Detection of the First Galaxies

The intrinsic Lyα emission lines of normal galaxies before reionization are much absorbed by the damping wing of the Gunn-Peterson trough, rendering their direct detection nearly impossible, if their intrinsic line widths are less than ∼ 100km/s. High redshift luminous quasars prior to the completion of cosmo-logical reionization at z ∼ 6, on the other hand, are capable of producing large HII regions around them (Strömgren spheres) to allow their intrinsic Lyα emission lines to be transmitted without overwhelming absorption (Cen & Haiman 2000). We suggest that targeted observations at the Strömgren spheres of known luminous quasars at z ≥ 6 would be able to detect Lyα emission lines of galaxies inside the Strömgren spheres largely unattenuated. A tunable, very narrowband filter of ∆λ λ ∼ 0.1% or a narrowband filter of ∆λ λ ∼ 1% with follow-up spectro-scopic identifications will be required. Such observations could directly observe the sources of cosmological reionization including possibly the Pop III galaxies at z = 6 − 20 by JWST. Possible applications include determinations of the ionization state of the intergalactic medium, the sizes of the Strömgren spheres, the ages of the quasars, the luminosity function of high redshift galaxies and its evolution, the spatial distribution of galaxies and its evolution, the biased distribution of galaxies around quasars and the anisotropy of quasar emission. Observations using Keck-class telescopes may already be made to enable a differentiation between a fully neutral and a 10% neutral intergalactic medium at z > 6.