Lyα cooling radiation from high-redshift halos

The baryons inside high-redshift halos with virial temperatures T ≳ 10^4 K cool radiatively as they condense inside dark matter potential wells. We show that the release of the gravitational binding energy, over the halo assembly timescale, results in a significant and detectable Lyα flux. At the limiting line flux ≈10^-19 ergs s^-1 cm^-2 arcsec^-2 of the Next Generation Space Telescope, several sufficiently massive halos, with velocity dispersions σ ≳ 120 km s^-1, would be visible per 4' × 4' field. The halos would have characteristic angular sizes of ≈10'', would be detectable in a broadband survey out to z ≈ 6-8, and would provide a direct probe of galaxies in the process of forming. They may be accompanied by He+ Lyα emission at the ≈10% level, but remain undetectable at other wavelengths. Our predictions are in good agreement with the recent finding of two Lyα "blobs" at z = 3.1 by Steidel et al.