The UDF05 follow-up of the Hubble Ultra Deep Field.: I.: The faint-end slope of the Lyman break galaxy population at z ∼ 5

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 similar to 5 Lyman break galaxies (LBGs) down to z(850) = 28.5 (29.25) mag in NICP12 (HUDF). We construct the rest-frame 1400 angstrom LBG luminosity function (LF) over the range M-1400 = [-21.4, -17.1], i.e. down to similar to 0.04L(*) at z similar to 5, and use Subaru Deep Field results (Yoshida et al. 2006) to constrain our LF at the bright end (M-1400 >= -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 similar to 5; (2) under consistent assumptions for dust properties and intergalactic absorption, the HUDF is similar to 30% underdense in z similar to 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 alpha similar to -1.6, similar to the faint-end slope of the LF of z similar to 3 and z similar to 6 LBGs. Our study therefore supports no variation in the faint end of the LBG LF over the whole redshift range z similar to 3 to z similar to 6. Based on a comparison with semianalytical models, we speculate that the z similar to 5 LBGs might have a top-heavy IMF.