Relative Sizes of X-ray and Optical Images of Elliptical Galaxies; Correlation with X-ray Luminosity
Abstract
Optical parameters of elliptical galaxies are tightly correlated, but their x-ray parameters vary widely. The x-ray luminosity Lx ranges over more than an order of magnitude for ellipticals having similar optical luminosity LB. The source of this scatter has been elusive. We show here that the dispersion in Lx for fixed optical luminosity LB correlates strongly with the dimensionless ratio of the sizes of the x-ray and optical images, rex/re. Specifically, we find that (Lx/LB) is proportional to (rex/re)0.60 0.30, a version of the correlation that is independent of distance. This correlation may be a natural result of mergings and tidal truncations that are expected during the formation and early evolution of ellipticals in groups of galaxies. The radial structure of x-ray images also varies: some are compact (e.g. NGC 4649, 7626, 5044), others diffuse (e.g. NGC 4636, 1399).
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