The formation of ring galaxies

Abstract

The conditions under which a head-on collision between a disk galaxy and a spherical galaxy can lead to ring formation are investigated, using the impulsive approximation. The spherical galaxy is modeled as a polytrope of indexn=4 and radiusR S and the disk galaxy as an exponential disk whose surface density is given by\(\sigma (r) = \sigma _c e^{ - 4r/R_D } \), where σ c is the central density andR D is the radius of the disk. The formation and properties of the rings are closely related to the fractional change in binding energy of the disk galaxy, given by ΔU/❘U❘=γ D β D , where (GM S 2 R D )/(V2M D R S 2 ),M S andM D being the masses of the spherical and disk galaxies, respectively, and β D ≡β D (n, σ, ɛ,i) is a function of the models of the two galaxies, the ratio of the radii of the two galaxies ɛ=R S /R D , and the angle of inclinationi, of the disk to the direction of relative motion of the two galaxies.