The UDF05 Follow-up of the Hubble Ultra Deep Field. I. The Faint-End Slope of the Lyman Break Galaxy Population at z ~ 5

Abstract

We present the UDF05 HST program, which consists of three disjoint fields-NICP12, NICP34, plus the HUDF-with deep ACS (F606W, F775W, and F850LP) and NICMOS (F110W and F160W) imaging. Here we use the ACS data for the NICP12 and HUDF fields to implement a (V-i)-(i-z) selection criterion that allows us to identify a sample of 101 (133) z~5 Lyman break galaxies (LBGs) down to z₈₅₀=28.5 (29.25) mag in NICP12 (HUDF). We construct the rest-frame 1400 Å LBG luminosity function (LF) over the range M₁₄₀₀=[-21.4,-17.1], i.e. down to ~0.04L_* at z~5, and use Subaru Deep Field results (Yoshida et al. 2006) to constrain our LF at the bright end (M₁₄₀₀>=-22.2). We show that (1) different assumptions regarding the LBG SED distribution, dust properties, and intergalactic absorption result in a 25% variation in the number density of LBGs at z~5 (2) under consistent assumptions for dust properties and intergalactic absorption, the HUDF is ~30% underdense in z~5 LBGs relative to the NICP12 field, a variation which is well explained by cosmic variance; and (3) the faint-end slope of the LF does not depend on the input parameters, and has a value of α~-1.6, similar to the faint-end slope of the LF of z~3 and z~6 LBGs. Our study therefore supports no variation in the faint end of the LBG LF over the whole redshift range z~3 to z~6. Based on a comparison with semianalytical models, we speculate that the z~5 LBGs might have a top-heavy IMF.

Based on data obtained with the Hubble Space Telescope operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-26555.