Infrared spectroscopy of faint 15 μm sources in the Hubble Deep Field South: First hints at the properties of the sources of the IR background

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Abstract
We present a spectroscopic analysis of a sample of 21 galaxies with drawn from a 25 square arcmin ultra-deep ISOCAM survey at 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 H[Nii]. Additional optical data come from the ESO VLT/FORS2 and NTT/EMMI, primarily targeting [Oii], [Oiii] and H β 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. H α line emission is detected in virtually all the observed objects down to a flux limit of erg cm-2 s-1 (corresponding to erg s-1 at for , and ). 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 H α at ) and one type-II quasars (with inverted [Nii]/H α ratio at ), while we suspect the presence of an AGN in two further sources (an Ultra-Luminous IR Galaxy, ULIRG, at and a luminous galaxy at ). The H α luminosities indicate star formation rates (SFR) in the remaining sources between 0.5 and 20 , assuming a Salpeter IMF between 0.1 and 100 and without extinction corrections. We find good correlations between the mid-IR, the radio and H α luminosities, confirming the mid-IR light as a good tracer of star formation (while the SFR based on H α 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 ( to 300 ). We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales t SF 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 t SF 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.
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