A European Southern Observatory Very Large Telescope Survey of Near-Infrared (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 Very Large Telescope (VLT) and Hubble Space Telescope imaging survey aimed at the identification of 4.5<z<6 galaxies. In the VLT data, a set of broadband and intermediate-band filters has been used to select 13 high-z candidates in a Z_AB<=25 mag 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_AB<=27.2 mag sample obtained from the Hubble Deep Field (HDF) campaigns. The VLT final sample consists of 13 high-z candidates, four of which are identified with high confidence as z>4.5 galaxies. The resulting integral surface density of the Z_AB<25 candidates at z>4.5 is in the range 0.13-0.44 arcmin^-2, and that in the highest redshift bin 5<z<=6 is between 0.07 and 0.13 arcmin^-2. In the two HDFs, we identify 25 galaxies at I_AB<=27.2 in the range 4.5<=z<5 and 16 at 5<=z<=6, corresponding to surface densities of 3.1 and 2 arcmin^-2, respectively. We show that the observed Z_AB<25 UV luminosity density (LD) appears to drop by about 1 order of magnitude from z~=3 to 6. However, if we apply a threshold to obtain an absolute magnitude-limited sample, the UV LD is roughly constant up to z~=6. We finally show that recent semianalytic hierarchical models for galaxy formation, while predicting a nearly constant total UV LD up to z~=6, underpredict the observed UV LD at Z_AB<=25 and overpredict the I_AB<=27.2 one. This behavior can be understood in terms of a poor match to the slope of the UV luminosity function.

Based on observations obtained in service mode at the ESO VLT for the program 65.O-0445.