Tapan. K. Chatterjee

Puebla, Apartado Postal 1316, Puebla, Mexico

It is well known that, under favorable conditions, tidal interactions between a spiral galaxy and a more compact elliptical leads to the formation of embedded rings in the disk; in addition there are also nuclear rings which do not seem to have a tidal origin. The study of many disk-sphere encounters have led to a quadratic relationship between the collision parameters and the radial position of the rings; this relationship is of the parametric form ${\rm r_m^2 \propto \sigma / V^2}$, V being the relative velocity, $\sigma$ the projected density of the intruder and ${\rm r_m}$ the radial position of the density maximum of the ring. The same enables us to estimate the collision velocity (and thus guage the position of the perturber) on the basis of the radial distances at which rings can be formed due to tidal interactions. It implies that such rings cannot form interior to a limiting distance; and thus rings formed interior to this region are not favored to have a tidal origin. This reinforces the different characteristics of the two varieties of rings.