2026ApJ...998L..33L

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.