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Astronomy Astrophysics

By E. Vanzella, S. Cristiani, S. Arnouts, M. Dennefeld, A. Fontana, A. Grazian, M. Nonino, P. Petitjean and P. Saracco


Abstract. We report on low-resolution multi-object spectroscopy of 65 objects from I(AB) � 20 to I(AB) � 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 χ 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 σz = 0.16 whereas the latter provides σz = 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> � 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 � 2.1 andz � 2.7 and a pronounced low redshift peak around z � 0.58. An elliptical galaxy lensing a background object turns out to be at a redshift z = 0.577

Year: 2009
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