The optical spectra of spitzer 24 μm galaxies in the cosmic evolution survey field. II. Faint infrared sources in the zCosmos-Bright 10k catalog

We have used the zCOSMOS-bright 10k sample to identify 3244 Spitzer/MIPS 24 μm-selected galaxies with 0.06 mJy < S _{24 μm} ≲ 0.50 mJy and I _AB < 22.5, over 1.5 deg² 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 80% of our infrared (IR) sources, within the error bars. For up to 16% of objects, instead, the Hα λ6563/Hβ λ4861 ratios are too high for their IR/UV attenuations, which is probably a consequence of inhomogeneous 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 H _δ equivalent width versus D_n (4000) diagram for 1722 faint and bright 24 μm 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) × 10¹¹ L _·.