4. Available data and SSP templates
Stellar clusters in the Large Magellanic Cloud provide a convenient and, in practice, unique set of templates for young and intermediate age stellar populations spanning a wide range of metallicities (between 1/100 and solar, e.g. Sagar & Pandey 1989). The available [CO] data are summarized in Fig. 3. The large scatter of the points at a given age reflects variations of metallicities, statistical effects related to the intrinsic small number of luminous red stars in the clusters and, possibly, field contamination (see e.g. Chiosi et al. 1986, Santos et al. 1997). A further complication occurs beyond 600 Myr, when carbon stars appear. These often display a very red spectrum with strong continuum emission from the envelope which dilutes the CO bands and produce an anti-correlation between J-K colours and [CO] index (Persson et al. 1983). Note in particular that the mild anti-correlation between CO index and age visible in Fig. 3 also reflects the fact that the older clusters are, on average, less metallic than the youngest one's.
For the purposes of this paper, the most important fact is that several of the LMC clusters with ages 100 Myr display [CO] indices much larger than those predicted by the models based on the Geneva's tracks. In other words, a highly metallic stellar population of 100-1000 Myr could have a [CO] similar to that of a 10-100 Myr cluster of the same metallicity, as indeed predicted by models including the whole AGB evolution (see Sect. 3).
Therefore, finding a galaxy with a very deep CO index does not necessarily imply that its stellar population must be younger than 100 Myr, as sometimes assumed in literature.
Fig. 3 also shows results from a wider set of data, including old stellar systems (Galactic globular clusters and ellipticals) and starburst galaxies. In general, the only clear observational result is that objects with [CO] significantly larger than 0.18 cannot be old stellar systems of (sub)solar metallicities, but require younger stellar populations or, alternatively, old stellar populations much more metallic than ellipticals. Encouragingly, several starburst galaxies do indeed display CO indices significantly larger than the above threshold but, in most cases, within the range covered by LMC clusters of ages 109 yr (see Fig. 3).
On the other hand, however, other well studied starbursters have values of [CO] lower than 0.18 and more similar to ellipticals and bulges. This probably reflects metallicity variations, i.e. weaker CO features can be associated with young stellar systems of lower metallicities.
© European Southern Observatory (ESO) 2000
Online publication: May 3, 2000