Radial Migration in the LMC

Updated 10 November 2025

The paper demonstrates that tidal interactions from the SMC drive high-amplitude migration waves in the LMC disc, with net migration fluxes reaching 40–50% M*/Gyr.
It utilizes simulation-based tracking of guiding radii and observational chemodynamical inversion to quantify migration strength, with uncertainties around 1.5 kpc for younger stars.
The study shows that these tidal migration episodes reshape metallicity gradients and alter the LMC's orbital dynamics within the Milky Way's halo.

The radial migration strength of the Large Magellanic Cloud (LMC) refers to the quantification of the amplitude and mechanisms of changes in the galactocentric orbital radii of its stars, as well as the migration of the LMC itself as a satellite orbiting the Milky Way. This topic encompasses both internal (secular, bar/spiral-driven) and external (tidal, satellite-driven) migration processes within the LMC’s stellar disc, as well as the large-scale orbital dynamics of the LMC around the Milky Way, and the dynamical “stirring” of both the LMC itself and the Galactic halo. Recent high-resolution simulations, spectroscopic surveys, and analytic modeling have established that tidal interactions—particularly with the Small Magellanic Cloud (SMC)—dominate radial mixing within the LMC disc, while the LMC’s own orbit around the Galaxy is sensitive to the Milky Way’s outer halo structure and total mass.

1. Definitions and Quantitative Measures

Radial migration in the context of the LMC is quantified using several precise definitions:

For LMC disc stars, the guiding radius $R_g(t)$ is the radius of the circular orbit with the same angular momentum as the star. To mitigate epicyclic noise, the time-dependent guiding radius is extracted as $R_g(t)=[R_{\mathrm{max}}(t)+R_{\mathrm{min}}(t)]/2$ , tracking the running envelopes of a star’s galactocentric distance over time (Hebrail et al., 22 Oct 2025).
The radial migration flux (or churning flux) $F(R, t)$ at a radius $R$ during interval $\Delta t$ is the mass fraction of disc stars whose guiding radius crosses $R$ per $\Delta t$ :

$F(R, t) = \frac{1}{\Delta t}\sum_{i\in \Delta R_g>0} m_i$

(outward flux), with analogous definition for inward flux $F_{\mathrm{in}}$ ; the net flux is $F_{\mathrm{net}} = F - F_{\mathrm{in}}$ . Values are typically reported as percent of the total disc stellar mass per Gyr (Hebrail et al., 22 Oct 2025).

For studies using resolved stellar populations, migration strength $\sigma_{\mathrm{RM}}(\tau)$ is measured as the dispersion in radial displacements for mono-age cohorts:

$\sigma_{\mathrm{RM}}(\tau) = 1.5\,\mathrm{MAD}\left[R_{\mathrm{current}} - R_{\mathrm{b}}(\tau)\right]$

where $R_{\mathrm{b}}$ is the birth radius inferred from chemodynamical inversion, and $\tau$ is the look-back time (Lu et al., 4 Nov 2025).

The LMC’s orbital excursion around the Milky Way is described by the radial difference between apo- and pericentre, $\Delta R = R_{\mathrm{apo}} - R_{\mathrm{peri}}$ , with a dimensionless “migration strength” $S=\Delta R/R_{\mathrm{peri}}$ (Zhang et al., 2012).

2. Internal Disc Migration: Secular and Tidal Drivers

Secular processes within the LMC disc—namely bar and spiral-driven resonant migration—produce moderate radial mixing, strongly dependent on the Toomre $Q$ parameter:

Isolated (non-interacting) LMC models with $Q$ from 1.0 to 1.5 display net migration fluxes of $F_{\mathrm{net}}\lesssim 15$ – $20\%\,M_*/\mathrm{Gyr}$ , with lower $Q$ (colder, more unstable discs) experiencing larger churning associated with bar formation and spiral activity (Hebrail et al., 22 Oct 2025).
The correspondence between resonance regions of the bar and the mixing regions in the metallicity profile underscores the central role of non-axisymmetric features in driving secular radial migration (Hebrail et al., 22 Oct 2025).
Unlike the Milky Way, where secular migration yields a smooth, continuous increase in $\sigma_{\mathrm{RM}}(\tau)\propto\tau^{0.5}$ [Frankel et al. 2019], the LMC’s secular background level is regularly interrupted by strong tidal events (Lu et al., 4 Nov 2025).

3. Tidally-Induced Migration: Migration Waves from SMC Interactions

Tidal encounters with the SMC drive episodic, high-amplitude migration within the LMC:

Each pericentre passage of the SMC triggers coherent, galaxy-spanning “migration waves” in the LMC disc. These are characterized by an initial strong inward flux (as tidal forces pull orbits centrally), rapidly followed at closest approach by an outward rebound (as vertical energy is redistributed to radial motion). The result is a wave-like structure in $F(R, t)$ that propagates from the center to the outskirts (Hebrail et al., 22 Oct 2025).
In fully interacting simulations, net migration fluxes reach $40$– $50\%\,M_*/\mathrm{Gyr}$ per wave—dwarfing the secular rates—almost independent of $Q$ in the tested range $1.0\leq Q\leq 1.5$ . For context, even the highest secular rates reach only $15$– $20\%\,M_*/\mathrm{Gyr}$ (Hebrail et al., 22 Oct 2025).
The temporal and radial structure of these waves is tightly coupled to the SMC–LMC separation. They oscillate more rapidly at smaller $R$ (reflecting shorter dynamical times) and more slowly at larger radii.
Migration strength $\sigma_{\mathrm{RM}}(\tau)$ , as measured from chemodynamically inferred birth radii, exhibits marked enhancements at $\tau \sim 0.5$ , $2$, and $5$ Gyr—epochs coincident with known SMC passages and star-formation enhancements in the LMC history (Lu et al., 4 Nov 2025).

