  We suggest a possible mechanism of ultra diffuse galaxy formation: the UDGs
may occur as a result of a central collision of galaxies. If the galaxies are
young and contain a lot of gas, the collision may kick all the gas off the
systems and thus strongly suppress any further star formation. As a result, the
galaxies now have a very low surface brightness and other properties typical of
the ultra diffuse galaxies. We use the Coma cluster (where numerous UDGs were
recently discovered) to test the efficiency of the process. The mechanism works
very well and can transform a significant fraction of the cluster population
into ultra diffuse galaxies. The UDGs formed by the process concentrate towards
the center of the cluster, and their globular cluster systems remain undamaged,
in accordance with observational results. The projected surface density of UDGs
in the cluster may help us to recognize the mechanism of UDG formation, or
clarify relative contributions of several possible competitive mechanisms at
work.
