First spectroscopic evidence for carbon stars outside the local group: Properties of a massive star cluster in NGC 7252

We present near-infrared [ $1{-}2.3\,\mu$m] spectroscopy of the massive intermediate age star cluster W3 in the merger remnant and proto-elliptical galaxy NGC 7252, obtained with the NTT telescope. This cluster has an age when the integrated near-infrared properties of a stellar population are dominated by the cool and luminous Asymptotic Giant Branch (AGB). We compare the data with instantaneous burst model predictions from new evolutionary synthesis models that include: (i) the computation of the evolution through the thermally pulsing AGB (TP-AGB) for low- and intermediate-massive stars, with the initial mass and metallicity dependent formation of carbon stars; (ii) spectroscopic data from a new stellar library in which differences between static red giants, variable oxygen rich TP-AGB stars and carbon stars are accounted for. The new evolutionary model predicts that the contribution of carbon rich stars to the luminosities in the near-IR passbands is a strong function of metallicity. The comparison of the data to the models clearly shows that carbon stars are present: for the first time, carbon rich star spectral features are thus detected directly outside the Local Group galaxies. Good fits to the available optical/near-IR photometry and the near-IR spectrum of NGC 7252-W3 are found for an age of 300–400 Myr and $A_{V}\,\simeq\,0.6{-}0.8$. The models show that these parameters depend weakly on the model metallicity in the range of $Z/Z_{\odot}=0.4{-}1$, with higher likelihood for solar metallicity models. At solar metallicity, a mixture of carbon rich and oxygen rich stars is predicted. The strength of the near-IR molecular bands that originated from oxygen rich AGB stars can be used to constrain the absolute Teff scale of these objects, i.e. a relation between colour and Teff. We found that, in the framework of our set of evolutionary tracks, the data are more consistent with the temperature scale calibrated on Long Period Variables than on giant stars. At a given colour, variable AGB stars have a lower Teff than static (or quasi-static) M giants