An empirical limit on extremely high redshift galaxies

We apply the Lyman absorption signature to search for galaxies at redshifts z ≈ 6-17 using optical and infrared images of the Hubble Deep Field. The infrared images are sensitive to a point-source 5 σ detection threshold of AB(22,000) = 23.8, which, adopting plausible assumptions to relate rest-frame ultraviolet flux densities to unobscured star formation rates, is easily sufficient to detect the star formation rates expected for massive elliptical galaxy formation to quite high redshifts. For q₀ = 0.5, the infrared images are sensitive to an unobscured star formation rate of M = 100 h^-2 M_⊙ yr^-1 to redshifts as large as z = 17, and for q₀ = 0, the infrared images are sensitive to an unobscured star formation rate of M = 300 h^-2 M_⊙ yr^-1 to redshifts as large as z = 14. The primary result of the analysis is that only one extremely high redshift galaxy candidate is identified at the 5 σ level of significance (and four at the 4 σ level). This implies a strict upper limit to the surface density of extremely high redshift galaxies of less than 1.5 arcmin^-2 to a limiting magnitude threshold AB(22,000) = 23.8. This also implies a strict upper limit to the volume density of extremely high redshift galaxies if (and only if) such galaxies are not highly obscured by dust.