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 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.
