Effects of intergalactic absorption on the visibility of young galaxies

Abstract

Intergalactic H I clouds that produce the Lyα absorption lines in QSO spectra with rest-frame equivalent widths greater than about 0.5 Å accumulate along the line of sight to be optically thick at wavelengths below the Lyman limit due to Lyman continuum absorption. Because the number of clouds along the line of sight increases rapidly with redshift, the cumulative effect of Lyman line absorption will also extinguish the spectra of source objects with a redshift of zs > 5.5 that are observed at wavelengths below λ ∼ 8000 Å before the Lyman limit eventually enters this wavelength region at zs ∼ 8. The intergalactic absorption places a definite upper bound on observable redshift of zs ≈ 5.5, above which no objects can be probed in optical surveys, either by spectroscopy or by I-band photometry. A question of whether galaxies would form at much earlier epoches can be answered only by searches at longer wavelengths where the intergalactic absorption is less. Show more