THE PHOTOMETRIC REDSHIFT DISTRIBUTION AND EVOLUTIONARY PROPERTIES OF GALAXIES UP TO Z-SIMILAR-TO-4.5 IN THE FIELD OF THE QUASAR BR-1202-0725

We present a deep BVrIK multicolor catalog of galaxies in the field of the high-redshift (z = 4.7) quasar BR 1202-0725. Reliable colors have been measured for galaxies selected down to R = 25. Taking advantage of the wide spectral coverage of the galaxies in the field, we compare the observed colors with those predicted by spectral synthesis models including UV absorption by the intergalactic medium and dust reddenin g. The choice of the optical filters has been optimized to define a ro bust multicolor selection of galaxies at 3.8 less than or equal to z l ess than or equal to 4.5. Within this interval the surface density of galaxy candidates with z similar to 4 in this field is 1 arcmin(-2). P hotometric redshifts have been derived for the galaxies in the held wi th the maximum likelihood analysis using the GISSEL library of similar to 10(6) synthetic spectra. The accuracy of the method used has been discussed and tested using galaxies in the Hubble Deep Field with know n spectroscopic redshifts and accurate photometry. A peak in the redsh ift distribution is present at z similar or equal to 0.6 with relative ly few galaxies at z > 1.5. At variance with brighter surveys (I < 22. 5) is a tail in the distribution toward high redshifts up to z similar to 4. The luminosity function at z similar to 0.6 shows a steepening for M-B > -19. This increase is reminiscent of that found in the most recent estimates of the local luminosity function, where a similar vol ume density is reached about 2 mag fainter. The observed cosmological UV luminosity density is computed in the overall redshift interval z = 0.3-4.5, reaching a value similar to 2 x 10(19) W Hz(-1) Mpc(-3) at z similar to 0.8. Including recent local estimates, it appears that the UV luminosity density changes by a factor of similar to 2.5 in the ov erall redshift interval z = 0.1-4, not including correction for fainte r undetected galaxies. Thus we find that the evidence of a marked maxi mum in the luminosity density at z similar to 1-1.5 for galaxies with R less than or equal to 25 is weak. Using the GISSEL libraries we have derived in a homogeneous way the physical parameters connected with t he fitted spectral energy distributions. Thanks to this new approach, the problem of the star formation history of the universe is dealt wit h in a consistent way, taking into account the dust and metallicity di stributions derived from the spectrophotometric properties of each gal axy in the sample. The bulk of the blue intermediate-redshift populati on with z = 0.4-1 mostly consists of very young star-forming galaxies with a median starburst age of the order of a few times 10(8) yr and t ypical mass in luminous stars similar to 2 x 10(8) M-.. The presence o f this young population is in contrast with the pure luminosity evolut ionary model based on a single high-formation redshift. The cosmologic al mass in formed stars per unit comoving volume at z similar to 3 is already similar to 20% of that formed at z = 0.5 in our magnitude-limi ted sample. Predictions based on the standard hierarchical clustering models are smaller, although not far from that derived from the observ ations.