CO emissions from optically selected galaxies at z∼0.1-0.2: Tight anti-correlation between molecular gas fraction and 4000A break strength

Abstract

We performed 12CO(J=1-0) (hereafter, CO) observations toward 12 normal star-forming galaxies with stellar masses of M∗=1010.6-1011.3M∗ at z=0.1-0.2 with the 45m telescope at the Nobeyama Radio Observatory. The samples were selected with Dn(4000), that is, the strength of the 4000A break, instead of the commonly used far-infrared (FIR) flux. We successfully detected the CO emissions from eight galaxies with signal-to-noise ratio larger than three, demonstrating the effectiveness of the Dn(4000)-based sample selection. For the first time, we find a tight anti-correlation between Dn(4000) and molecular gas fraction (fmol) using literature data of nearby galaxies in which the galaxies with more fuel for star formation have younger stellar populations. We find that our CO-detected galaxies at z∼0.1-0.2 also follow the same relation as nearby galaxies. This implies that the galaxies evolve along this Dn(4000)-fmol relation, and that Dn(4000) seems to be able to be used as a proxy for fmol, which requires many time-consuming observations. Based on the comparison with the model calculation with a population synthesis code, we find that star formation from metal enriched gas and its quenching in the early time are necessary to reproduce galaxies with large Dn(4000) and non-zero gas fraction.