Relative sizes of X-ray and optical images of elliptical galaxies: Correlation with X-ray luminosity

Optical parameters of elliptical galaxies are tightly correlated, but their X-ray parameters vary widely. The X-ray luminosity L-X ranges over more than 1 order of magnitude for ellipticals having similar optical luminosity L-B. The source of this scatter has been elusive. We show here that the dispersion in L-X for fixed optical luminosity L-B correlates strongly with the dimensionless ratio of the sizes of the X-ray and optical images, r(eX)/r(e). A distance-independent variant of this correlation is L-X/L-B proportional to (r(eX)/r(e))(0.60+/-0.30). This correlation may be a natural result of mergings and tidal truncations that occurred during the early formation of ellipticals in groups of galaxies. The shapes of the X-ray images also vary: some are compact (e.g., NGC 4649, 7626, 5044), others diffuse (e.g., NGC 4636, 1399).