Dwarf Satellite Galaxies in the Modified Dynamics

Abstract

In the modified Newtonian dynamics (MOND) the inner workings of dwarf satellites can be greatly affected by their mother galaxy—over and beyond tidal effects. Because of MOND's nonlinearity, a system's internal dynamics can be altered by an external field in which it is immersed (even when this field, by itself, is constant in space). As a result, the size and velocity dispersion of the satellite vary as the external field varies along its orbit. A notable outcome of this is a substantial increase in the dwarf's vulnerability to eventual tidal disruption—rather higher than Newtonian dynamics (with a dark matter halo) would lead us to expect for a satellite with given observed parameters. The space of system parameters of the dwarf may be divided according to three main criteria: It can be either external- or internal-field dominated; it can be in the adiabatic or in the impulsive regime; and it can be in the tidal or nontidal regime. The Milky Way's dwarf satellites populate all these regions of parameter space, and a single dwarf on an eccentric orbit can traverse several of them. The situation is particularly transparent in the external-field dominated, adiabatic regime where the dynamics can be described analytically: because of the variation in the external field strength with the galactocentric distance of the dwarf, R, its size changes as R-1, and the internal velocities change as R. As the dwarf approaches the center it puffs up, becoming susceptible to tidal disruption. Adiabaticity is lost roughly at the same R at which tidal effects become important. The behavior near and in the impulsive regime is studied numerically.