Identifying Red Supergiants in the Local Group Using JWST Photometry. I. NGC 6822, Sextans A, NGC 300, WLM, and IC 1613

Abstract

Red supergiants (RSGs) are crucial for studying the properties and evolution of massive stars. It is representative to conduct a census of RSGs across the Local Group, which spans a broad metallicity range. However, identifying RSGs in distant and metal-poor galaxies remains challenging mainly due to contamination of foreground dwarfs and observational limitations. In this work, we perform PSF photometry on publicly released JWST/NIRCam images of five Local Group galaxies: NGC 6822, Sextans~A, NGC 300, WLM, and IC 1613 using the DOLPHOT NIRCam module. We find an optimal color-color diagram (CCD) for metal-poor environments, that is F115W $-$ F200W versus F356W $-$ F444W, which clearly separates RSGs from foreground dwarfs. By using the CCD, we identify 208, 135, and 22 RSG candidates in NGC 6822, Sextans A, and NGC 300, respectively, free from contamination by foreground dwarfs and oxygen-rich asymptotic giant branch stars (O-AGBs). In addition, 40 and 14 RSG candidates are directly selected on the CMD in WLM and IC 1613, respectively. Compared with previous works, the number of RSG candidates within the same luminosity range and sky region increases significantly, demonstrating the advantages of JWST in constructing a more complete RSG sample in the Local Group thanks to its high spatial resolution and photometric quality. In addition, catalogs of O-AGBs and carbon-rich AGBs (C-AGBs) are provided as by-products.

Figures (10)

• Figure 1: The observed fields of view of NGC 6822, Sextans A, NGC 300, WLM, and IC 1613. The blue frames mark the JWST fields used for photometry in this study, overlaid on GALEX ultraviolet images.
• Figure 2: Various CMDs based on the photometric results for NGC 6822, IC 1613, Sextans A, NGC 300, and WLM. In each CMD, the red dashed line denotes the magnitude of TRGB, while the sloped dashed lines $l_1$, $l_2$, and $l_3$ represent the boundaries of RSGs, O-AGBs, and C-AGBs. Sources brighter than the black dashed line are analyzed in this study.
• Figure 3: Matching the observed CMD with the isochrones from the PARSEC model, shown here as an example for NGC 6822. The left and middle panels show the observed and model CMDs, respectively, while the right panel presents the matched CMD. In the left and middle panels, the blue region indicates the area used to calculate the maximum density point of the red clump, which is marked by the orange star. In the right panel, the green, orange, blue, red and black dots represent the PARSEC isochrones with stellar masses of $\leq 2~M_\odot$, $2$--$6~M_\odot$, $6$--$8~M_\odot$, $8$--$30~M_\odot$, and $30$--$40~M_\odot$, respectively.
• Figure 4: Foreground dwarfs on the JWST CMD, shown here for NGC 6822 as an example. The black dots represent the observed data, the yellow dots represent foreground dwarfs from the Besançon model, and the blue dots represent observed foreground dwarfs in 2025ApJ...979..208L.
• Figure 5: The optimal CCD for metal-poor environments, F115W $-$ F200W versus F356W $-$ F444W, shown here for NGC 6822 as an example. In both panels, the gray dots represent all analyzed sources, and the solid line indicates the adopted boundary of the dwarf branch. In the left panel, the blue and orange dots indicate the removed foreground dwarfs and identified RSGs by 2025ApJ...979..208L, respectively, the yellow triangles show the dwarf tracks from the MARCS stellar atmosphere model, and the green dashed lines represent the dwarf tracks from the MARCS model within [Fe/H]$=-1.0\pm 0.5$.