Galaxies M32 and NGC 5102 Confirm a Near-infrared Spectroscopic Chronometer

We present near-infrared (NIR) IRTF/SpeX spectra of the intermediate-age galaxy M32 and the post-starburst galaxy NGC 5102. We show that features from thermally pulsing asymptotic giant branch (TP-AGB) and main-sequence turnoff (MSTO) stars yield similar ages to those derived from optical spectra. The TP-AGB can dominate the NIR flux of a coeval stellar population between similar to 0.1 and similar to 2 Gyr, and the strong features of (especially C-rich) TP-AGB stars are useful chronometers in integrated light studies. Likewise, the Paschen series in MSTO stars is strongly dependent on age and is an indicator of a young stellar component in integrated spectra. We define four NIR spectroscopic indices to measure the strength of absorption features from both C-rich TP-AGB stars and hydrogen features in main-sequence stars, in a preliminary effort to construct a robust chronometer that probes the contributions from stars in different evolutionary phases. By comparing the values of the indices measured in M32 and NGC 5102 to those in the Maraston stellar population synthesis models for various ages and metallicities, we show that model predictions for the ages of the nuclei of M32 and NGC 5102 agree with previous results obtained from integrated optical spectroscopy and color-magnitude diagram analysis of the giant branches. The indices discriminate between an intermediate-age population of similar to 3-4 Gyr, a younger population of less than or similar to 1 Gyr, and can also detect the signatures of very young less than or similar to 100 Myr populations.