Stellar Birth Radii in the LMC: Insights into Chemodynamics, Radial Migration, and Star Formation Across the Disk

TL;DR

This work investigates the chemodynamic history and radial migration of the Large Magellanic Cloud by inferring stellar birth radii $R_b$ from ages and abundances, validated on a hydrodynamical LMC–SMC–MW simulation. The inferred $R_b$ reveals inside-out formation for older stars and distinct star-formation episodes at ~5, 3, and 1 Gyr that shape metallicity gradients and drive episodes of radial migration, with enhanced migration contemporaneous with these SF bursts. The analysis also finds that most $ ext{α}$-enriched stars form 2–3 Gyr ago at birth radii 2–4 kpc and that the LMC lacks the Milky Way–style [$ ext{α}$/M]-[Fe/H] bimodality, likely due to centrally concentrated SF during gradient steepening. Together, these results constrain the LMC’s assembly and interaction history with the SMC and offer a framework for testing chemodynamic scenarios in external disk galaxies.

Abstract

The LMC and SMC are interacting dwarf galaxies that offer a valuable testbed for studying the effects of galactic mergers. We investigate the chemodynamic history of the LMC in the context of its interaction with the SMC by inferring stellar birth radii, first validated on a hydrodynamical simulation tailored to reproduce their interaction history. Using inferred birth radii and stellar ages, we identify signatures of dynamical and chemical evolution across the LMC disk. We find that the LMC's metallicity gradient steepened around 5, 3, and 1 Gyr ago, coinciding with enhanced star formation (SF) episodes. These events exhibit distinct spatial patterns -- initially concentrated in the inner disk at 5 Gyr, expanding outward by 3 Gyr, and becoming widespread with renewed central activity at 1 Gyr -- likely reflecting changes in spin alignment between the interacting disks if the enhancements of SF tracks the pericenter passages of the SMC to the LMC. The inferred radial migration strength of the LMC shows notable enhancements at 0.5, 2, and 5 Gyr. The most $α$-enriched stars form 2-3 Gyr ago at birth radii of 2-4 kpc, the only epoch when star formation is broadly distributed across the disk. Finally, unlike the Milky Way, the LMC lacks a clear [$α$/M]-[Fe/H] bimodality. This is likely due to its more centrally concentrated star formation during these periods, compared to the MW's more extended outer-disk star formation enhancements. These findings place strong constraints on the LMC's assembly history and its interaction with the SMC.

Figures (10)

• Figure 1: Key plots for inferring the metallicity evolution of the simulated LMC galaxy. Left: The blue line shows the range of metallicity, Range[Fe/H], as a function of age, using a bin width of 0.5 Gyr. The range is measured using the difference between the 95$^{\rm th}$-percentile and the 10$^{\rm th}$-percentile of [Fe/H] in mono-age populations to account for more stars in the inner galaxy compared to the outer. The red solid line shows the true metallicity gradient, $\nabla$[Fe/H], as a function of look-back time, measured using the true $R_b$ and [Fe/H] in each mono-age population. The red dashed line is the inferred metallicity gradient using Equation \ref{['eq1']}. We can reproduce some of the key features, such as the steepening, in the metallicity gradient, and the median absolute deviation from the true metallicity gradient is 6%, or 0.002 dex/kpc. Right: The evolution of metallicity at the galactic center is shown in the black solid line. The background shows the histogram of the age-metallicity relation for this simulated galaxy. Again, the most metal-enriched stars at every age bin should represent the metallicity of the LMC center at that look-back time, assuming a negative metallicity gradient is present at all ages after the disk has started to form.
• Figure 2: Histogram of the true birth radii from the simulated LMC vs the inferred birth radii using the method described in Section \ref{['subsec:recover_sim']}, colored by the number of stars (top) and the median age (bottom). We are able to successfully infer birth radii with a bias of -0.6 kpc and a variance of 1.5 kpc for stars $<$ 6 Gyr.
• Figure 3: Metallicity-$R_b$ relation for stars in the LMC simulation during interaction phase between the LMC and SMC (1.5-2 Gyr; left), isolation phase (5-5.5 Gyr; middle), and settling phase (6.5-7 Gyr; right). The solid lines show the true metallicity gradient, and the dashed line shows the inference from Range[Fe/H]. The linear relation does fit the outer galaxy well, as the Metallicity-$R_b$ relations are not exactly linear, especially in the inner galaxy. This causes the bias for inferring $R_b$ for the outer galaxy (see Figure \ref{['fig:2']}).
• Figure 4: Left: The median and the 1.5*MAD of the birth radii distribution as a function of age using the true $R_b$ ($R_{b, truth}$) and age shown in blue, and the inferred $R_b$ ($R_{b, inf.}$) and true age convolved with a 20% uncertainty in black. Right: The true migration strength (blue points) compared to the migration strength measured using inferred $R_b$ (black crosses with errorbars) for the simulation. 1.5 kpc is subtracted from the strength using inferred $R_b$ to take into account the variance in our inference (see Figure \ref{['fig:2']}). The uncertainties are measured with bootstrapping $R_b$ with a width of 1.5 kpc, and age with a width of 20% of its value. Both relations using inferred $R_b$ extend beyond 6 Gyr due to the imposed age uncertainty. We are able to reproduce the general shape for both the $R_b$ distributions and the radial migration strength.
• Figure 5: Similar to Figure \ref{['fig:1']} but with the LMC observational data. The red solid line shows the inferred metallicity gradient as a function of look-back time, derived using the range in metallicity. The orange shaded areas are enhancements in star formation (SF) at 0.5, 1.1, 2, 3, and 5 Gyr Massana2022. The gray dashed line shows the metallicity gradient as a function of age taken from Povick2023. The observed metallicity gradient as a function of age is shallower than the inferred metallicity gradient at birth due to radial migration. Steepening in the metallicity gradient, defined as where the local minima are, is observed at or close to the times of increased SF around 1 and 5 Gyr. This is also seen in the MW Lu2024 and MW-like simulations Buck2023Ratcliffe2024.