Lyα LINE SPECTRA OF THE FIRST GALAXIES: DEPENDENCE ON OBSERVED DIRECTION TO THE UNDERLYING COLD DARK MATTER FILAMENT

摘要

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 Lyα photons, they are surrounded by plentiful neutral hydrogen with a typical optical depth for Lyα of ～ 10~5 (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 Lyα emissions from the first galaxies, examining their dependence on viewing angles. Solving the Lyα 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 Lyα source is observed with a narrowband filter, the difference in apparent Lyα line luminosities among viewing angles can be a factor of approx > 40 for an extreme case. Furthermore, we evaluate the predicted physical features of the Lyα 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 Lyα emissions from the first galaxies whose CDM filament axes almost face us can be detected with S/N approx > 10.