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

TL;DR

This study demonstrates that near-infrared spectral features from TP-AGB and MSTO stars can reliably determine the ages of galaxies, aligning with optical and CMD analyses, and introduces indices for age diagnostics in integrated light.

Contribution

The paper develops and tests NIR spectroscopic indices as robust chronometers for galaxy stellar populations, validated against models and previous optical studies.

Findings

NIR features from TP-AGB and MSTO stars yield consistent galaxy ages.

Indices can distinguish between populations of different ages, including very young stars.

Model predictions agree with optical and CMD age estimates.

Abstract

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 turn-off (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 ~0.1 and ~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 stongly 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 (2005) 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 CMD analysis of the giant branches. The indices discriminate between an intermediate age population of ~3-4 Gyr, a younger population of <1 Gyr, and can also detect the signatures of very young (<100 Myr) populations.