Differential evolution of the UV luminosity function of Lyman break galaxies from z ∼ 5 to 3

We report the ultraviolet luminosity function (UVLF) of Lyman break galaxies at z similar to 5 derived from a deep and wide survey using the prime focus camera of the 8.2 m Subaru telescope (Suprime-Cam). Target fields consist of two blank regions of the sky, namely, the region including the Hubble Deep Field-North and the J0053+1234 region, and the total effective surveyed area is 1290 arcmin(2). Applications of carefully determined colour selection criteria in V - I-c and I-c - z' yield a detection of 853 z similar to 5 candidates with z'(AB) < 26.5 mag. The UVLF at z similar to 5 based on this sample shows no significant change in the number density of bright (L greater than or similar to L-z=3*) LBGs from that at z similar to 3, while there is a significant decline in the LF's faint end with increasing look-back time. This result means that the evolution of the number densities is differential with UV luminosity: the number density of UV luminous objects remains almost constant from z similar to 5 to 3 (the cosmic age is about 1.2 to 2.1 Gyr) while the number density of fainter objects gradually increases with cosmic time. This trend becomes apparent thanks to the small uncertainties in number densities both in the bright and faint parts of LFs at different epochs that are made possible by the deep and wide surveys we use. We discuss the origins of this differential evolution of the UVLF along the cosmic time and suggest that our observational findings are consistent with the biased galaxy evolution scenario: a galaxy population hosted by massive dark haloes starts active star formation preferentially at early cosmic time, while less massive galaxies increase their number density later. We also calculated the UV luminosity density by integrating the UVLF and at z similar to 5 found it to be 38.8(-4.1)(+6.7) per cent of that at z similar to 3 for the luminosity range L > 0.1L(z=3)* . By combining our results with those from the literature, we find that the cosmic UV luminosity density marks its peak at z = 2-3 and then slowly declines towards higher redshift.