Homogeneity of stellar populations in early-type galaxies with different X-ray properties

We have found that 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 predates the epoch when the dichotomy of the potential structure was established.