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

TL;DR

This study demonstrates that the 4000 Å break strength (Dn(4000)) can serve as an effective proxy for molecular gas fraction in galaxies at z~0.1-0.2, revealing a tight anti-correlation and providing insights into galaxy evolution.

Contribution

It introduces a novel Dn(4000)-based selection method for observing molecular gas in galaxies and confirms the relation across different redshifts, enhancing understanding of galaxy evolution.

Findings

Successful detection of CO in 8 out of 12 galaxies.

Confirmed tight anti-correlation between Dn(4000) and molecular gas fraction.

Galaxy evolution follows the Dn(4000)-fmol relation over cosmic time.

Abstract

We performed 12CO(J=1-0) (hereafter, CO) observations towards 12 normal star-forming galaxies with stellar mass of Mstar=10^10.6-10^11.3 Msun at z=0.1-0.2 with the 45-m telescope at the Nobeyama Radio Observatory (NRO). The samples are selected with Dn(4000) that is a strength of the 4000 \AA break, instead of commonly used far-infrared (FIR) flux. We successfully detect the CO emissions from eight galaxies with signal-to-noise ratio (S/N) 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 of nearby galaxies. This implies that the galaxies evolve along this Dn(4000)-fmol relation, and that Dn(4000) seems 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.