Low-mass protogalaxies and absorption lines in quasi-stellar objects.

The possibility is explored that the many weak unidentified absorption lines in high-redshift QSO spectra are L-alpha lines of hydrogen. The likely existence of a metagalactic flux of ionizing radiation indicates that hydrogen clouds at high redshifts must be thoroughly ionized. Observations of PHL 957 show that the suggested clouds may be low-mass protogalaxies. A simple model for the clouds' collapse and for the flux of ionizing radiation is used to compute the distribution of expected L-alpha lines in the spectrum of a QSO as a function of wavelength and H I column density. The computed distribution is in accord with observations of the two most distant QSOs, PHL 957 and 4C 05.34. Crudities in the theory and stronger tests of its validity are also discussed.