Response of the LMC's Bar to a Recent SMC Collision and Implications for the SMC's Dark Matter Profile

TL;DR

The paper investigates the unusual LMC bar by testing the hypothesis that a recent collision with the SMC drove the bar’s $ ext{$ obreak 1$}-$1.5 kpc offset, its $ ext{$ obreak 5$–$15^ op$}$ tilt, and the suppressed central gas inflows. Using Besla2012’s N-body SPH simulations, it compares a collisionless Model 1 with a collision Model 2 (impact parameter ≈ $2$ kpc, collision ~ $100$ Myr ago) and demonstrates that the observed bar features emerge only in Model 2, with an offset decaying to match the observed $ ext{$0.76^ op$}$ kpc around $150$–$200$ Myr post-collision, and a tilt of $ ext{$8.6^ op$}$ consistent with observations. The bar’s pattern speed slows by a factor of about two after the collision, emphasizing the disequilibrium state of the LMC/bar system and cautioning against interpreting a single pattern-speed value as representative of secular evolution. A semi-analytic impulse-torque model constrains the SMC’s pre-collision mass within $2$ kpc to be $M_{ m SMC}(<2 m{kpc}) ext{ around } (0.8$–$2.4) imes 10^9 M_\odot$, implying the SMC was dark matter dominated; a central-gas framework further provides a general method to compare gas distributions across interaction histories. Overall, the work links the LMC bar’s current morphology to its interaction history, offering a novel probe of the SMC’s inner mass profile and highlighting the need for higher-resolution, physics-rich simulations in the future.

Abstract

The LMC's stellar bar is offset from the outer disk center, tilted from the disk plane, and does not drive gas inflows. These properties are atypical of bars in gas-rich galaxies, yet the LMC bar's strength and radius are similar to typical barred galaxies. Using N-body hydrodynamic simulations, we show that the LMC's unusual bar is explainable if there was a recent collision (impact parameter $\approx$2 kpc) between the LMC and SMC. Pre-collision, the simulated bar is centered and co-planar. Post-collision, the simulated bar is offset ($\approx$1.5 kpc) and tilted ($\approx8.6^\circ$). The simulated bar offset reduces with time, and comparing with the observed offset ($\approx0.8$ kpc) suggests the timing of the true collision to be 150-200 Myr ago. 150 Myr post-collision, the LMC's bar is centered with its dark matter halo, whereas the outer disk center is separated from the dark matter center by $\approx1$ kpc. The SMC collision produces a tilted-ring structure for the simulated LMC, consistent with observations. Post-collision, the simulated LMC bar's pattern speed decreases by a factor of two. We also provide a generalizable framework to quantitatively compare the LMC's central gas distribution in different LMC-SMC interaction scenarios. We demonstrate that the SMC's torques on the LMC's bar during the collision are sufficient to explain the observed bar tilt, provided the SMC's total mass within 2 kpc was $(0.8-2.4) \times 10^9$ M$_\odot$. Therefore, the LMC bar's tilt constrains the SMC's pre-collision dark matter profile, and requires the SMC to be a dark matter-dominated galaxy.

Figures (15)

• Figure 1: The distance between the stellar center of mass of the LMC and SMC for the B12 Model 1 (magenta dash-dot line) and Model 2 (blue solid line) simulation, as a function of time. The solid black line denotes the "present day" in the simulation, which is $\approx$ 1 Gyr after the Clouds cross the virial radius of the MW's halo. In Model 2, the SMC collides with the LMC (impact parameter $\approx$ 2 kpc) at the epoch denoted by the black dashed line. The collision occurred $\approx$ 100 Myr before the present day. In Model 1, the Clouds remain far from each other with their closest separation being $> 25$ kpc. The horizontal dotted line shows the observed separation between the Clouds. In section \ref{['sec:limitations']}, we discuss how the difference between the simulated and observed separation at present day affects our conclusions.
• Figure 2: The visualization of the SMC's orbit about the LMC for the Model 2 simulation, plotted in the LMC's frame of reference over the past 1 Gyr as the galaxies orbit the MW. The left (right) panel shows the face-on (edge-on) projection of the surface density ($\Sigma_\ast$) of the LMC's simulated stellar disk at the present day. The magenta curve shows the orbit of the SMC's stellar center of mass with the arrow indicating the position and direction of SMC's motion 1 Gyr ago. The magenta cross marks the location of the SMC when the two galaxies collide ($\approx$100 Myr ago, impact parameter of $\approx 2$ kpc). The magenta square marks the location of the SMC at present day in the simulation. On its closest approach to the LMC, the SMC's orbital plane is inclined to the LMC's disk plane by around 50$^\circ$. With such an orbital geometry, the SMC can affect both the vertical and the in-plane motion of the LMC's bar.
• Figure 3: Same as Figure \ref{['fig:orbit_vis']}, but for the Model 1 simulation. In Model 1, the LMC and SMC remain much farther away as compared to Model 2 (note the difference in axis scales from Figure \ref{['fig:orbit_vis']}). The magenta cross marks the location of the SMC at its closest approach to the LMC ($\approx$ 27 kpc). In Model 1, we do not expect the SMC to significantly perturb the LMC's bar due to the much larger separation between the two galaxies as compared to Model 2 (where the closest approach is $\approx$ 2 kpc). We use Model 1 as a control simulation for comparison with the bar properties of Model 2.
• Figure 4: The offset of the simulated LMC bar at the epoch of infall into the MW (left panel), LMC-SMC collision (middle panel) and present day (right panel) is illustrated using the surface density of stars ($\Sigma_\ast$) in the LMC disk. The LMC stellar disk is plotted in the XY plane in the LMC's frame of reference. The solid-magenta ellipse (bar ellipse) is an isodensity elliptical contour with a semi-major axis equal to the bar radius. The magenta star is the geometric center of the bar ellipse. The dashed-green ellipse (outer disk ellipse) is an isodensity elliptical contour with a semi-major axis equal to the radius where the surface density profile of the disk drops by a factor of 100. The green circle is the geometric center of the outer disk ellipse. The separation between the center of the bar ellipse and the center of the outer disk ellipse is defined as the bar offset. The LMC's bar develops a large offset ($\approx 1.5$ kpc) at present day. The offset is small at the epochs of MW infall and the LMC-SMC collision, indicating that the LMC's bar develops a large offset post SMC collision. The small offset at the MW infall epoch is equivalent to the offset found in Model 1 at the present day (where the Clouds do not collide).
• Figure 5: The LMC bar's offset as a function of time. The red solid line is the measured bar offset in the Model 2 simulation and the shaded yellow band around it is the measurement error. The black dash-dot line marks the observed bar offset of 0.76 kpc (R25) with the purple shaded band being the $3-\sigma$ error on the observations. The vertical black dashed line marks the LMC-SMC collision epoch and the vertical black solid line marks the present day epoch in Model 2. The LMC's bar develops a large offset just as the SMC collides with the LMC. The present-day offset is larger than observed, suggesting that the true collision must have occurred 150-200 Myr ago such that the bar offset has sufficiently decayed, but is still present. The blue shaded band denotes the $1-\sigma$ spread on either side of the mean bar offset measured in the Model 1 simulation. In Model 1, where the LMC and SMC remain far away, the bar does not develop an offset to an extent seen in Model 2 and observations at the level of $7-\sigma$ and $3-\sigma$ respectively in statistical significance.