Collisions of young disc galaxies in the early universe
  In the local universe, disc galaxies are generally well evolved and Toomre
stable. Their collisions with satellite galaxies naturally produce ring
structures, which has been observed and extensively studied. In contrast, at
high redshifts, disc galaxies are still developing and clumpy. These young
galaxies interact with each other more frequently. However, the products of
their collisions remain elusive. Here we systematically study the minor
collisions between a clumpy galaxy and a satellite on orbits with different
initial conditions, and find a new structure that is different from the local
collisional ring galaxies. The clumpness of the target galaxy is fine-tuned by
the values of Toomre parameter, $Q$. Interestingly, a thick and knotty ring
structure is formed without any sign of a central nucleus in the target galaxy.
Our results provide a promising explanation of the empty ring galaxy recently
observed in R5519 at redshift $z=2.19$. Moreover, we show that the clumpy state
of the collided galaxy exists for a much longer timescale, compared to isolated
self-evolved clumpy galaxies that have been widely investigated.
