The Photometric Redshift Distribution and Evolutionary Properties of Galaxies up to z ～ 4.5 in the Field of the Quasar BR 1202?0725*

摘要

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 reddening. The choice of the optical filters has been optimized to define a robust multicolor selection of galaxies at 3.8 ≤ z ≤ 4.5. Within this interval the surface density of galaxy candidates with z ~ 4 in this field is 1 arcmin-2. Photometric redshifts have been derived for the galaxies in the field with the maximum likelihood analysis using the GISSEL library of ~106 synthetic spectra. The accuracy of the method used has been discussed and tested using galaxies in the Hubble Deep Field with known spectroscopic redshifts and accurate photometry. A peak in the redshift distribution is present at z 0.6 with relatively 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 ~ 4. The luminosity function at z ~ 0.6 shows a steepening for MB -19. This increase is reminiscent of that found in the most recent estimates of the local luminosity function, where a similar volume 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 ~2 × 1019 W Hz-1 Mpc-3 at z ~ 0.8. Including recent local estimates, it appears that the UV luminosity density changes by a factor of ~2.5 in the overall redshift interval z = 0.1–4, not including correction for fainter undetected galaxies. Thus we find that the evidence of a marked maximum in the luminosity density at z ~ 1–1.5 for galaxies with R ≤ 25 is weak. Using the GISSEL libraries we have derived in a homogeneous way the physical parameters connected with the fitted spectral energy distributions. Thanks to this new approach, the problem of the star formation history of the universe is dealt with in a consistent way, taking into account the dust and metallicity distributions derived from the spectrophotometric properties of each galaxy in the sample. The bulk of the blue intermediate-redshift population 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 108 yr and typical mass in luminous stars ~2 × 108 M☉. The presence of this young population is in contrast with the pure luminosity evolutionary model based on a single high-formation redshift. The cosmological mass in formed stars per unit comoving volume at z ~ 3 is already ~20% of that formed at z = 0.5 in our magnitude-limited sample. Predictions based on the standard hierarchical clustering models are smaller, although not far from that derived from the observations.