Type I shell galaxies as a test of gravity models

TL;DR

This study compares shell formation in Type I shell galaxies across different gravity models, revealing how dark matter and alternative theories influence shell properties and formation timescales, providing potential observational tests for gravity theories.

Contribution

It offers the first theoretical comparison of shell formation in different gravity models, including Newtonian with dark matter, MOG, and MOND, highlighting their distinct effects on shell dynamics.

Findings

Dark matter models produce shorter shell formation timescales.

Non-Newtonian models allow for multi-generation shell formation.

Shell distances and velocities can distinguish between gravity theories.

Abstract

Shell galaxies are understood to form through the collision of a dwarf galaxy with an elliptical galaxy. Shell structures and kinematics have been noted to be independent tools to measure the gravitational potential of the shell galaxies. We compare theoretically the formation of shells in Type I shell galaxies in different gravity theories in this work because this is so far missing in the literature. We include Newtonian plus dark halo gravity, and two non-Newtonian gravity models, MOG and MOND, in identical initial systems. We investigate the effect of dynamical friction, which by slowing down the dwarf galaxy in the dark halo models limits the range of shell radii to low values. Under the same initial conditions, shells appear on a shorter timescale and over a smaller range of distances in the presence of dark matter than in the corresponding non-Newtonian gravity models. If galaxies are embedded in a dark matter halo, then the merging time may be too rapid to allow multi-generation shell formation as required by observed systems because of the large dynamical friction effect. Starting from the same initial state, in the dark halo model the observation of small bright shells should be accompanied by large faint ones, while for the case of MOG, the next shell generation patterns iterate with a specific time delay. The first shell generation pattern shows a degeneracy with the age of the shells and in different theories, but the relative distance of the shells and the shell expansion velocity can break this degeneracy.