The formation of faint ring structures

Abstract

In a previous paper (Chatterjee, 1984), we studied the formation of ring galaxies due to head-on collisions between disk and spherical galaxies. The rings so formed are quite prominent and well defined. However, in the domain of rings, there are many faint rings which are ill defined and are so faint that they lie on the borderline of detectability. In this paper the formation of such rings is investigated.

Using the impulsive approximation technique, in this paper we study an off-center collision between an exponential model disk galaxy and a polytropic (n=4) model spherical galaxy, in which the spherical galaxy moves normal to the plane of the disk galaxy and grazes its periphery. The radii of the two galaxies are taken to be equal, but due to the high central concentration of the spherical galaxy its radius is effectively one-third that of the disk galaxy. It is found that, as a result of the collision, the disk galaxy expands and attains an oblate shape. If the fractional change in binding energy of the disk galaxy, ΔU/|U|, lies between 0.001 to 0.01, then a faint ring structure is formed which does not close on itself. If ΔU/|U|∼0.01, then in addition to the faint ring a bridge and tail are formed. The ability of such low-energy collisions to affect even minor structural changes in the disk is due to the preferentially higher energy changes occurring in the side nearer to the perturbing galaxy than in the opposite side. Consequently, the structural changes produced are asymmetric.

Such a faint ring may elude detection on photographs, but it will be revealed by sensitive photometric observations. There are several examples of such faint ring structures embedded in disks, which have been detected by observation.