EFFECTS OF INTERGALACTIC ABSORPTION ON THE VISIBILITY OF YOUNG GALAXIES

Intergalactic H I clouds that produce the Ly alpha absorption lines in QSO spectra with rest-frame equivalent widths greater than about 0.5 Angstrom 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 z(s) > 5.5 that are observed at wavelengths below lambda similar to 8000 Angstrom before the Lyman limit eventually enters this wavelength region at z(s) similar to 8. The intergalactic absorption places a definite upper bound on observable redshift of z(s) approximate to 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.