Uncovering Multiple Populations in Globular Clusters with Washington Photometry

Globular Clusters (GCs), long considered as ideal Simple Stellar Populations, are now known to harbor a wide variety of chemical inhomogeneities. Multiple populations (MP) are being found in a growing number of Galactic globular clusters (GCs) via both photometric and spectroscopic techniques. Indeed, it has been suggested that a GC is an object that possesses MP. A definitive investigation of MP in GCs will undoubtedly provide a profound improvement in our understanding of their formation and evolution.However, most studies employ either high resolution VLT spectroscopy, HST photometry or inefficient filters from the ground. A ground-based photometric system which is both efficient and effective would be especially excellent for uncovering MP. We demonstrate that the Washington system meets these goals. The Washington C filter, in addition to being specifically designed for the purpose of detecting MPs, is both much broader and redder than competing UV filters, making it far more efficient at detecting MPs and much less sensitive to reddening and extinction.Our analysis of the well-studied GC NGC 1851 shows indeed that the C filter is both very efficient and effective at detecting its previously discovered MPs in the RGB and SGB, using relatively little telescope time on only a 1-meter telescope. Remarkably, we have also detected an intrinsically broad MS best characterized by two distinct but heavily overlapping populations that cannot be explained by binaries, field stars, or photometric errors. Detailed analysis shows that the MS distribution is in very good agreement with that seen on the RGB. This is the first time MPs in a MS have been discovered from the ground, and just as strikingly, using only a 1-meter telescope. The Washington system thus proves to be a very powerful tool for investigating MPs, and holds particular promise for extragalactic objects where photons are limited.