Metallicity Distribution in M31 and M33 Based on the Tip of the Red Giant Branch Near-infrared Color

Abstract 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.