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

TL;DR

This paper demonstrates that near-infrared imaging of asymptotic giant branch stars can reliably reconstruct detailed star formation histories of galaxies, even at large distances, using minimal stellar samples and matching optical data.

Contribution

It introduces a method to derive star formation histories from NIR AGB stars that matches optical CMD results, enabling studies of distant galaxies with upcoming telescopes.

Findings

AGB stars effectively trace star formation history over 8 Gyr.

Only about 1000 AGB stars needed for reliable SFH recovery.

Identified a major star formation burst 3-5 Gyr ago in M31's halo.

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 $\sim8$ 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 $M_{\rm F814W} \lesssim +2$), and our spatially-matched SFH based only on modeling AGB stars on a NIR CMD ($\sim7.7\times10^{3}$ stars with $M_{\rm J} \lesssim -6$). We find that only $\sim1000$ 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 ($D_{\rm M31, projected} \sim20-30$ kpc) using $\sim10^4$ 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 $\sim1-3$ Gyr ago; (ii) $M_{\star}\approx3_{-1}^{+5}\times10^9 M_{\odot}$ 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 ($\sim14$ Gyr ago) in galaxies to large distances ($D\gtrsim20$ Mpc) with JWST, Roman, and Euclid.