Post-Starburst Populations Near and Far — The Potential of Near-IR Spectroscopy

Abstract

Abstract. The efficient use of near-IR data in studies of external stellar populations depends on our ability to recognize the nature of the predominant sources of light, and to interprete these findings in terms of age and metallicity. Here we focus on elderly post-starburst populations, with ages of 10 8 − 10 9 yrs. New models confirm that they are indeed expected to display specific spectral signatures in the near-IR, due to variable M stars of the asymptotic giant branch and to carbon stars. The signatures depend on age and metallicity. We summarize the status of current quantitative predictions and emphasize the importance of an empirical calibration of the spectral synthesis models. OverviewNear-IR data has been used very often to confirm the presence of an "evolved stellar component" in starburst galaxies. The need to study stellar populations deeply embedded in dust is also frequently invoked to justify near-IR observations. But what exactly are these cool stellar populations seen between 1 and 3 µm? Population synthesis models tell us that red supergiants are the main sources 10 7 − 10 8 yrs after a star formation episode; during the following 10 9 yrs, stars of the upper, thermally pulsing asymptotic giant branch (TP-AGB) become predominant; and finally giants of the early AGB (E-AGB) and of the first red giant branch (RGB) outweigh other populations. The efficient use of near-IR data depends on our ability to distinguish these categories from each other and to interprete them in terms of age and metallicity.Very strong CO absorption (2.3 µm) is a clear signature of a red supergiant population ([1] initiated the systematic exploration of CO in cool stars; [19] analysed early observations in starbursts; [9] discussed the diagnostic power of the feature and its limitations). But the dependence of the red supergiant population on metallicity is strong [14] and not reproduced by stellar evolution models unless ad hoc corrections are invoked ([12], [17]). Therefore the threshold above which the CO absorption becomes an unambiguous signature of a supergiant population is uncertain. On the other hand, it has recently been claimed that high quality, high spectral resolution near-IR observations allow for relative age-dating of young post-starburst populations to within 2 − 3 Myr, at least at solar metallicity (Gilbert, this workshop).