Homogeneity of Stellar Populations in Early-Type Galaxies with Different   X-ray Properties
  We have found the stellar populations of early-type galaxies are homogeneous with no significant difference in color or Mg2 index, despite the dichotomy between X-ray extended early-type galaxies and X-ray compact ones. Since the X-ray properties reflect the potential gravitational structure and hence the process of galaxy formation, the homogeneity of the stellar populations implies that the formation of stars in early-type galaxies predat es the epoch when the dichotomy of the potential structure was established. 
T. Kodama (Durham, UK), K. Matsushita (Tokyo Metropolitan, Japan)