Warped & Hooked: Mapping the Magellanic Clouds in 3D using Red Clump stars

TL;DR

This work presents a high-fidelity, population-corrected 3D map of the Magellanic Clouds by employing ~2.3 million RC stars from Gaia DR3, complemented by DELVE-MC SFHs to account for metallicity and age-driven luminosity variations. The authors develop a robust RC selection and extinction-correction pipeline, quantify crowding effects, and derive second-order distance corrections using spatially resolved SFHs, achieving median distances of $D_{ m LMC} = $ $50.62 \\pm 2.32$ kpc and $D_{ m SMC} = $ $60.75 \\pm 2.85$ kpc across wide areas. They map a detailed LMC geometry, obtaining $i = $ $25.32^{\\circ} \\pm 0.10^{\\circ}$ and $\\theta = $ $142.34^{\\circ} \\pm 0.21^{\\circ}$ for the inner disk, and reveal a striking azimuthal, U-shaped warp with amplitudes up to ~7 kpc at ~15 kpc, consistent with a tidal response to a recent LMC–SMC encounter. The warp is shown to be globally coherent and azimuthally extended, challenging prior expectations of localized distortions, and providing stringent constraints for dynamical models of the Magellanic system and its orbital history.

Abstract

The Large and Small Magellanic Clouds (LMC and SMC) are the Milky Way's nearest interacting galaxy pair, offering a unique laboratory for studying tidal effects on galactic disks. Despite extensive survey efforts, the three-dimensional geometry of the Clouds, particularly the putative warp of the LMC, remains poorly constrained due to incompleteness in their crowded centers and the low stellar density of their peripheries, which demand wide-field coverage. Using red-clump (RC) stars as standard candles, corrected for age- and metallicity-dependent population effects with empirically calibrated color-magnitude relations and spatially resolved star-formation histories, we construct the most detailed distance map of the Magellanic System to date. Based on $\sim$2.3 million RC stars from \textit{Gaia} DR3 combined with modern reddening maps, we measure median heliocentric distances of $50.62 \pm 2.32$~kpc for the LMC (to $\sim$23$^{\circ}$) and $60.75 \pm 2.85$~kpc for the SMC (to $\sim$12$^{\circ}$). The maps reveal substructures including the LMC Northern Arm, southern hooks, the Magellanic Bridge, and SMC peripheral over-densities, with refreshed distance estimates. Fitting the LMC disk within $7^{\circ}$ yields a global inclination of $i = 25.32^{\circ} \pm 0.10^{\circ}$ and line-of-nodes position angle of $θ= 142.34^{\circ} \pm 0.21^{\circ}$. Most strikingly, we find the LMC periphery is warped azimuthally into a U-shaped structure reaching vertical amplitudes of $\sim$7~kpc at radii of $\sim$15~kpc. We interpret this warp as a tidal response to the most recent close passage of the SMC $\sim$300 Myr ago, providing new constraints on the orbital history and dynamical evolution of the Magellanic Clouds.

Figures (15)

• Figure 1: Left: Combined reddening map of the Magellanic Clouds (MCs) plotted in Magellanic Stream (MS) Coordinates Nidever2008 using a combination of the OGLE-IV map (inner LMC & SMC denoted by the dashed white circles) and the recalibrated SFD map in their outer peripheries. The small circle inside the SMC inner region denotes the 47 Tuc region where we adopt SFD values. This reddening map is available publicly on GitHub \ref{['https://github.com/slateroden/XMCreddeningmap']}. Right: 2D density map of the final derived RC sample of the MCs.
• Figure 2: Example localized proper motion (PM) cuts in the nearest neighbor spatial search for the LMC (top) and SMC (bottom). The gold circles define the local 2$\sigma$ mas yr$^{-1}$ PM cut centered on the median PM of the local nearest neighbor searches.
• Figure 3: Zoomed in CMDs of representative grid cells showing the selection procedure of Magellanic RC stars. The solid (dashed) red polygons are the limits of the best fit mean gaussian(s) in the LMC (SMC). The top marginal histograms show the distribution in $(BP - RP)_{o}$ color, while the right marginal histograms show the distribution in Gaia$G_{o}$-band apparent magnitude. The solid black curves show the total combined fits. The blue (red) dashed lines show the best fit color Gaussian(s) components over the RC (RGB). The solid grey histograms indicate our selection of individual RC star candidates in magnitude and color. Only stars that pass both selections in magnitude and color are adopted as Magellanic RC star candidates. Top Left: Example cell in the main disk of the LMC. Top Right: Example cell in the main body of the SMC. Bottom Left: Example cell in the dense inner bar of the LMC. Bottom Right: Example cell in the eastern SMC presenting the bi-modal RC population.
• Figure 4: Gaia's flux-limited completeness modeled as a sigmoid function, whose shape parameter $\alpha$ is derived from the slope of the faint-end LF falloff. We estimate the 50% completeness limits for Gaia in the LMC and SMC to be approximately $\sim$20.9 and $\sim$21.0 mag, respectively. The non–extinction-corrected $G$-band magnitude distributions for the RC stars selected in the LMC bar (black) and SMC center (red) are also shown, demonstrating that our RC selection remains more than 90% complete at these magnitudes.
• Figure 5: Top: Crowding-dominated completeness map of RC stars ($G \sim 18.5$--$20.0$ mag) in the LMC. Bottom: Same as above, but for the SMC. Values near unity indicate regions where essentially all stars present are detected, whereas values near zero correspond to areas where most stars are missed due to crowding. The dashed ellipse (LMC) and dashed circle (SMC) mark the central regions used to assess potential completeness-driven biases in our RC selection.