If the universe was reionized by O and B stars in an early population of galaxies , the associated supernovae should have enriched the universe to a mean metallicity \bar { Z } = 10 ^ { -5 } ( 1 + n _ { rec } ) , where n _ { rec } is the mean number of times that each baryon recombined during the reionization era .
This is consistent with recent observations of the metallicity in the Ly \alpha forest at z \simeq 3 .
The mean surface brightness observable at present from the galaxies that produced these heavy elements , in the rest-frame wavelengths 1216 { \AA } < \lambda \lesssim 2500 { \AA } , should be \sim 10 ^ { -6 } ( \bar { Z } / 10 ^ { -4 } ) ( \Omega _ { b } h ^ { 2 } / 0.02 ) ~ { } { photons } { cm% } ^ { -2 } { s } ^ { -1 } { arcsec } ^ { -2 } .
Most of this radiation should be emitted at z > 5 , before reionization was complete .

These high-redshift galaxies may be detectable in near-infrared photometric surveys , identifying them via the Gunn-Peterson trough ( analogous to the use of the Lyman limit cutoff to search for galaxies at z \sim 3 , where the Ly \alpha forest blanketing is smaller ) .
Their spectrum may also be characterized by a strong Ly \alpha emission line .
However , the spectra of galaxies seen behind intervening gas that is still neutral should show the red damping wing of the Gunn-Peterson trough , with a predictable profile that obstructs part of the Ly \alpha emission .

The low-mass galaxies formed before reionization might constitute a distinctive population ; we discuss the signature that this population could have in the faint number counts .
Although most of these galaxies should have merged into larger ones , those that survived to the present could be dwarf spheroidals .