Two Species of the Local Group Dwarf Spheroidals

Abstract

We analyzed 10 dwarf spheroidal galaxies (dSphs) in the Local Group, and found two distinct sequences on the $M_{\mathrm{vir}}/L$ -- $M_{\mathrm{vir}}$ plane: $M_{\mathrm{vir}}/L\propto M_{\mathrm{vir}}^{1.6}$ for $M_{\mathrm{vir}}<10^8{M}_{\odot }$ whereas $M_{\mathrm{vir}}/L\simeq const.$ for $M_{\mathrm{vir}}>10^8{M}_{\odot }$ ($M_{\mathrm{vir}}$ 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 $10^8{M}_{\odot }$ 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.

4 pages LaTeX, using paspconf.sty, 1 eps figure. Contributed poster presented at the IAU Symp. 192, ``The Stellar Content of Local Group Galaxies''