A VLT/FORS2 spectroscopic survey in the HDF-S

We report on low-resolution multi-object spectroscopy of 65 objects from I(AB) similar or equal to 20 to I(AB) similar or equal to 25 in the HDF-S obtained with the VLT Focal Reducer/low dispersion Spectrograph (FORS2). 18 objects belong to the HDF-S proper, i.e. the WFPC2 deep area. 15 high-redshift galaxies with 2.0 < z < 3.5 (10 in the HDF-S proper) have been identified. The spectroscopic redshifts are in good agreement with the photometric ones derived from a chi(2) minimization technique comparing the observed spectral energy distribution with synthetic libraries and with a new neural network (NN) approach. The dispersion with the former method is sigma(z) = 0.16 whereas the latter provides sigma(z) = 0.13. No catastrophic difference is encountered. The inferred star formation rates of the individual objects range from tens to a few hundreds of M-. yr(-1) and the global star formation rate of the Universe at < z > similar or equal to 2.4 is estimated to be 0.15 M-. yr(-1) Mpc(-3) with a statistical error of 0.04. Evidence for large-scale structure is found with two groups' redshifts observed at z similar or equal to 2.1 and z similar or equal to 2.7 and a pronounced low redshift peak around z similar or equal to 0.58. An elliptical galaxy lensing a background object turns out to be at a redshift z = 0.577.