Scattered Lyα radiation around sources before cosmological reionization

The spectra of the first galaxies and quasars in the universe should be strongly absorbed shortward of their rest-frame Ly alpha wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Ly alpha line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape owing to the Hubble expansion of the surrounding intergalactic H I. We calculate the resulting brightness distribution and the spectral shape of the diffuse Ly alpha line emission around high-redshift sources before the intergalactic medium was reionized. Typically, the Ly alpha photons emitted by a source at z(s) similar to 10 scatter over a characteristic angular radius of similar to 15 " around the source and compose a line that is broadened and redshifted by similar to 10(3) km s(-1) relative to the source. The scattered photons are highly polarized. Detection of the diffuse Ly alpha halos around high-redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales, where the Hubble how is smooth and the gas is neutral, the Ly alpha brightness distribution can be used to determine the cosmological mass densities of baryons and matter.