Tidally-induced radial migration waves in LMC-like galaxies

TL;DR

This work investigates how tidal interactions shape radial migration and the radial metallicity distribution in LMC-like galaxies using six high-resolution KRATOS N-body simulations (three isolated, three interacting) across Toomre parameters $Q=1.0,1.2,1.5$. By defining a planar reference frame and tracking guiding radii $R_g(t)$, the authors map non-axisymmetric patterns, radial migration fluxes, and metallicity evolution, revealing tidally triggered wave-like migration up to ~40% of disc mass per Gyr near pericentre. The results show that tidal interactions can dominate radial migration and metallicity mixing, with inner-disc metallicity drops of ~3–5% relative to the isolated maximum, and that corotation resonances modulate mixing zones and flux amplitudes. The findings highlight the significant role of tidal forces in disc evolution and point to the need for future hydrodynamic simulations to capture star formation, metallicity evolution, and disc asymmetries more realistically.

Abstract

Stellar radial migration has predominantly been examined in isolated disc galaxies where non-axisymmetric structures drive the process. By contrast, while tidal interactions are known for having an influence, their contribution remains comparatively under explored. The LMC, the nearest disc galaxy to the Milky Way (MW) and currently interacting with the SMC, provides a unique laboratory to investigate this interplay. We aim to quantify the impact of tidal interactions on radial migration and metallicity distribution in high-resolution simulations of LMC-like disc galaxies. We leverage a subsample of KRATOS, a suite of 28 pure $N$-body simulations of the LMC-SMC-MW system. Specifically, we use 6 simulations of both isolated and interacting LMC-like galaxies, exploring different values of the Toomre stellar parameter $Q$. These simulations allow to map the evolution of the stars' guiding radii $R_g(t)$ and compute radial migration fluxes in interacting systems and compare with their isolated counterparts, allowing to quantify the link between tidal interactions, radial migration, non-axisymmetric patterns, disc internal stability, and radial metallicity distribution. We present tidally-triggered wave-like radial migration fluxes reaching up to $\sim40\%$ of disc stellar mass per Gyr. This wave-like migration appears during the satellite's pericentre passages, almost independently of $Q$ and induces a metallicity drop of $\sim$3-5\% of the isolated galaxy's maximum metallicity in the inner disc. Additionally, in the isolated simulations, the extent of variation in the bar's resonance region coincides with the mixing zones in the metallicity distribution. We propose a novel description of a wave-like radial migration flux as a dynamical response of a galaxy undergoing tidal interactions and sketch its impact on the galaxy's metallicity distribution.

Figures (4)

• Figure 1: Top: Visualisation of the method described in halle2015quantifying to infer the $R_g (t)$ of the particles. Radial evolution of three random particles in the K9i simulation with respect to time. $R_g(t)$ (pink) is the mean of $R_{\rm max}(t)$ (dark brown) and $R_{\rm min}(t)$ (light brown), and its value at the start of the simulation is $R_g(t_i)$ (black horizontal dashed line). The times of the pericentre passages of $G_{\rm SMC}$ are shown in grey vertical dashed lines, and the evolution of the galactocentric radius of the particles $R(t)$ is the dark blue curve. The black vertical line represents the $t=0$ snapshot that is displayed on the bottom panel. Bottom: Visualisation of the method inferring the guiding radius $R_g(t)$ of a particle at $t=0$ (same legend as above, with the previous orbit of the particle in white). The shown particle is the particle with the highest radius of the top panel. $G_{SMC}$ is shown on the lower right hand corner but its particles are not tracked in this work. An animated visualisation is available at https://youtu.be/SvtrVxrCpwI.
• Figure 2: $A_2/A_0$ Fourier mode strength (top panel), difference between the churning fluxes outwards and inwards (central panel, counted positive outwards), and normalised metallicity evolution (bottom panel) maps with respect to time and galactocentric radius $R$ or guiding radius $R_g(t)$ for the K9i (interacting $Q=1.5$ simulation, top three panels) and K7 (isolated $Q=1.5$ simulation, bottom three panels) simulations. The grey area masks the first $0.6$ Gyr displayed, when the disc is not fully relaxed yet. For the K9i maps, the times of the pericentre passages of $G_{\rm SMC}$ are shown in white dashed lines, and its impact parameters are the white stars. The corotation radius is plotted in green and yellow for resonance with a bar or a weak central pattern, respectively.
• Figure 3: Same as Fig. \ref{['MAPS79']} for K6i (top three panels, $Q=1$ interacting simulation) and K4 (bottom three panels, $Q=1$ isolated simulation).
• Figure 4: Same as Figs. \ref{['MAPS79']} and \ref{['MAPS46']} for K3i (top three panels, $Q=1.2$ interacting simulation) and K1 (bottom three panels, $Q=1.2$ isolated simulation).