  We present a spectroscopic analysis of 21 galaxies with z=0.2-1.5 drawn from
a 25 sq. arcmin ultra-deep 15 micron ISOCAM survey centered in the HDFS.
VLT/ISAAC spectra are reported for 18 sources, aimed at detecting the
redshifted Ha. Optical data come from VLT/FORS2 and NTT/EMMI. Ha line emission
is detected in all the observed objects. Our analysis of emission lines,
morphology, and SEDs shows evidence for AGN in only two of these sources. The
Ha luminosities indicate star formation rates (SFR) between 0.5 and 20 Msun,
without extinction corrections. We find good correlations between the mid-IR,
the radio and Ha luminosities, confirming the mid-IR light as a good tracer of
star formation (while SFR based on Ha flux show large scatter and offset, still
to be understood). We have estimated the baryonic masses in stars by fitting
the optical-IR SED, and found that the host galaxies of ISO sources are massive
members of groups with SFR=10 to 300 Msun/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
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), assuming typical starburst timescales, we
infer that, with few exceptions, only a fraction of the galactic stars can be
formed in any single starburst, while several of such episodes during a
protracted SF history are required for the whole galactic build-up.
