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Newtonian limit of scalar-tensor theories and galactic dynamics: isolated and interacting galaxies

We use the Newtonian limit of a general scalar-tensor theory around a background field to study astrophysical effects. The gravitational theory modifies the standard Newtonian potential by adding a Yukawa term to it, which is quantified by two theoretical parameters: $λ$, the lenghtscale of the gravitational interaction and its strength, $α$. Within this formalism we firstly present a numerical study on the formation of bars in isolated galaxies. We have found for positive $α$ that the modified gravity destabilizes the galactic discs and leads to rapid bar formation in isolated galaxies. Values of $λ$ in the range $\approx 8$ -- 14 kpc produce strongest bars in isolated models. Then, we extent this work by considering tidal effects due to interacting galaxies. We send two spirals to collide and study the bar properties of the remnant. We characterize the bar kinematical properties in terms of our parameters ($λ, α$).