Chemodynamic History of the LMC

• LMC chemodynamics is the study of its evolving chemical composition, star formation, and gas flows shaped by both internal feedback and external tidal interactions.
• The history unfolds through distinct episodes of early enrichment, merger events, and interaction-driven starbursts, evidenced by metallicity gradients and spatially resolved abundances.
• High-resolution spectroscopy and dynamical modeling offer quantitative insights into stellar migration, galactic winds, and the cumulative effects of episodic outflows on the LMC’s evolution.

The chemodynamic history of the Large Magellanic Cloud (LMC) encapsulates the intertwined evolution of its chemical composition, gas flows, star formation, and orbital interactions, shaped over cosmic time by both internal mechanisms and external perturbations. This synthesis integrates spatially resolved stellar abundances, dynamical modeling, absorption-line diagnostics, and chemical evolution theory to reconstruct the LMC’s past and ongoing transformation across six key domains.

1. Foundations: Early Enrichment, Assembly, and Proto-LMC Character

The earliest phase of the LMC’s chemodynamic evolution is now accessible due to high-resolution abundance analyses of extremely metal-poor stars ([Fe/H] as low as –4.1), made possible by photometric pre-selection from Gaia DR3 and follow-up Magellan/MIKE spectroscopy (Chiti et al., 20 Jan 2024). These stars, with $[\mathrm{C}/\mathrm{Fe}]_{c} < 0$ and $[\alpha/\mathrm{Fe}]$ enhancements (e.g., $[\mathrm{Mg}/\mathrm{Fe}]\approx +0.4$), record chemical enrichment from one or a few Population III core-collapse supernovae, marking the LMC’s formation in a region isolated from the proto-Milky Way’s earliest enrichment pathways. Unlike the MW halo, the LMC lacks carbon-enhanced metal-poor (CEMP) stars below $[\mathrm{Fe}/\mathrm{H}]<-2.5$, implying a fundamentally different early IMF or nucleosynthetic channel.

At the same time, the ΛCDM paradigm predicts and observations confirm that the LMC accrued mass not solely in situ but also via minor mergers. The dissolved dwarf galaxy progenitor of the GC NGC 2005 provides direct chemical evidence for early hierarchical assembly, displaying lower $[\alpha/\mathrm{Fe}]$ and Zn than in situ LMC GCs at the same metallicity ([Fe/H] ≃ –1.75), consistent only with a low-SFE system that ceased star formation after a brief initial burst (Mucciarelli et al., 2021). Nevertheless, such mergers contributed negligibly (<1% by mass) to the present LMC’s metal content or age–metallicity relation.

2. Bulk Disk Chemical Evolution: Star Formation History and Metallicities

The subsequent chemical evolution of the LMC disk is characterized by two principal stellar components (Lapenna et al., 2012):

• LMC-R (Metal-Rich, ~84%): [Fe/H] ~ –0.48, [α/Fe] ~ –0.1, formed during a strong star formation burst 3–4 Gyr ago, plausibly triggered by the first close tidal interaction with the SMC. This event drove rapid gas inflows, centrally concentrated star formation, and efficient Fe enrichment by SNe Ia, suppressing the α-enhancement characteristic of earlier epochs.
• LMC-P (Metal-Poor, ~16%): [Fe/H] ~ –1.06, [α/Fe] ~ +0.2, representing stars formed during the preceding quiescent interval (5–12 Gyr ago) under low SFR (~0.1 M⊙ yr⁻¹) and relatively isolated conditions.

Minor stellar populations, including ancient GC-like (α-rich, [Fe/H] < –1.5), and highly α-poor/metal-rich NGC 1718-type stars, each constitute <1% of disk stars.

The overall present-day metallicity gradient from field red giants is $d[\mathrm{Fe}/\mathrm{H}]/dR = -0.0380 \pm 0.0022$ dex kpc⁻¹ out to $\sim$8 kpc (Povick et al., 2023), matching results from resolved main-sequence, red clump, and Cepheid populations.

