The stellar cluster system of the Magellanic Clouds

People involved at OAB: Origlia, Fusi Pecci

The spectral evolution of a Simple Stellar Population (SSP) and its most evident color glitches are ideal clocks for dating primeval galaxies and deriving a suitable, empirical relation between lookback time and redshift.

The empirical calibration of the clock which settles the spectral evolution of SSPs and its readability are the primary goals of our project. The globular cluster system of the Magellanic Clouds (MC) provides a unique opportunity to investigate the integrated spectrophotometric behavior of stellar populations as a function of both age and chemical composition. We are tackling these major astrophysical objectives by means of a coordinated spectrophotometric survey on a representative sample of MC clusters, aimed at determining with great accuracy and in a homogeneous way their age, metallicity and overall integrated spectral properties.

During a number of successful observing runs with SOFI@NTT, our group secured high quality $J$, $H$, $K$ photometry of 20 LMC clusters spanning the age range between 50 Myr and a few Gyr. Popolous and complete near-IR CMDs covering the entire RGB extension have been obtained. The high quality and homogeneity of such an IR database will provide the most accurate empirical determination of the occurrence of the so-called AGB and RGB phase transitions and their contribution to the cluster integrated light in each photometric IR band-pass and in bolometric. These empirical estimates compared to those of the models will allow to calibrate the integrated magnitude and colour glitches in terms of age, i.e., to reliably calibrate the stellar clock which is the fundamental engine of any evolutionary synthesis technique. The results based on the first set of data have been recently published (see Ferraro et al. 2004) and demonstrated that the full development of the RGB occurs at ages around 700 Myr and is a relatively fast event (duration $\sim 300$ Myr).

The correct reading of the age from a SSP requires the accurate knowledge of the global metallicity. This major piece of information is still lacking, namely a self-consistent metallicity scale and a detailed description of the abundance patterns of MC clusters. In order to fill such a gap, we are performing a spectroscopic survey at medium-high resolution in the visual (by using UVES+GIRAFFE at the ESO-VLT) and in the IR (by using SOFI@NTT) for a representative sample of MC clusters. Such high quality spectroscopic data coupled with the photometric database secured by our group over the last 10 years will allow to calibrate the evolutionary clock, a fundamental tool to trace the history of star formation and evolution of primordial galaxies.

This research is in collaboration with F. Ferraro (Bologna University), V. Testa (INAF-Rome Obs.), C. Maraston (Max-Planck Munich, Germany).