The optical spectra of Spitzer 24 micron galaxies in the COSMOS field: II. Faint infrared sources in the zCOSMOS-bright 10k catalogue

TL;DR

This study analyzes the spectral properties of faint infrared galaxies in the COSMOS field, revealing dust extinction behaviors, metallicity trends, and star formation modes across different redshifts and luminosities.

Contribution

It provides a detailed spectral analysis of faint IR galaxies, highlighting dust, metallicity, and star formation characteristics, and identifies a transition in star formation modes at specific IR luminosities.

Findings

Dust extinction explained by common reddening laws for 80% of sources.

Approximately 16% of galaxies show high Halpha/Hbeta ratios due to dust distribution.

A transition in star formation modes occurs at IR luminosity of about 3x10^11 Lsun.

Abstract

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.