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 (M-vir)-metallicity relation among the Local Group dwarf spheroidal (dSph) galaxies is examined, Hirashita, Takeuchi, & Tamura showed that the dSph galaxies can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass (M-vir less than or similar to 10(8) M.) and high-mass (M-vir greater than or similar to 108 M.) groups. We see that both the mass-metallicity and mass-luminosity relations of the high-mass dSph galaxies are understood as a low-mass extension of giant elliptical galaxies. On the contrary, we find that the classical galactic wind model is problematic when applied to the low-mass dSph galaxies, 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 dSph galaxies 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 dSph galaxies but also suggest that the mass-metallicity and mass-luminosity relations be understood in a consistent context.