The Deepest Near-infrared Color-Magnitude Diagram from the Ground

Left: GeMS/GSAOI image of NGC 6624 reveals individual stars to the cluster’s core and covers a field more than 90 arcseconds across. Right: The color-magnitude diagrams of NGC6624 obtained from the Gemini observations.

Adaptive optics enables accurate and extremely deep photometry of crowded fields, as new work using observations from Gemini demonstrates. Sara Saracino (University of Bologna) and colleagues measured thousands of stars in the globular cluster NGC 6624 using the Gemini Multi-conjugate adaptive optics System (GeMS) and the Gemini South Adaptive Optics Imager (GSAOI), finding an age of 12.0 +/- 0.5 gigayears. The near-infrared color-magnitude diagrams (using the J and Ks photometry) each span more than 8 magnitudes. The team detects the main sequence knee for the first time in a purely near-infrared color-magnitude diagram, at Ks ~ 20. They find clear evidence for mass segregation, which confirms that NGC 6624 is at an advanced stage of dynamical evolution. The work is featured in a Gemini press release and will be published in The Astrophysical Journal. A preprint is available.