A Near Infrared Spectroscopic Chronometer Suitable for JWST Confirmed with Nearby Galaxies

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Scientific paper

The thermally-pulsing asymptotic giant branch (TP-AGB) dominates the near-infrared (NIR) flux of a coeval stellar population between 0.1 and 1 Gyr, and hence can serve as a useful chronometer. Here we present NIR spectra of the intermediate-age galaxy M32 and the post-starburst galaxy NGC 5102 with the SpeX spectrograph on the NASA Infrared Telescope Facility, and show that spectral features due to the TP-AGB and other evolutionary phases can be used to determine the mean stellar population ages of these two galaxies. Specifically, we define four NIR spectroscopic indices that measure the strength of stellar absorption features: two Paschen-series lines and two indices sensitive to carbon-rich TP-AGB stars. By comparing the observed values of these indices 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 3-4Gyr, a younger population of 1 Gyr, and also detect the signatures of very young 100 Myr populations. This technique will be useful for studying the formation history of galaxies in the early universe with the James Webb Space Telescope.

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