A Galaxy X-Ray Fundamental Plane?

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Abstract
We suggest that the radii and luminosities of the X-ray-emitting halos of elliptical galaxies define a fundamental plane with the star velocity dispersions, as for the corresponding optical observables. Since the X-ray-emitting material usually is at larger radius than the stars, this can be interpreted as additional evidence for a relation between the space distributions of stellar and dark mass, in analogy to the Tully-Fisher relation and flat rotation curves for spiral galaxies. There is, however, a class of elliptical galaxies with low X-ray luminosities for which the validity of the present statement is not clear: these elliptical galaxies are endowed with inconspicuous dark halos, quite unlike the standard picture for spiral galaxies in which universal massive dark halos are observed.
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