The GeMS/GSAOI Galactic Globular Cluster Survey (G4CS)

We present the first results from the GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) of the Milky Way globular clusters (GCs) NGC3201 and NGC2298. Using the Gemini South Adaptive Optics Imager (GSAOI) behind the Gemini Multi-conjugate adaptive optics System (GeMS) on the 8.1-meter Gemini South telescope, we have collected deep near-IR observations of the clusters, resolving their stellar populations down to Ks = 21 Vega mag. Point spread function (PSF) photometry was performed utilizing a spatially variable PSF to overcome AO correction variations across the field of view. The resulting near-IR photometric catalogs were augmented with optical and near-ultraviolet photometry from the Hubble Space Telescope (HST). We apply proper motion cleaning and differential reddening corrections before utilizing the characteristic color-luminosity difference between the lower main sequence knee (MSK) and main sequence turnoff (MSTO) to determine the ages. We find that the Ks vs. F606W-Ks and F336W vs. F336W-Ks filter combinations provide the most diagnostic power and we use them to derive the stellar population ages, distances and reddening values for both clusters. We fit three sets of isochrone models to the two color combinations using a pseudo-χ2 approach. A weighted average of the results gives best-fit absolute ages of 12.2 ± 0.5 Gyr and $13.2 ± 0.4 Gyr for NGC3201 and NGC2298, respectively. Our derived parameters are in good agreement with recent age determinations of the too clusters, with our constraints on the ages ranking among the most statistically robust. These findings demonstrate the power of GeMS/GSAOI as a tool for the exploration of both cluster characteristics and their constituent stellar populations.