Ly alpha cooling radiation from high-redshift halos

The baryons inside high-redshift halos with virial temperatures T greater t han or similar to 10(4) K cool radiatively as they condense inside dark mat ter potential wells. We show that the release of the gravitational binding energy, over the halo assembly timescale, results in a significant and dete ctable Ly alpha flux. At the limiting line flux approximate to 10(-19) ergs s(-1) cm(-2) arcsec(-2) of the Next Generation Space Telescope, several su fficiently massive halos, with velocity dispersions sigma greater than or s imilar to 120 km s(-1), would be visible per 4' x 4' field. The halos would have characteristic angular sizes of approximate to 10 ", would be detecta ble in a broadband survey out to z approximate to 6-8, and would provide a direct probe of galaxies in the process of forming. They may be accompanied by He+ Ly alpha emission at the approximate to 10% level, but remain undet ectable at other wavelengths. Our predictions are in good agreement with th e recent finding of two Ly alpha "blobs" at z = 3.1 by Steidel et al.