Probing The Galactic Bulge With Deep Adaptive Optics Imaging: The Age Of Ngc 6440

摘要

We present first results of a pilot project aimed at exploiting the potentiality of ground-based adaptive optics imaging in the near-infrared to determine the age of stellar clusters in the Galactic bulge. We have used a combination of high-resolution adaptive optics (ESO-VLT NAOS-CONICA) and wide-field (ESO-NTT-SOFI) photometry of the metal-rich globular cluster NGC 6440 located toward the inner bulge, to compute a deep color-magnitude diagram from the tip of the red giant branch down to J ～ 22, 2 magnitudes below the main-sequence turnoff (TO). The magnitude difference between the TO level and the red horizontal branch has been used as an age indicator. It is the first time that such a measurement for a bulge globular cluster has been obtained with a ground-based telescope. From a direct comparison with 47 Tuc and with a set of theoretical isochrones, we concluded that NGC 6440 is old and likely coeval to 47 Tuc. This result adds a new evidence that the Galactic bulge is ≈2 Gyr younger at most than the pristine, metal-poor population of the Galactic halo.