Galactic rotation and the intergalactic medium

The observed spectrum of the isotropic diffuse X-ray background has often been invoked to infer the possible existence of a hot intergalactic medium at a density of the order of that required for closure of the Universe. We consider here the effect of such a hot isotropic gas on the rotation of galaxies, with the coupling between the intergalactic medium and the galaxies being provided by galactic magnetic fields. The calculation provides the basis for an indirect observational test which bears on the existence of the hot intergalactic medium. We point out that the basic physics involved should also pertain to variable X-rays stars emitting by mass accretion and, perhaps, to the Coma cluster of galaxies