Mass–Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range

The virial mass (Mvir)–metallicity relation among the Local Group dwarf spheroidal galaxies (dSphs) is examined. Hirashita, Takeuchi, & Tamura showed that the dSphs can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass (Mvir . 10M⊙) and highmass (Mvir & 10M⊙) groups. We see that both the mass–metallicity and the mass– luminosity relations of the high-mass dSphs are understood as a low-mass extension of giant ellipticals. On the contrary, we find that the classical galactic-wind model is problematic to apply to the low-mass dSphs, whose low binding energy is comparable to that released by several supernova explosions. A strongly regulated star formation in their formation phase is required to reproduce their observed metallicity. Such regulation is naturally expected in a gas cloud with the primordial elemental abundance according to Nishi & Tashiro. A significant scatter in the mass–metallicity relation for the low-mass dSphs is also successfully explained along with the scenario of Hirashita and coworkers. We not only propose a new picture for a chemical enrichment of the dSphs, but also suggest that the mass–metallicity and the mass–luminosity relations be understood in a consistent context. Subject headings: galaxies: dwarf — galaxies: evolution — galaxies: formation — galaxies: fundamental parameters — Local Group