We report the results of near-infrared VLT-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 .
H _ { \alpha } emission is detected in 11 out of 12 objects down to a flux limit of 7 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } , corresponding to a luminosity limit of 10 ^ { 41 } erg s ^ { -1 } at z = 0.6 , ( for an H _ { o } = 50 and \Omega = 0.3 cosmology ) .
From the H _ { \alpha } luminosities in these galaxies we derive estimates of the star formation rate in the range 2–50 M _ { \odot } / yr for stellar masses 1–100 M _ { \odot } .
The raw H _ { \alpha } –based star formation rates are an order of magnitude or more lower than SFR ( FIR ) 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 \sim 1.5 and expected to contribute significantly to the cosmic far-IR extragalactic background .