The optical spectra of 24 μm galaxies in the COSMOS field. I. Spitzer MIPS bright sources in the zCOSMOS-bright 10k catalog

We study zCOSMOS-bright optical spectra for 609 Spitzer MIPS 24 μm-selected galaxies with S_{24 μm} > 0.30 mJy and I < 22.5 (AB mag) over 1.5 deg² of the COSMOS field. From emission-line diagnostics we find the following: (1) SFRs derived from the observed Hα λ6563 and Hβ λ4861 lines underestimate, on average, the total SFR by factors of ∼5 and 10, respectively. (2) Both the Calzetti et al. and the Milky Way reddening laws are suitable to describe the extinction observed in IR sources in most cases. (3) Some IR galaxies at z < 0.3 have low abundances, but many others with similar IR luminosities and redshifts are chemically enriched. (4) The average [O III] λ5007/Hβ λ4861 ratios of vL_v^{24 μm} > 10¹¹ L_⊙ galaxies at 0.6 < z < 0.7 are ∼0.6 dex higher than the average ratio of all zCOSMOS galaxies at similar redshifts. Massive star formation and AGNs could simultaneously be present in those galaxies with the highest ionizing fluxes. (5) Roughly of the galaxies with metallicity measurements at 0.5 < z < 0.7 lie below the general mass-metallicity relation at the corresponding redshifts. The strengths of the 4000 Å break and the Hδ EW of our galaxies show that secondary bursts of star formation are needed to explain the spectral properties of most IR sources. The LIRG and ULIRG phases occur, on average, between 10⁷ and 10 ⁸ yr after the onset of a starburst on top of underlying older stellar populations. These results are valid for galaxies of different IR luminosities at 0.6 < z < 1.0 and seem independent of the mechanisms triggering star formation.