We find a tight anti-correlation between Dn(4000), the strength of the 4000 ĒŠ break, and
molecular gas fraction (fmol) using literature data of z~0-2 galaxies suggesting that
galaxies with more fuel for star formation have younger stellar populations. The
relation is independent with respect to redshifts of galaxies. This implies that
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. A
population synthesis model can explain the trend qualitatively, but not
quantitatively. Our results suggest 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. This talk is based on Morokuma-Matsui et al., 2015, PASJ, 67, 36
and Morokuma-Matsui et al. in prep..