3. Radial Gradients, Migration, and Bursts: Temporal and Spatial Patterning

Analysis of age–abundance relations, birth radii reconstruction, and APOGEE data reveals that the LMC’s radial metallicity gradient is not static but exhibits a pronounced U-shaped time evolution (Lu et al., 4 Nov 2025, Povick et al., 2023):

• Early (t > 8 Gyr): Steep gradients ($\sim$–0.044 dex kpc⁻¹) signal inside-out growth and high SFR.
• Intermediate (8–3 Gyr): Progressive flattening (to $\sim$–0.015 dex kpc⁻¹), reflecting reduced SFR and efficient radial mixing.
• Recent (<2–3 Gyr): A dramatic steepening ($\sim$–0.035 to –0.050 dex kpc⁻¹) coincident with starburst onset and increased α-enhancement, closely synchronized with SMC pericenter passages.

Spatially, star formation episodes are strongly concentrated in the inner disk at 5 Gyr, shift outward and become widely distributed by 3 Gyr, and recent bursts (1 Gyr) display both renewed central activity and broad disk coverage. The pattern and timing of these events are traceable to the orbital geometry and spin alignment of the interacting LMC–SMC pair.

Quantifying migration by the distribution width $$\sigma_{\mathrm{RM}}(\tau) = 1.5\,\mathrm{MAD}(R-R_b)$$, enhancements at 0.5, 2, and 5 Gyr indicate episodic, burst-associated dynamical stirring, with typical migration amplitudes up to 3 kpc (shallower in age dependence but larger at recent times than the Milky Way).

4. Multi-phase Galactic Winds and Environmental Influences

The LMC hosts a broad, symmetric multiphase galactic wind, detected via HST UV absorption toward both a disk O-star and a background AGN at 3.2 kpc from the kinematic center (Barger et al., 2015). Both low-ionization (Si II, O I) and high-ionization (Si IV, C IV, O VI) phases are present, with line-of-sight outflow velocities up to ~100 km s⁻¹ and perpendicular velocities ≲110 km s⁻¹. The mass and mass-loss rates are:

Component	$M_{\mathrm{gas}}$ ($\mathrm{M}_\odot$)	$\dot{M}$ ($\mathrm{M}_\odot\,\mathrm{yr}^{-1}$)
Low-ionization	$\gtrsim1.5\times 10^7$	$\gtrsim 0.41$
High-ionization	$\gtrsim1.4\times10^6$	$\gtrsim 0.04$
Total	$\gtrsim1.6\times10^7$	$\gtrsim 0.45$

Ionization diagnostics ([Si II/O I] ~ +0.50, $x(\mathrm{H}^+)\geq72\%$, high $N_\mathrm{CIV}/N_\mathrm{SiIV}\sim3.5$) indicate photoionized low-ion and collisionally ionized high-ion phases with metallicity $Z_\mathrm{LMC}\sim0.5\,Z_\odot$.

The outflow marginally exceeds the escape velocity only locally ($\sim90$ km s⁻¹), so much of the gas is ultimately removed from the disk through the combined action of tidal forces (SMC, Milky Way) and ram-pressure stripping by the MW hot halo. This outflow, especially episodic after recent central bursts, is a critical mechanism depleting the LMC’s star-forming fuel, enriching the Magellanic Stream and the MW’s circumgalactic medium, and producing cumulative metal ejection $\gtrsim10^5\,M_\odot$.

