Survival or Destruction: Effects of Spheroidal Satellite Collisions on Bars in Milky Way-Like Galaxies

Abstract

Although stellar bars are prevalent in local galaxies, unbarred galaxies constitute a significant fraction, particularly at high redshifts. While some galaxies are unbarred by nature due to stability against the bar instability, several mechanisms capable of transforming barred galaxies into unbarred systems have also been proposed, such as central mass concentration, specific dark matter halo morphologies and tidal interactions. Regarding galactic interactions, mergers can undoubtedly disrupt bars while potentially destroying the entire disk. However, the effects of pure collisions (non-merging scenarios) on bars remain unclear, with limited existing studies yielding contradictory conclusions. Here we aim to systematically investigate the disruptive effects of collisions on bars hosted by Milky Way-like galaxies using N-body/SPH simulations. We model collisions between the barred galaxy and a spherical intruder, conducting multiple simulations by varying interaction parameters, with mass ratios set at 1:3, 1:5, and 1:15. We find that bars are remarkably robust, with most interactions failing to significantly reduce their strength or pattern speed. Only off-center high-inclination retrograde collisions can effectively destroy bars, while central high-inclination collisions can substantially decrease the pattern speed. Such destruction and deceleration primarily arise from gravitational forces rather than gas-related processes. Notably, compared to collisions occurring along the bar's major axis, those along the minor axis cause greater weakening but can slow the bar's natural deceleration. Furthermore, changes in mass resolution do not significantly affect the results when the resolution is better than ~10^5 Solar mass.

Figures (8)

• Figure 1: Upper-left panel: face-on stellar surface density map of the fiducial barred target galaxy at 0 Gyr. Upper middle-left panel: edge-on schematic diagram of the collision simulation, showing $m$, $V_{\rm 0}$, $b$, and $\theta$. Upper middle-right panel: face-on schematic diagram for illustrating some specific scenarios: prograde (retrograde) collisions and major(minor)-axis collisions. Upper-right panel: the bar strength ($A_{\rm 2,max}$) evolution of the target galaxy during isolated evolution or in the collision simulation 'm30v500b5t90ret', with the phase before bar formation (-2 Gyr to 0 Gyr) marked with gray shading. The horizontal yellow dashed line corresponds to $A_{\rm 2,max}=0.2$, with points above it colored magenta and the remainder in cyan. Lower panels: face-on stellar surface density map at different times in 'm30v500b5t90ret'.
• Figure 2: The evolution of bar strength $A_{\rm 2,max}$, length $r$, and pattern speed $\Omega_{\rm p}$ over time for 12 fiducial collision simulations. The upper-left corner of each panel is marked with row and column indices for easy navigation. The upper-right corner displays the simulation label, indicating the values of $m$, $V_{\rm 0}$, $b$, and $\theta$. Prograde and retrograde simulations are annotated with 'pro' and 'ret', respectively. The 'pro/ret' suffix only appears in simulations where $b \neq 0$. In the $A_{\rm 2,max}$ sub-panel, points above the bar existence threshold of 0.2 are colored magenta, while the rest are colored cyan. For times where $A_{\rm 2,max}>0.1$, the values of $r$ and $\Omega_{\rm p}$ are calculated; but if $A_{\rm 2,max}<0.2$, $r$ and $\Omega_{\rm p}$ points are rendered semi-transparently. The gray vertical dashed line marks the time of the collision, which is the single collision event for all simulations. The black dashed line shows the evolution of bar parameters in the isolated simulation (see Section \ref{['subsec:models']}).
• Figure 3: Evolution of the median bar strength over time for simulations sharing a specific parameter value in Figure \ref{['fig:2']} and Figure \ref{['fig:S1']}. The shaded region behind each median scatter point indicates the range from the minimum to the maximum value.
• Figure 4: The evolution of bar strength $A_{\rm 2,max}$, length $r$, and pattern speed $\Omega_{\rm p}$ over time for the 12 collision simulations mentioned in Section \ref{['sec:discussions']}. Prograde, retrograde, major-axis, minor-axis, gas-free, medium-resolution, and low-resolution simulations are annotated with 'pro', 'ret', 'maj', 'min', 'f0', 'mid', and 'low', respectively. Since there is no need to compare these 12 simulations with the isolated scenario, the evolution of the bar parameters in the isolated collision is not shown, unlike in Figure \ref{['fig:2']}.
• Figure 5: The evolution of bar strength $A_{\rm 2,max}$, length $r$, and pattern speed $\Omega_{\rm p}$ over time for the 24 sibling simulations of the simulations shown in Figure \ref{['fig:2']}. The meaning of each element in this figure is the same as in Figure \ref{['fig:2']}.