Bar Formation During a Gaia-Sausage-Enceladus-like Merger Event

TL;DR

This study investigates how galaxy mergers, especially those similar to Gaia-Sausage-Enceladus, influence the formation and survival of galactic bars through N-body simulations, revealing that merger mass ratio significantly affects bar outcomes.

Contribution

It provides the first detailed analysis of how specific merger parameters impact bar formation and survival, highlighting the dominant role of mass ratio.

Findings

Bar survival decreases with higher merger mass ratios.

Mergers with low mass ratios generally preserve bars.

Intermediate ratios can weaken bars over time.

Abstract

Bars are among the most prominent galactic structures, yet their formation mechanisms remain incompletely understood. They can form either internally, via dynamical instabilities, or externally, triggered by interactions with other galaxies. The impact of mergers on bar formation and survival, however, has not been thoroughly investigated. To explore the influence of mergers on bars, we construct a suite of \textit{N}-body merger pairs where a Gaia-Sausage-Enceladus-like radially biased satellite disk galaxy merges with a central disk galaxy during its bar formation. With the central galaxy fixed, the satellite varies in merger parameters: the mass ratio $m/M$ relative to the central galaxy, the impact parameter $b$, and the orbital inclination angle $\theta_i$ relative to the central disk. We find that the bar survival probability decreases with increasing $m/M$. Mergers with $m/M\lesssim1/10$ generally preserve the forming bar, whereas those with ${m/M}\geq1/2$ tend to destroy it, producing more early-type-like remnants. For intermediate mass ratios ($1/5 \leq m/M \leq 1/3$), several models yield ``weakening bars'', in which the bar survives the merger but gradually decays during subsequent secular evolution, possibly due to interactions between nested double bars formed from merger debris. In contrast to $m/M$, $b$ and $\theta_i$ have only secondary and stochastic effects on bar survival. The different influences of these three merger parameters can be naturally explained by the tidal force exerted by the satellite on the forming bar, which tends to weaken the bar when the satellite crosses it nearly perpendicular to its major axis.