Probing the Galactic Bulge with deep Adaptive Optics imaging: the age of NGC 6440

TL;DR

This study demonstrates the use of ground-based adaptive optics imaging to accurately determine the age of the globular cluster NGC 6440 in the Galactic Bulge, revealing it is as old as 47 Tuc and supporting the idea that the Bulge is slightly younger than the Halo.

Contribution

First ground-based adaptive optics measurement of the age of a bulge globular cluster using deep near-infrared photometry and color magnitude diagram analysis.

Findings

NGC 6440 is old and likely coeval with 47 Tuc.

The Galactic Bulge is at most ~2 Gyr younger than the Galactic Halo.

Ground-based AO can effectively determine cluster ages in the Bulge.

Abstract

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 towards the inner Bulge, to compute a deep color magnitude diagram from the tip of the Red Giant Branch down to J~22$, two magnitudes below the Main Sequence Turn Off (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.