Very large telescope-ISAAC near-infrared spectroscopy of ISO-selected hubble deep field south galaxies

We report the results of near-infrared Very Large Telescope (VLT)Infrared Spectrometer and Array Camera (ISAAC) spectroscopy of a sample of 12 galaxies at z = 0.4-1.4, drawn from the ISOCAM survey of the Hubble Deep Field South. We find that the rest-frame R-band spectra of the ISOCAM galaxies resemble those of powerful dust-enshrouded starbursts. Her emission is detected in 11 out of 12 objects down to a flux limit of 7 x 10(-17) ergs cm(-2) s(-1), corresponding to a luminosity limit of 10(41) ergs s(-1) at z = 0.6 (for an H-0 = 50 and Omega = 0.3 cosmology). From the H alpha luminosities in these galaxies we derive estimates of the star formation rate (SFR) in the range 2-50 M. yr(-1) for stellar masses 1-100 M.. The raw H alpha-based SFRs are an order of magnitude or more lower than SFR (far-infrared) estimates based on ISOCAM LW3 fluxes. If the H alpha emission is corrected for extinction, the median offset is reduced to a factor of 3. The sample galaxies are part of a new population of optically faint but infrared-luminous active starburst galaxies, which are characterized by an extremely high rate of evolution with redshift up to z similar to 1.5 and are expected to contribute significantly to the cosmic far-infrared extragalactic background.