Tidal interactions and the massive halo hypothesis

Abstract

THE idea that galaxies may be surrounded by large amounts of unseen material has become very popular. The arguments favouring such massive haloes are almost all based on the dynamical analysis of galaxies either individually or in groups, and they have been cogently assembled by Einasto et al.¹ and by Ostriker et al.². They concluded that galaxies may well be surrounded by large (∼ 100 kpc) or very large (∼ 0.5 Mpc) ‘isothermal haloes’ whose mass varies as the radius within which it is measured. (We take H₀ = 50 km s⁻¹ Mpc⁻¹). Their views have subsequently received support from Arecibo observations which found flat HI rotation curves to very large radii for several spiral galaxies³, and from analyses of the dynamics of new samples of binary galaxies⁴ and of groups of galaxies⁵. The most extreme form of the massive halo hypothesis postulates very large haloes which are the dominant constituent of the Universe, and in which the observed galaxy is little more than contaminating debris left over from the formation of the unseen object⁶. It is disquieting that there seems to be little chance of direct observation of this dark material, and this letter points out that dynamical constraints can be put on either the extent or the universality of galactic haloes.