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

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 $θ_i$ 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 $θ_i$ 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.

Figures (8)

• Figure 1: Temporal evolution of the bar strength $A_2$ (black) and pattern speed $\Omega_\mathrm{p}$ (red) for the central galaxy evolved in isolation. A prominent bar forms within $1\ \mathrm{Gyr}$ and subsequently undergoes secular growth accompanied by a corresponding decline in pattern speed.
• Figure 2: Illustration of the merger parameters with stereoscopic projections: the mass ratio $m/M$, impact parameter $b$, and orbital inclination angle $\theta_i$. The initial separation between the two galaxies is fixed as $d=125\ \mathrm{kpc}$ along the $X$-axis, while the positions in other directions vary according to $b$ and $\theta_i$. The satellite's incident velocity is fixed as $-100\ \mathrm{km/s}$ along the $X$-axis.
• Figure 3: Face-on views of several representative models. The top row shows the isolated model. The second row depicts a minor merger, where the satellite's effect on the bar is negligible. The third row presents a model in which the bar survives in a non-trivial merger. The fourth row illustrates a weakening bar that survives the merger but gradually decays subsequently in secular evolution. The fifth row shows a merger-produced elliptical-like system with negligible rotation, resulting from a violent merger. The bar strengths and pattern speeds of the central galaxy are annotated in the lower-left corner of each panel.
• Figure 4: Left: Pattern speed $\Omega_\mathrm{p}$ as a function of bar strength $A_2$, with three models of very high pattern speed omitted for clarity (one with $\Omega_\mathrm{p}\sim40\ \mathrm{km\,s^{-1}\,kpc^{-1}}$ and two with $\Omega_\mathrm{p}\sim60\ \mathrm{km\,s^{-1}\,kpc^{-1}}$). The color of each point indicates the merger mass ratio. The red line marks the pattern speed of the isolated model, $\Omega_\mathrm{p, isolated}$. The cyan and yellow shaded regions denote low pattern speeds ($\Omega_\mathrm{p}<0.25\,\Omega_\mathrm{p, isolated}$) and weak bars ($A_2<0.15$), respectively. Models with an average azimuthal streaming velocity–to–dispersion ratio $V/\sigma<1$ within $10<R/\mathrm{kpc}<20$ are shown as unfilled circles. Right: Histogram of $\Omega_\mathrm{p}$, showing one peak near $\Omega_\mathrm{p, isolated}$ and another near $\Omega_\mathrm{p}=0~\mathrm{km\,s^{-1}\,kpc^{-1}}$.
• Figure 5: Distribution of final bar strength as a function of the merger parameters $(m/M,~b,~\theta_i)$. The mass ratio $m/M$ is shown on the horizontal axis. At each value of $m/M$, the seven vertical lines correspond to different inclination angles $\theta_i$, increasing from left to right ($\theta_i$ from $0^\circ$ to $90^\circ$). The color of each point indicates the impact parameter $b$. The yellow shaded region marks the weak-bar regime, defined as $A_2 < 0.15$. For comparison, the final bar strength of the isolated model is indicated by the red horizontal line. Models with negligible pattern speed ($\Omega_\mathrm{p}<0.25\,\Omega_\mathrm{p, isolated}$) are shown as crosses and are primarily located at higher $m/M$.