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 Lyα wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyα line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyα line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyα photons emitted by a source at zs ∼ 10 scatter over a characteristic angular radius of ∼ 15 around the source and compose a line which is broadened and redshifted by ∼ 103 km s−1 relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyα 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 flow is smooth and the gas is neutral, the Lyα brightness distribution can be used to determine the cosmological mass densities of baryons and matter. Subject headings: cosmology: theory – line: profiles