Variations in the Milky Way’s Stellar Mass Function at Fe/H < −1

Abstract We present the first determination of the Galactic stellar mass function (MF) for low-mass stars (0.2–0.5 M ⊙ ) at metallicities Fe/H ≲ −1. A sample of ∼53,000 stars was selected as metal-poor on the basis of both their halolike orbits and their spectroscopic Fe/H from Gaia DR3 BP/RP (XP) spectra. These metallicity estimates for low-mass stars were enabled by calibrating Gaia XP spectra with stellar parameters from SDSS-V. For −1.5 < Fe/H < −1, we find that the MF below 0.5 M ⊙ exhibits a “bottom-heavy” power-law slope of α ∼ −1.6. We tentatively find that at even lower metallicities the MF becomes very bottom-light, with a near-flat power-law slope of α ∼ 0 that implies a severe deficit of low-mass stars. This metallicity-dependent variation is insensitive to the adopted stellar evolution model. These results show that the Galactic low-mass MF is not universal, with variations in the metal-poor regime. A further calibration of XP metallicities in the regime of M < 0.5 M ⊙ and Fe/H < −1.5 will be essential to verify these tentative low-metallicity trends.