Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) I. The final word on the origin of NGC6388 and NGC6441

TL;DR

CARMA provides a homogeneous, systematic approach to determine globular cluster ages, revealing that NGC6388 and NGC6441 are likely formed in-situ, not accreted, refining our understanding of the Milky Way's assembly history.

Contribution

This paper introduces the CARMA framework for precise age measurements of globular clusters and applies it to resolve the debated origin of NGC6388 and NGC6441.

Findings

NGC6388 and NGC6441 are coeval with in-situ clusters.

Their ages match in-situ field stars in the age-metallicity plane.

The clusters are not likely accreted from past mergers.

Abstract

We present CARMA, the Cluster Ages to Reconstruct the Milky Way Assembly project, that aims at determining precise and accurate age measurements for the entire system of known Galactic globular clusters and at using them to trace the most significant merger events experienced by the Milky Way. The strength of CARMA relies on the use of homogeneous photometry, theoretical isochrones, and statistical methods, that will enable to define a systematic-free chronological scale for the complete sample of Milky Way globulars. In this paper we describe the CARMA framework in detail, and present a first application on a sample of six metal-rich globular clusters with the aim of putting the final word on the debated origin of NGC6388 and NGC6441. Our results demonstrate that this pair of clusters is coeval with other four systems having a clear in-situ origin. Moreover, their location in the age-metallicity plane matches the one occupied by in-situ field stars. Such an accurate age comparison enabled by the CARMA methodology rules out the possibility that NGC6388 and NGC6441 have been accreted as part of a past merger event.