Tidal Enhancement of Internal Structures

Extract

Observational evidence indicates that the frequency of internal structures (such as rings, bars, spirals, etc.) is preferentially enhanced for spirals in binary systems (e.g. Elmegreen et al. 1990). In this context we study the tidal effects produced due to two spiral galaxies in a near grazing relative orbit with a near parabolic relative velocity, as such barely bound encounters are the most frequent ones. Both the spirals are endowed with an inner and an outer ring.

The spiral galaxies (of radius R) are modeled by an exponential disk of scale length α = 4/R, with a (static) thickness (Chatterjee 1990), and a spherical polytropic bulge (n=0, 3, 4 equally weighted combination) containing 1/3 of the mass; about 10% of the mass of the disk contains gas particles. Two polytropic rings (n=1; Ostriker 1964), are projected on the disk at 1/4 (inner) and 3/4 (outer) of its radius. Though the rings are gaseous, they are here treated as density enhancements. The gravitational potential is softened with softening constants of ∊ = r_∘/5, r_∘/3, and 0.8r_∘, for the bulge, stellar and gaseous components of the disk, respectively. Here r_∘ is the radius containing 75% of the total mass of the spiral. The softening constant for the polytropic rings is 0.35, while the mutual gravitational interaction is softened with a softening constant of r_∘/4.