Ultra-deep GEMINI near-infrared observations of the bulge globular
  cluster NGC 6624
  We used ultra-deep $J$ and $K_s$ images secured with the near-infrared GSAOI
camera assisted by the multi-conjugate adaptive optics system GeMS at the
GEMINI South Telescope in Chile, to obtain a ($K_s$, $J-K_s$) color-magnitude
diagram (CMD) for the bulge globular cluster NGC 6624. We obtained the deepest
and most accurate near-infrared CMD from the ground for this cluster, by
reaching $K_s$ $\sim$ 21.5, approximately 8 magnitudes below the horizontal
branch level. The entire extension of the Main Sequence (MS) is nicely sampled
and at $K_s$ $\sim$ 20 we detected the so-called MS "knee" in a purely
near-infrared CMD. By taking advantage of the exquisite quality of the data, we
estimated the absolute age of NGC 6624 ($t_{age}$ = 12.0 $\pm$ 0.5 Gyr), which
turns out to be in good agreement with previous studies in the literature. We
also analyzed the luminosity and mass functions of MS stars down to M $\sim$
0.45 M$_{\odot}$ finding evidence of a significant increase of low-mass stars
at increasing distances from the cluster center. This is a clear signature of
mass segregation, confirming that NGC 6624 is in an advanced stage of dynamical
evolution.