4. Methods for Measuring Migration Strength

Rigorous quantification of radial migration strength in the LMC employs both simulation- and observation-driven approaches:

$N$ -body simulations (e.g. the KRATOS suite) directly track guiding radius evolution for tens of thousands of tracer particles. Migration fluxes are evaluated at high temporal resolution ( $\Delta t<2.6$ Myr) using definitions of $F(R, t)$ as above (Hebrail et al., 22 Oct 2025).
For observational data, migration strength is statistically reconstructed for mono-age bins using a robust estimator (1.5 $\times$ MAD) on the displacement $\Delta R=R_{\mathrm{current}}-R_{\mathrm{birth}}$ . Birth radii are obtained via inversion of age-metallicity gradients constrained by APOGEE-RGB stars and current [Fe/H]– $R$ profiles. Underlying measurement biases are assessed via hydrodynamical simulations, yielding uncertainties of $\sim1.5$ kpc for $\sigma_{\mathrm{RM}}$ for stars younger than 6 Gyr (Lu et al., 4 Nov 2025).
Typical values for the LMC migration strength:
- $\tau=0.5$ Gyr: $\sigma_{\mathrm{RM}}\simeq 1.2\pm0.3$ kpc
- $\tau=2$ Gyr: $\sigma_{\mathrm{RM}}\simeq 2.1\pm0.4$ kpc
- $\tau=5$ Gyr: $\sigma_{\mathrm{RM}}\simeq 3.7\pm0.6$ kpc
The LMC’s mixing amplitude grows with look-back time as a sublinear power law: $\sigma_{\mathrm{RM}, \mathrm{LMC}}(\tau) = 4.4(\tau/8\,\mathrm{Gyr})^{0.4}$ kpc (Lu et al., 4 Nov 2025).

$\tau$ [Gyr]	$\sigma_{\mathrm{RM}}$ [kpc]	Migration mode
0.5	$1.2\pm0.3$	Recent SMC-induced
2	$2.1\pm0.4$	SMC passage/starburst
5	$3.7\pm0.6$	Peak tidal churning + starburst

5. Impact on Metallicity Gradients and Observable Structure

Radial migration induced by SMC interactions has direct implications for the chemodynamical evolution of the LMC:

Migration waves rearrange the spatial metallicity profile by inwardly transporting more metal-poor stars, yielding transient central metallicity drops of $3$– $5\%$ of the original $Z_0$ per event. Outward waves conversely steepen the outer metallicity gradient, but the net effect over Gyr timescales is to flatten the radial gradient beyond what is seen in isolation (Hebrail et al., 22 Oct 2025).
At several epochs (notably around 5 Gyr ago), large-scale migration coincides with a steepening of the metallicity gradient and a centrally concentrated starburst, consistent with simultaneous tidal churning and rejuvenated star formation (Lu et al., 4 Nov 2025).
The absence of a clear [ $\alpha$ /M]–[Fe/H] bimodality in the LMC, in contrast to the Milky Way, has been attributed to the more centrally concentrated spatial pattern of star formation and the dominance of discrete, SMC-driven migration episodes (Lu et al., 4 Nov 2025).
Predicted transient metallicity dips at each pericentre could be observable in spatially resolved spectroscopic surveys (e.g. Gaia + APOGEE), offering a method to constrain past SMC–LMC interactions.

6. Orbital Migration and the LMC in the Galactic Halo Context

The notion of “radial migration strength” also applies to the LMC as a massive satellite orbiting the Milky Way:

The LMC’s orbit is characterized by the radial excursion $\Delta R=R_{\mathrm{apo}}-R_{\mathrm{peri}}$ and migration strength $S=\Delta R/R_{\mathrm{peri}}$ . For physically plausible Galactic potentials, these excursions range from $90$ to $160$ kpc, corresponding to $S=1.8$ –$3.2$ (Zhang et al., 2012).
The LMC’s pericenter is essentially fixed at the present $\sim50$ kpc, while $R_{\mathrm{apo}}$ and thus $S$ depend on the Galactic halo’s mass profile and the adopted local circular velocity $V_0$ . Flatter outer potentials and lower $V_0$ both yield larger migration strengths.
A higher halo mass ( $M_{\mathrm{vir}}\gtrsim 1.4\times10^{12}\,M_\odot$ ) and $V_0\gtrsim 228$ km s $^{-1}$ admit multi-orbit LMC histories with $S\sim2$ –$2.4$, while steeper potentials and low $V_0$ suggest a first-passage scenario ( $S\lesssim1$ ) (Zhang et al., 2012).

7. Broader Cosmological and Dynamical Implications

The radial migration strength of the LMC, on both internal and orbital scales, has profound implications:

Tidal churning surpasses secular mechanisms, moving $40$– $50\%$ of LMC disc stars through migration waves per Gyr, fundamentally altering the age, chemical, and morphological structure of the galaxy (Hebrail et al., 22 Oct 2025).
The “sloshing” and radial mixing of the Galactic halo induced by the LMC’s infall drives the Galaxy out of equilibrium beyond $r\gtrsim 30$ kpc, with observed North–South dipoles in halo-star $v_{\mathrm{GSR}}$ of $38\pm10$ km s $^{-1}$ , corresponding to radial excursions $\Delta r\sim 12\pm3$ kpc per orbit (Erkal et al., 2020).
Accurate modeling of the LMC’s migration strength provides stringent constraints on the Milky Way’s dark matter profile at large radii and on the dynamical evolution of the Magellanic System.
Recurrent tidal migration episodes imprint directly observable features (metallicity dips, altered morphology, disk flaring) that can be compared against next-generation integral field and spectroscopic surveys, refining the chronology and mass scale of past interaction events.