Origin Of Tidal Structures In Modified Gravity

TL;DR

This paper demonstrates through MOND N-body simulations that many tidal structures traditionally attributed to galaxy mergers can also form via non-merging encounters, challenging conventional merger-based formation theories.

Contribution

It provides the first self-consistent MOND N-body simulation of a Milky Way-like galaxy flyby, showing formation of various tidal features without mergers.

Findings

Tidal structures like arms, streams, and shells can form in non-merging galaxy encounters.

MOND predicts weaker dynamical friction, allowing close flybys without mergers.

Many observed tidal features may result from non-merging interactions, not just mergers.

Abstract

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.