A numerical study of interactions and stellar bars

TL;DR

This study uses numerical simulations to compare how stellar bars in disc galaxies form and evolve due to internal processes versus external interactions, revealing differences in bar properties and formation triggers.

Contribution

It provides a detailed numerical comparison of bar development from internal instabilities and external fly-by interactions, highlighting their distinct effects on bar characteristics.

Findings

Bars affected by interactions tend to be slower and more boxy.

Interaction-triggered bars host hotter stellar discs.

Slow fly-by interactions can also induce bar formation.

Abstract

For several decades it has been known that stellar bars in disc galaxies can be triggered by interactions, or by internal processes such as dynamical instabilities. In this work, we explore the differences between these two mechanisms using numerical simulations. We perform two groups of simulations based on isolated galaxies, one group in which a bar develops naturally, and another group in which the bar could not develop in isolation. The rest of the simulations recreate 1:1 coplanar fly-by interactions computed with the impulse approximation. The orbits we use for the interactions represent the fly-bys in groups or clusters of different masses accordingly to the velocity of the encounter. In the analysis we focus on bars' amplitude, size, pattern speed and their rotation parameter, ${\cal R}=R_{CR}/R_{bar}$. The latter is used to define fast (${\cal R}<1.4$) and slow rotation (${\cal R}>1.4$). Compared with equivalent isolated galaxies we find that bars affected or triggered by interactions: (i) remain in the slow regime for longer; (ii) are more boxy in face-on views; (iii) they host kinematically hotter discs. Within this set of simulations we do not see strong differences between retrograde or prograde fly-bys. We also show that slow interactions can trigger bar formation.