Chemical Compositions, Colors, and the Hubble Sequence (Proceedings of Japan-France Seminar on Chemical Evolution of Galaxies with Active Star Formation)

Chemical and photometric properties of galaxies with different Hubble type are computed based on an expanded method of evolutionary population synthesis. The comparison of model results with the observed properties reveals that the initial mass function (IMF) is universal (μ≅1) for all Hubble types except irregulars and the star formation rate (SFR) per unit mass decreases from ellipticals to irregulars. High SFRs induce a supernovae-driven wind at t<1Gyr in ellipticals and at 1Gyr<t<(T_G-1)Gyr in SQs, respectively; whereas SFRs are too low to expell the gas from galaxies within a galactic age T_G=15Gyr in spirals and irregulars. This well explains why there are two different sequences of galaxies; i.e., a sequence of gas-poor galaxies and that of gas-rich ones