Three-dimensional reddening maps of the Magellanic Clouds constructed by RR Lyrae stars

Abstract

We present the first three-dimensional reddening maps of the Large and Small Magellanic Clouds (LMC and SMC) constructed using fundamental-mode RR Lyrae stars from the OGLE-IV survey. By applying a period-amplitude-color relation and a period-luminosity-metallicity calibration in the OGLE photometric system, we derive intrinsic colors, color excess $E(V-I)$, and photometric distances for more than 20,000 RRab stars in the LMC and 3,000 in the SMC. Spatial variations in reddening are modeled using an adaptive quadtree scheme, where robust reddening-distance relations are fit within each partition and distances are iteratively updated to achieve self-consistency. The resulting maps reveal resolved dust structures across both galaxies, including steep reddening gradients in the central LMC and flatter profiles in the SMC. The construction of the three-dimensional reddening maps further reveals that high-extinction regions exhibit reddening behavior inconsistent with a uniform extinction law, implying localized variations in dust properties. The final maps comprise 205 partitions for the LMC and 67 partitions for the SMC, and are released together with a Python-based query tool and GeoJSON data products. These 3D maps provide a foundation for distance-dependent reddening corrections and for probing the structure and physical conditions of the Magellanic interstellar medium, and future high-precision and cadence RR Lyrae sample from Gaia DR4 will support higher-resolution mapping and deeper exploration of dust substructure.

Figures (6)

• Figure 1: Flow diagram illustrating the adaptive partitioning method.
• Figure 2: Overview of the spatial subdivision and fitting performance of the LMC reddening map. Panel (a): Spatial partitions of the LMC, with each subregion labeled by its red ID; a zoomed-in view of the central area is provided for clarity. Panel (b): Reddening gradients from the linear reddening–distance fits, color-coded by gradient magnitude. Star symbols mark representative star-forming regions, labeled according to the catalog of Henize1956. Panel (c): RMSE of the fits in each partition, where darker colors indicate larger residuals.
• Figure 3: Overview of the spatial subdivision and fitting performance of the SMC reddening map. Panel (a): Spatial partitions of the SMC, with each subregion labeled by its ID in red. Panel (b): Reddening gradients derived from the linear reddening–distance fits, color-coded by gradient magnitude. Star symbols mark representative star-forming regions, labeled according to the catalog of Henize1956. Panel (c): RMSE of the fits in each partition, with darker colors indicating larger residuals.
• Figure 4: Examples of reddening--distance relations spanning different gradient strengths in selected partitions of the LMC (top row) and SMC (bottom row). Each panel shows the measured $E(V-I)$ values with error bars and the best-fit linear relation (red line) for a single partition, with the partition ID, slope ($\alpha$) and RMSE.
• Figure 5: Zoomed-in view of the LMC region where the $R_I$ adjustment is applied. The plot shows the final $R_I$ values of individual sample stars, with different colors indicating different adjusted values: gray marks the initial $R_I$, while purple, red, green, and blue represent progressively smaller $R_I$ values.