Tapan K. Chatterjee

The formation of ring galaxies

AbstractThe 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 radiusRS and the disk galaxy as an exponential disk whose surface density is given by

A dynamical study of the frequency of interacting galaxies

\sigma (r) = \sigma _c e^{ - 4r/R_D }

Potential and force due to thin disk and spherical galaxies

, where σc is the central density andRD 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❘=γDβD, where (GMS2RD)/(V2MDRS2),MS andMD being the masses of the spherical and disk galaxies, respectively, and βD≡βD (n, σ, ɛ,i) is a function of the models of the two galaxies, the ratio of the radii of the two galaxies ɛ=RS/RD, and the angle of inclinationi, of the disk to the direction of relative motion of the two galaxies.Calculations are made for the caseRS=RD. Since practically the entire mass of the spherical galaxy, for the chosen model, lies within 1/3 of its radius, the radius of the spherical galaxy is effectively

Optimum merger time-scales and binary evolution

\tfrac{1}{3}

The formation of a ring galaxy during a head-on collision between a disk and a spherical galaxy

that of the disk galaxy. It is found that as a result of the collision, the innermost and the outer parts of the disk galaxy are not much affected, but the intermediate region expands and gets evacuated, leading to the crowding of stars in a preferential region forming a ring structure. The rings are best formed for a normal, on-axis collision. For this case, rings form when ΔU/|U| lies between