An ESO/VLT survey of NIR (Z<=25) selected galaxies at redshifts 4.5<z<6: constraining the cosmic star formation rate near the reionization epoch

Abstract

We present the results of a VLT and HST imaging survey aimed at the identification of 4.5<z<6 galaxies. In the VLT data, a set of broad and intermediate band filters has been used to select 13 high--$z$ candidates in a Z(AB)<25 catalog, over an area of about 30 arcmin2. Discrimination against lower redshift interlopers (mainly early--type galaxies at high redshift and cool Galactic stars) has been done combining morphological and spectral classification. This sample has been combined with a deeper I<27.2 sample obtained from the Hubble Deep Field campaigns. The VLT final sample consists of 13 high z candidates, 4 of which are identified with high confidence as z>4.5 galaxies. The resulting integral surface density of the Z<25 candidates at z>4.5z is in the range 0.13-0.44/arcmin^2 and that in the highest redshift bin 5<z<6 is between 0.07-0.13/arcmin^2. In the two HDFs, we identify at I<27.2 25 galaxies in the range 4.5<z<5 and 16 at 5<z<6$, corresponding to surface densities of 3.1 arcmin^-2 and 2 arcmin^-2, respectively. We show that the observed Z<25 UV luminosity density appear to drop by about one order of magnitude from z~3 to z~6. However, if we apply a threshold to obtain an absolute--magnitude limited sample, the UV luminosity density results to be roughly constant up to z~6. We finally show that recent semi-analytic hierarchical models for galaxy formation, while predicting a nearly constant total UV luminosity density up to z~6, under-predict the observed UV luminosity density at Z<25 and over-predicts the I<27.2 one. This behaviour can be understood in term of a poor match to the slope of the UV luminosity function.