We have used the zCOSMOS-bright 10k sample to identify 3244 Spitzer/MIPS 24 \mu m -selected galaxies with 0.06 < S _ { 24 \mu m } \raisebox { -3.698858 pt } { ~ { } \shortstack { $ < $ \ [ -0.07 cm ] $ \sim$ } } ~ { } 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 \sim 80 % of our infrared ( IR ) sources , within the error bars .
For up to 16 % of objects , instead , the H \alpha \lambda 6563 / H \beta \lambda 4861 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 \sim 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 H _ { \delta } equivalent-width versus D _ { n } ( 4000 ) diagram for 1722 faint and bright 24 \mu m 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 \sim 40 % of the less luminous sources .
Our results suggest a transition in the possible modes of star formation at total IR luminosities L _ { TIR } \approx ( 3 \pm 2 ) \times 10 ^ { 11 } L _ { \odot } .