Mass-Metallicity Relation of the Fossil Disk in Six Nearby Galaxies using JWST RGB Stars

Abstract

We here present the first ever mass-metallicity relation (MZR) of fossil disk populations using the resolved stellar populations in the James Webb Space Telescope Near-Infrared Camera (JWST/NIRCam) images of six nearby galaxies. Both the mass and metallicity were determined using the Red Giant Branch (RGB) stars, employing methods that have been well established to study the halo populations of nearby galaxies using the Hubble Space Telescope images. Specifically, we obtained the mass and metallicity by comparing the color, magnitude and number density near the tip of the RGB (TRGB) stars in the F115W$-$F200W vs F200W color-magnitude diagrams (CMDs) with that of the PARSEC isochrones of 4 and 13~Gyr. The RGB metallicities of the disks of nearby galaxies closely match those found in galaxies at redshifts $z>8$, with the metallicity range following a power-law relation with the fossil disk mass. The power-law index of the fossil disk MZR, $γ\approx0.26$, compares well with the typical value found in MZRs obtained using gas metallicities across redshifts. Through this work, we illustrate the utility of JWST/NIRCam images for obtaining the mass and metallicity of the fossil disks of nearby galaxies at precisions that were not possible earlier, which opens up a new method to carry out the near-field cosmological studies.