The optical spectra of Spitzer 24 micron galaxies in the COSMOS field:
  II. Faint infrared sources in the zCOSMOS-bright 10k catalogue
  We have used the zCOSMOS-bright 10k sample to identify 3244 Spitzer/MIPS
24-micron-selected galaxies with 0.06< S(24um)< 0.50 mJy and I(AB)<22.5, over
1.5 deg^2 of the COSMOS field, and studied different spectral properties,
depending on redshift. At 0.2<z<0.3, we found that different reddening laws of
common use in the literature explain the dust extinction properties of around
80% of our infrared (IR) sources, within the error bars. For up to 16% of
objects, instead, the Halpha/Hbeta ratios are too high for their IR/UV
attenuations, which is probably a consequence of inhomogenous dust
distributions. In only a few of our galaxies at 0.2<z<0.3 the IR emission could
be mainly produced by dust heated by old rather than young stars. Besides, the
line ratios of ~22% of our galaxies suggest that they might be
star-formation/nuclear-activity composite systems. At 0.5<z<0.7, we estimated
galaxy metallicities for 301 galaxies: at least 12% of them are securely below
the upper-branch mass-metallicity trend, which is consistent with the local
relation. Finally, we performed a combined analysis of the Hdelta
equivalent-width versus Dn(4000) diagram for 1722 faint and bright 24um
galaxies at 0.6<z<1.0, spanning two decades in mid-IR luminosity. We found
that, while secondary bursts of star formation are necessary to explain the
position of the most luminous IR galaxies in that diagram, quiescent,
exponentially-declining star formation histories can well reproduce the
spectral properties of ~40% of the less luminous sources. Our results suggest a
transition in the possible modes of star formation at total IR luminosities
L(TIR)=(3 +/-2)x10^11 Lsun.