5. Diagnostic Element Ratios and Star Formation Pathways

High-resolution spectroscopy of bar and inner disk red giants reveals systematic chemical fingerprints (Swaelmen et al., 2013). Key features include:

• α-elements: $[\mathrm{O}/\mathrm{Fe}]$ and $[\mathrm{Mg}/\mathrm{Fe}]$ in the LMC bar and disk are suppressed by $\sim$0.3 dex relative to the MW at $-1.5\lesssim[\mathrm{Fe}/\mathrm{H}]\lesssim-0.5$, e.g., $[\mathrm{Mg}/\mathrm{Fe}]\simeq0$; $[\mathrm{Si}/\mathrm{Fe}]$, $[\mathrm{Ca}/\mathrm{Fe}]$, $[\mathrm{Ti}/\mathrm{Fe}]$ more closely track MW values.
• Neutron-capture elements: Persistent r-process enhancement (e.g., $[\mathrm{Eu}/\mathrm{Fe}]$ up to +0.5 at [Fe/H]<–0.8); strong late-time s-process signals (Ba, La) with $[\mathrm{Ba}/\mathrm{Eu}]$ rising steeply at [Fe/H]>–0.8, indicating significant metal-poor AGB enrichment.
• First-/Second-peak s-process elements: Systematically low [Y+Zr/Ba+La], with bar offsets in [Y/Fe], $\sim$+0.3, +0.2" title="" rel="nofollow" data-turbo="false" class="assistant-link">Zr/Fe compared to the disk, reflecting enhanced AGB yields in central starbursts.
• Cu: Flat, low [Cu/Fe] ~ –0.6 across all [Fe/H], in contrast to MW, pointing to a deficit in massive star nucleosynthesis.

The “knee” in $[\alpha/\mathrm{Fe}]$ occurs at much lower metallicity ($\sim$–1.2) than in the MW disk, a signature of slower chemical evolution and more prompt SNe Ia enrichment. Selective element loss (e.g., Ca via anisotropic SN II ejection) is necessary to reconcile models with observed $[\mathrm{Ca}/\mathrm{Fe}]$\,$<-$0.2 at [Fe/H]>–0.6 (Bekki et al., 2012).

6. Synthesis: Interaction-Driven and Episodic Chemodynamics

Reconstruction of stellar birth radii, migration, and abundance patterns delineates a chemodynamic history dominated by three major interaction/burst epochs at ~5, 3, and 1 Gyr ago (Lu et al., 4 Nov 2025):

• 5 Gyr: Centrally focused burst ($R_b<2$ kpc); strong metallicity gradient steepening (–0.055 dex kpc⁻¹), presumably from gas inflow driven by spin-aligned pericenter passage with the SMC.
• 3 Gyr: Burst with broad radial distribution ($R_b=2$–6 kpc), weaker gradient steepening (–0.045 dex kpc⁻¹), reflects misaligned or perpendicular LMC–SMC encounter geometry.
• 1 Gyr: Widespread but centrally enhanced SF, with renewed gradient steepening (–0.050 dex kpc⁻¹).

These episodes coincide with maxima in [α/Fe], [N/Fe], [Al/Fe], and represent synchronized evolutionary jumps also mirrored in the SMC (Povick et al., 2023). The absence of [$\alpha$/M]–[Fe/H] bimodality in the LMC, contrasting with the MW, is a direct outcome of its centrally concentrated, interaction-dominated SFH and lack of extended low-α, outer disk star formation.

The starburst-driven metal-enriched outflows, shaped by feedback and environment, serve both to regulate the LMC’s ISM metallicity gradient and to modulate its continued SF, with the Magellanic Stream and circumgalactic enrichment as observable downstream consequences.

7. Implications and Broader Context

The integrated chemodynamic history of the LMC demonstrates the critical roles of interaction-driven inflows/outflows, environment-dependent early enrichment, and short timescale starbursts in dwarf galaxy evolution. Minor mergers have contributed only trace stellar populations (as in NGC 2005 (Mucciarelli et al., 2021)), while mass-dominant old and young disk populations document the timing and spatial footprint of SMC-induced transformation.

These findings underscore the LMC as a laboratory for understanding the general mechanisms of disk assembly and chemical evolution in low-mass galaxies. The synergy of resolved stellar abundances, galactic wind diagnostics, and validated inversion methodologies (Lu et al., 4 Nov 2025, Barger et al., 2015) provides stringent boundary conditions for galaxy formation models, revealing both the commonality and diversity of chemodynamic pathways among nearby dwarfs and the Milky Way.