2025AJ....170....2L

This study investigates the metallicity distribution in M31 and M33 by using the near-infrared color index J - K of the tip of the red giant branch (TRGB) of the member stars from Y. Ren et al. after removing foreground dwarf stars by a near-infrared J - H versus H - K diagram as well as Gaia astrometric measurements. We employ the Voronoi binning technique to divide the galaxy into subregions, the Poisson noise method to determine the TRGB position in the J - K versus K diagram, and the bootstrap method to estimate uncertainties. The TRGB positions are calculated to be J - K = 1.195 ± 0.002 and 1.100 ± 0.003, and K = 17.615 ± 0.007 and 18.185 ± 0.053 for M31 and M33, respectively, consistent with previous results. The J - K gradient of M31 is found to be -0.0055 kpc^–1 at R_GC = (0, 24) kpc and -0.0002 kpc^–1 at R_GC = (24, 150) kpc. Notably, two dust rings are evident at 11.5 and 14.5 kpc, in agreement with previous studies. The J - K of M33 is analyzed in four directions and generally shows a trend of gradually decreasing from the inside to the outside, however, it shows an increasing trend of 0.022 kpc^–1 in the inner 0-2 kpc in the west. Through the color-metallicity relation of the TRGB, the metallicity gradient of M31 is -0.040 ± 0.0012 dex kpc^–1 for R_GC < 30 kpc and -0.001 ± 0.0002 dex kpc^–1 for R_GC > 30 kpc, and for M33, it is -0.269 ± 0.0206 dex kpc^–1 for R_GC < 9 kpc.