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Abstract 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 × 10 7 stars with M F814W ≲ +2), and our spatially matched SFH based only on modeling AGB stars on an NIR CMD (∼7.7 × 10 3 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 ∼10 4 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 ⋆ ≈ 3 − 1 + 5 × 1 0 9 M ⊙ 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.
Lee et al. (Thu,) studied this question.