2025ApJ...995..135L

We demonstrate how near-infrared (NIR) imaging of resolved luminous asymptotic giant branch (AGB) stars can be used to measure well-constrained star formation histories (SFHs) across cosmic time. Using UKIRT J and K-band imaging of M31, we first show excellent agreement over the past ∼8 Gyr between the Panchromatic Hubble Andromeda Treasury SFH of M31's outer disk derived from a deep optical color-magnitude diagram (CMD; ∼3.3 × 10⁷ stars with M_F814W ≲ +2), and our spatially matched SFH based only on modeling AGB stars on an NIR CMD (∼7.7 × 10³ stars with M_J ≲ -6). We find that only ∼1000 AGB stars are needed for reliable SFH recovery, owing to their excellent age sensitivity in the NIR. We then measure the spatially resolved SFH of M31's inner stellar halo (D_{M31,projected} ∼ 20-30 kpc) using ∼10⁴ AGB stars. We find: (i) a dominant burst of star formation across M31's stellar halo 3-5 Gyr ago and lower level, spatially distributed star formation ∼1-3 Gyr ago; (ii) ${M}_{\star }\approx {3}_{-1}^{+5}\times 1{0}^{9}{M}_{\odot }$ formed over the past ∼8 Gyr. We discuss some caveats and the enormous potential of resolved AGB stars in the NIR for measuring SFHs back to ancient epochs (∼14 Gyr ago) in galaxies to large distances (D ≳ 20 Mpc) with JWST, Roman, and Euclid.