Two Species of the Local Group Dwarf Spheroidals

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We analyzed 10 dwarf spheroidal galaxies (dSphs) in the Local Group, and found two distinct sequences on the M vir / L -- M vir plane: M vir / L ∝ M vir¹.⁶ for M vir < 10⁸ M whereas M vir / L ≃ const. for M vir > 10⁸ M (M vir and L are the virial mass and the total luminosity of a dSph, respectively). We interpret the disconuity as the threshold for the gas in dSphs to be blown away by successive supernovae. We succeeded to give a quantitative explanation of the discontinuity mass of 10⁸M as blow-away condition. We further derived the above relation for the low-mass dSphs, assuming that the initial star formation rate of the dSphs is proportional to the inverse of the cooling time. The relation of high-mass dSphs is also explained along with the same consideration, with the condition that the gas cannot been blown away effectively.