  Ring galaxies are rare in the Universe. A head-on or off-centre collision
between an intruder galaxy and a disc galaxy leads to a collisional ring galaxy
(CRG) when the intruder-to-target mass ratio (ITMR) is over 0.1 in Newtonian
dynamics. Within the framework of Milgromian dynamics, the strong equivalence
principle is violated due to the presence of an external field. When two
galaxies collide towards each other, the dynamical mass of the phantom dark
halo in a galaxy is suppressed by the external field induced by the other
galaxy. As a consequence of such suppression, the gravitational perturbation
for the target galaxy introduced by a collision is weakened. In this case, a
minor collision may not be capable of generating a CRG. In this work, we
address this issue and perform a series of numerical simulations of collisions
by tuning the values of ITMR. We find that the critical ITMR is 0.5 in MOND,
which is much larger than that in Newtonian dynamics. The observed massive ring
galaxies, such as Arp 147, can be effectively interpreted by CRGs in MOND. This
interpretation does not necessitate the presence of dark matter halos for
either the target or intruder galaxies. Moreover, for a larger inclination
angle or a larger impact parameter, the off-centred ring structure is fainter.
The larger critical ITMR indicates that it is harder to form a CRG in Milgrom's
Modified Newtonian Dynamics (MOND). To account for the observed ring structures
of the NGC 922-like galaxies in MOND, it is necessary to invoke other scenarios
than a dry minor collision.
