Metallicity Distribution in the LMC and the SMC Based on the Tip-RGB Colors

Abstract

The color index ( J − K ) _0 of tip-red-giant branch (TRGB) is used to study the metallicity distribution in the Large and Small Magellanic Cloud. With the most complete and pure sample of red member stars so far, the areas are divided into 154 and 70 bins for the LMC and SMC, respectively, with similar number of stars by the Voronoi binning. For each bin, the position of TRGB on the near-infrared color–magnitude diagram, specifically ( J − K ) _0 / K _0 , is determined by the poison-noise weighted method. Converting the color index of TRGB into metallicity, the metallicity gradients in the LMC and the SMC are obtained in four major directions. For the LMC, the gradient to the north is −0.006 ± 0.004 dex kpc ^−1 , to the south −0.010 ± 0.005 dex kpc ^−1 , to the east −0.006 ± 0.003 dex kpc ^−1 , and to the west −0.010 ± 0.003 dex kpc ^−1 . The farthest distance extends to 16 kpc. For the SMC, the gradients to the north, south, east, and west are −0.017 ± 0.031 dex kpc ^−1 , −0.016 ± 0.007 dex kpc ^−1 , −0.003 ± 0.002 dex kpc ^−1 , and −0.004 ± 0.003 dex kpc ^−1 , respectively. The farthest distance for the SMC extends to 27 kpc. The gradient is large from the center to 1 kpc.

Keywords

• Red giant tip
• Magellanic Clouds
• Metallicity