We present the UDF05 project , a HST Large Program of deep ACS ( F606W , F775W , F850LP ) and NICMOS ( F110W , F160W ) imaging of three fields , two of which coincide with the NICP1-4 NICMOS parallel observations of the Hubble Ultra Deep Field ( HUDF ) .
In this first paper we use the ACS data for the NICP12 field , as well as the original HUDF ACS data , to measure the UV Luminosity Function ( LF ) of z \sim 5 Lyman Break Galaxies ( LBGs ) down to very faint levels .
Specifically , based on a V - i , i - z selection criterion , we identify a sample of 101 and 133 candidate z \sim 5 galaxies down to z _ { 850 } = 28.5 and 29.25 magnitudes in the NICP12 and in the HUDF fields , respectively .
Using an extensive set of Monte Carlo simulations we derive corrections for observational biases and selection effects , and construct the rest-frame 1400 Å LBG LF over the range M _ { 1400 } = [ -21.4 , -17.1 ] , i.e .
down to \sim 0.04 L _ { * } at z \sim 5 , and complement it with data from the Subaru Deep Field ( SDF ) from Yoshida et al .
( 62 ) to extend it to the brighter end ( M _ { 1400 } \geq - 22.2 ) .
We show that : ( i ) Different assumptions regarding the SED distribution of the LBG population , dust properties and intergalactic absorption result in a 25 % variation in the number density of LBGs at z \sim 5 ; ( ii ) Under consistent assumptions for dust properties and intergalactic absorption , the HUDF is about 30 % under-dense in z \sim 5 LBGs relative to the NICP12 field , a variation which is well explained by cosmic variance ; ( iii ) The faint-end slope of the LF is independent of the specific assumptions for the input physical parameters , and has a value of \alpha \sim - 1.6 , similar to the faint-end slope of the LF that has been measured for LBGs at z \sim 3 and z \sim 6 .
Our study therefore supports no variation in the faint-end of the LBG LF over the whole redshift range z \sim 3 to z \sim 6 .
Based on a comparison with semi-analytical models , we speculate that the z \sim 5 LBGs might have a top-heavy IMF .