Searching for the Earliest Galaxies Using the Gunn-Peterson Trough and the Lyα Emission Line

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 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 Lyα forest at z ~= 3. The mean surface brightness observable at present from the galaxies that produced these heavy elements, in the rest-frame wavelengths 1216 Å < λ <~ 2500 Å, should be ~10^-6(Z¯/10^-4)(Ω_bh²/0.02) 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 ~ 3, where the Lyα forest blanketing is smaller). Their spectrum may also be characterized by a strong Lyα 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 Lyα 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.