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<~10⁸ M_solar) and high-mass (M_vir>~10⁸ M_solar) 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.