X-ray observations reveal extended halos around early-type galaxies which enable us to trace the dark matter distribution around the galaxies (see Mathews and Brighenti 2003 for a review). X-ray luminosities, LX of massive early-type galaxies are 1040−1042 erg s−1 in 0.3–2 keV. The correlation plot between LX and B-band luminosity LB shows a large scatter in the sense that LX varies by 2 orders of magnitudes for the same LB, in the brightest end (log LB ≳ 10.5). The amount of the X-ray hot gas in early-type galaxies is typically a few % of the stellar mass, in contrast to clusters of galaxies which hold ~5 times more massive gas than stars. Matsushita (2001) showed that X-ray luminous galaxies are characterized by extended X-ray halo with a few tens of re, similar to the scale of galaxy groups, so the presence of group-size potentials would be strongly linked with the problem of large LX scatter.
Semianalytic Predictions for Statistical Properties of X-Ray Clusters of Galaxies in Cold Dark Matter Universes