detectability of the Ly? emissions from the first galaxies, examining their dependence on viewing angles. Solving the Ly? line transfer problem in an anisotropically expanding H I halo, we show that the escape probability from the H I halo is the largest in the direction along the filament axis. If the Ly? source is observed with a narrowband filter, the difference in apparent Ly? line luminosities among viewing angles can be a factor of 40 for an extreme case. Furthermore, we evaluate the predicted physical features of the Ly? sources and flux magnification by the gravitational lensing effect due to clusters of galaxies along the filament. We conclude that by using next-generation space telescopes such as JWST, the Ly? emissions from the first galaxies whose CDM filament axes almost face us can be detected with S/N 10.