Changes in sizes and shapes of spherical galaxies in head-on collisions

Head-on collisions of two identical spherical galaxies are studied for two initial velocities, (1) nearly equal to and (2) greater than the capture velocity. Orbits of about 500 representative stars are computed, taking into account the effects of dynamical friction in the motion of the galaxies. From the computer studies the changes in the structure of the galaxies are deduced. The galaxies contract at closest approach and expand as they recede from each other. When the initial velocity is nearly equal to the capture velocity, the mean radius expands to almost double its size and the galaxies have a prolate structure until the closest approach with the longer axis in the direction of motion. The prolate structure is destroyed as the galaxies recede. For larger collision velocity, the mean radius expands by 50% and the galaxies are prolate until the closest approach, becoming distinctly oblate after the collision. The fractional increase in the binding energy is 0.46 in the first case and 0.30 in the second case.