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 \lambda _ { eff } = 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 H _ { \alpha } + [ Nii ] .
Additional optical data come from the ESO VLT/FORS2 and NTT/EMMI , primarily targeting [ Oii ] , [ Oiii ] and H _ { \beta } 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 _ { \alpha } line emission is detected in virtually all the observed objects down to a flux limit of 7 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } 
( corresponding to L _ { H _ { \alpha } } > 10 ^ { 41 } erg s ^ { -1 } at z = 0.6 for H _ { o } = 65 , \Omega _ { \Lambda } = 0.7 and \Omega _ { m } = 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 H _ { \alpha } at z=1.57 ) and one type-II quasars ( with inverted [ Nii ] /H _ { \alpha } 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 H _ { \alpha } luminosities indicate star formation rates ( SFR ) in the remaining sources between 0.5 and 20 M _ { \odot } / yr , assuming a Salpeter IMF between 0.1 and 100 M _ { \odot } and without extinction corrections .
We find good correlations between the mid-IR , the radio and H _ { \alpha } luminosities , confirming the mid-IR light as a good tracer of star formation ( while the SFR based on H _ { \alpha } 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 \sim 10 to 300 M _ { \odot } / yr ) .
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 .