Development of Near-Infrared Spectroscopic Chronometers for Galaxy Evolution: Critical Testing With Nearby Galaxies

I present preliminary results in the development of a near-infrared (NIR) spectroscopic chronometer for studying stellar populations in galaxies. By examining NIR IRTF/SpeX spectroscopy of the young post-starburst galaxy NGC 5102, and the compact elliptical M32, I have defined six indices that measure the strength of features due to thermally-pulsing asymptotic giant branch (TP-AGB) and main sequence turn-off (MSTO) stars, which are prominent in the integrated spectra of young populations. Two indices measure equivalent widths of Paschen lines originating in the atmospheres of hot stars, and are sensitive to ages up to ∼100 Myr; three indices measure the flux ratios of narrow bands surrounding molecular absorption breaks due to very cool and luminous TP-AGB stars; and the final index is a blended line also sensitive to TP-AGB stars. The TP-AGB is known to dominate the NIR flux of stellar populations between ∼100 Myr and 3 Gyr, and therefore the indices are particularly sensitive to ages in this regime. To demonstrate that the NIR indices can be used as chronometers, I compare the values measured in NGC~5102 and M32 to those in the Maraston (2005) stellar population models for a grid of ages and metallicities. The model-derived star formation histories for both galaxies are consistent with previous studies with established optical spectroscopic techniques. Moreover, in the case of NGC~5102 the indices are able to detect the presence of two distinct stellar populations with ages ∼20--40 Myr and several hundred Myr. I also address the issue of TP-AGB contribution in stellar population models, which is largely uncertain due to poorly understood processes in the evolution of TP-AGB stars, such as mass loss and convective burning. I find that the TP-AGB contribution in the Maraston (2005) models is accurate within error limits on the measured indices. Therefore, more work will be needed to place strict limits on this important ingredient in stellar population models. The NIR indices in M32 are also examined, and model comparisons predict an SSP-equivalent age of ∼2.75 Gyr, consistent with optical studies. A more complicated star formation history in M32 cannot be ruled out, however, because the sensitivity of the indices declines at intermediate ages.