Measuring Star Formation Histories from Asymptotic Giant Branch Stars: A Demonstration in M31

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 $\sim8$ Gyr between the PHAT SFH of M31's outer disk derived from a deep optical color-magnitude diagram (CMD; $\sim3.3\times10^{7}$ stars with $M_{F814W} \lesssim +2$), and our spatially-matched SFH based only on modeling AGB stars on a NIR CMD ($\sim2.3\times10^{4}$ stars with $M_{J} \lesssim -5$). We find that only hundreds of 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 ($D_{M31, projected} \sim20-30$ kpc) using $\sim10^4$ AGB stars. We find: (i) a dominant burst of star formation across M31's inner stellar halo from $4-5$ Gyr ago and lower level, spatially distributed star formation $\sim1-2$ Gyr ago; (ii) a younger 'quenching time' in the vicinity of NGC 205 ($\sim1$ Gyr ago) than near M32 ($\sim1.6$ Gyr ago); (iii) $M_{\star}\sim4\pm0.5\times10^9 M_{\odot}$ formed over the past $\sim8$ Gyr. We discuss some caveats and the enormous potential of resolved AGB stars in the NIR for measuring SFHs back to ancient epochs ($\sim13$ Gyr ago) in galaxies to large distances ($D\gtrsim20$ Mpc) with JWST, Roman, and Euclid.