Stabilizing a Cold Disk with a 1/r Force Law

Extract

Massive “dark” halos are currently believed to surround individual galaxies and systems of galaxies primarily because: (a) individual galaxies exhibit “flat” rather than Keplerian rotation curves (see the review by Faber and Gallagher 1979); (b) the measured ratio of mass to light in systems of galaxies continues to increase roughly linearly with radius as one observes larger and larger systems (Rood 1982); and (c) rapidly spinning “cold” stellar disks are not dynamically stable if they are fully self-gravitating (Ostriker and Peebles 1973). It can be shown, however, that properties (a) and (b) of galaxies and systems of galaxies can be explained without invoking dark halos if one assumes that the force of gravity has the form where r is the distance between the attracting bodies of masses M and m, and the scale “a” must assume a value a ∼ 1 kpc. (See Milgrom 1982 and Bekenstein, these proceedings, for a discussion along similar lines.) Newton's Law is retrieved on scales « a but the force is proportional to 1/r on scales » a. Equation (1) should not be adopted in place of Newtonian gravity to explain the dynamical properties of galaxies unless it also explains property (c) without invoking the presence of dark halos.