Infrared spectroscopy of faint 15 mu m sources in the Hubble Deep Field South: first hints at the properties of the sources of the IR background

We present a spectroscopic analysis of a sample of 21 galaxies with z = 0.2-1.5 drawn from a 25 square arcmin ultra-deep ISOCAM survey at lambdaeff =15 mu m centered in the WFPC-2 Hubble Deep Field South. Near-infrared spectra are reported for 18 ISO sources, carried out with ISAAC on the VLT, aimed at detecting the redshifted Halpha +[NII]. Additional optical data come from the ESO VLT/FORS2 and NTT/EMMI, primarily targeting [OII], [OIII] and Hbeta for further physical insight. Although not numerous in terms of areal density in the sky, this population of very luminous IR sources has been recently found to be responsible for a substantial fraction of the extragalactic background light energy density. Halpha line emission is detected in virtually all the observed objects down to a flux limit of 7 x 10-17 erg cm-2 s-1 (corresponding to LH_alpha > 1041 erg s-1 at z = 0.6 for H0 = 65, OmegaLambda=0.7 and Omegam = 0.3). Our analysis (including emission line, morphology, and SED properties) shows clear evidence for AGN activity in only two of these sources: one type-I (with broadened Halpha at z=1.57) and one type-II quasars (with inverted [NII]/Halpha ratio at z=1.39), while we suspect the presence of an AGN in two further sources (an Ultra-Luminous IR Galaxy, ULIRG, at z=1.27 and a luminous galaxy at z=0.69). The Halpha luminosities indicate star formation rates (SFR) in the remaining sources between 0.5 and 20 Msun/yr, assuming a Salpeter IMF between 0.1 and 100 Msun and without extinction corrections. We find good correlations between the mid-IR, the radio and Halpha luminosities, confirming the mid-IR light as a good tracer of star formation (while the SFR based on Halpha flux show some large scatter and offset, which are still to be understood). We have estimated the baryonic masses in stars with a newly-developed tool fitting the overall optical-IR continuum, and found that the host galaxies of ISO sources are massive members of groups with typically high rates of SF (SFR ~ 10 to 300 Msun/yr). We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales tSF for the stellar build-up, which turn-out to be widely spread in these objects between 0.1 Gyrs to more than 10 Gyr. The faint ISOCAM galaxies appear to form a composite population, including moderately active but very massive spiral-like galaxies, and very luminous ongoing starbursts, in a continuous sequence. From the observed tSF and assuming typical starburst timescales, we infer that, with few exceptions, only a fraction of the galactic stars can be formed in any single starburst event, while several of such episodes during a protracted SF history are required for the whole galactic build-up