  The baryons inside high-redshift halos with virial temperatures above 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
time-scale, results in a significant and detectable Ly alpha flux. At the
limiting line flux of approximately 10^-19 erg/s/cm^2/asec^2 of the Next
Generation Space Telescope, several sufficiently massive halos, with velocity
dispersions above sigma=120 km/s, would be visible per 4'x4' field. The halos
would have characteristic angular sizes of approximately 10 asec, would be
detectable in a broad-band survey out to redshifts z=6-8, and would provide a
direct probe of galaxies in the process of forming. They may be accompanied by
HeII Ly alpha emission at the 10 percent level, but remain undetectable at
other wavelengths. Our predictions are in good agreement with the recent
finding of two Ly alpha ``blobs'' at z=3.1 by Steidel et al. (1999).
