The AGB phase-transition outside the local group:: K-band observations of young star clusters in NGC 7252

We have extended the study of the young star clusters observed in the merger remnant galaxy NGC 7252 by obtaining K band photometry for these clusters. Our K band data significantly complement the optical photometry and spectroscopy in the literature: K band data are fundamental to study the Asymptotic Giant Branch (AGB) population of these clusters, since the AGE phase transition (occuring between the age of similar to 200 Myr and similar to1 Gyr) causes abrupt changes in the near-infrared luminosity of the clusters while producing only small changes in the optical. Therefore, the e.g. V-K colour is ideal to study this evolutionary phase of stellar populations. For the present analysis we present models for Simple Stellar Populations which include the contribution of the AGE stellar phase, calibrated with the young and intermediate age star clusters of the Magellanic Clouds. The comparison with the colour distribution of the NGC 7252 star clusters shows that they are indeed intermediate age clusters undergoing the AGE phase transition. The AGE phase transition is observed for the first time outside the Local Group. Most of the studied clusters span the very narrow age range 300-500 Myr, and likely base metallicities 0.5-1 Z(circle dot). A very important exception is the cluster W32, which has already completed its AGE epoch, its colours being consistent with an age of similar to1-2 Gyr. This impacts on the duration of the merger-induced starburst. The strengths of the magnesium and iron lines in the spectrum of the best observed cluster W3, and in the spectrum of the diffuse central light of NGC 7252, do not show an overabundance in alpha -elements, in contrast to the bulk stellar population of elliptical galaxies.