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

Abstract

If the universe was reionized by O and B stars in an early population of galaxies, the associated super- novae should have enriched the universe to a mean metallicity where is the mean Z1 10~5(1 ) n rec ), n rec number of times that each baryon recombined during the reionization era. This is consistent with recent observations of the metallicity in the Lya 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 Ae should be photons cm~2 s~1 arcsec~2. Most of this radiation j ( 2500 Ae , D10~6(Z1 /10~4)() b h2/0.02) 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 cuto† to search for galaxies at z D 3, where the Lya forest blanketing is smaller). Their spectrum may also be characterized by a strong Lya 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 pro-le that obstructs part of the Lya 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 gal- axies should have merged into larger ones, those that survived to the present could be dwarf spheroidals. Subject headings: galaxies: formation E galaxies: stellar content E large-scale structure of universe E quasars: absorption lines E surveys