Origin Of Tidal Structures In Modified Gravity
  The missing mass problem has not been solved decisively yet. Observations
show that if gravity is to be modified, then the MOND theory is its excellent
approximation on galactic scales. MOND suggests an adjustments of the laws of
physics in the limit of low accelerations. Comparative simulations of
interacting galaxies in MOND and Newtonian gravity with dark matter revealed
two principal differences: 1) galaxies can have close flybys without ending in
mergers in MOND because of weaker dynamical friction, and 2) tidal dwarf
galaxies form very easily in MOND. When this is combined with the fact that
many interacting galaxies are observed at high redshift, we obtain a new
perspective on tidal features: they are often formed by non-merging encounters
and tidal disruptions of tidal dwarf galaxies. Here we present the results from
our self-consistent MOND $N$-body simulation of a close flyby of two galaxies
similar to the Milky Way. It turns out that most types of the structures that
are traditionally assigned to galaxy mergers can be formed by non-merging
encounters, including tidal arms, bridges, streams, shells, disk warps, thick
disks, and most probably also disks of satellites. The success of MOND in
explaining the dynamics of galaxies hints us that this way of formation of
tidal structures should be considered seriously.
