The UDF05 Follow‐up of the Hubble Ultra Deep Field. I. The Faint‐End Slope of the Lyman Break Galaxy Population atz∼ 5

We present the UDF05 HST program, which consists of three disjoint fields-NICP12, NICP34, plus the HUDFwith 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 z 850 28:5 (29.25) mag in NICP12 ( HUDF). We construct the rest-frame 1400 8 LBG luminosity function (LF) over the range M 1400 21:4; 17:1, i.e. down to $0.04L at z $ 5, and use Subaru Deep Field results 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 $ 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.