The role of impact parameter in typical close galaxy flybys

Abstract

Close galaxy flybys, interactions during which two galaxies inter-penetrate, are frequent and can significantly affect the evolution of individual galaxies. Equal-mass flybys are extremely rare and almost exclusively distant, while frequent flybys have mass ratios 0.1 or lower, with a secondary galaxy penetrating deep into the primary. This can result in comparable strengths of interaction S between the two classes of flybys and lead to essentially the same effects. To demonstrate this, emphasize and explore the role of the impact parameter $b$, we performed a series of N-body simulations of flybys with varying relative $b$ ranging from 0.114 to 0.272 of the virial radius of the primary. Two-armed spirals form during flybys, with radii of origin correlated with $b$ and strengths well approximated with an inverted S-curve. The impact parameter does not affect the shape of induced spirals, and the lifetimes of a distinguished spiral structure appear to be constant, 2 Gyr. Bars, with strengths anti-correlated with $b$, form after the encounter is over in simulations with interaction strengths $S\geq0.076$, but they are short-lived except for the stronger ones with $S\geq0.129$. We showcase an occurrence of double bar that survives for a long time in one of the simulations. Effects on the pre-existing bar instability are diverse. There is no uniform correlation between these effects and $b$, as they are secondary effects, happening later in a post-flyby stage. Bulges are resilient to flybys, while dark matter halos can significantly spin up in the amount anti-correlated with $b$. There is an offset angle between the angular momentum vector of the dark matter halo and that of a disc, and it correlates linearly with b. Flybys remain an important pathway for structural evolution within galaxies in the local Universe.