An Empirical Limit on Extremely High Redshift Galaxies

Abstract

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 sigma 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 q_0 = 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 q_0 = 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 sigma level of significance (and four at the 4 sigma 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555. Based on observations made at the Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation.