The ACS Survey of Galactic Globular Clusters. VII. Relative Ages
Antonio Marin-Franch, Antonio Aparicio, Giampaolo Piotto, Alfred, Rosenberg, Brian Chaboyer, Ata Sarajedini, Michael Siegel, Jay Anderson,, Luigi R. Bedin, Aaron Dotter, Maren Hempel, Ivan King, Steven Majewski,, Antonino P. Milone, Nathaniel Paust, I. Neill Reid

TL;DR
This study provides precise relative ages for 64 Galactic globular clusters using HST data, revealing two distinct populations that support a two-phase formation history of the Milky Way halo.
Contribution
The paper introduces a homogeneous method for determining relative ages of globular clusters with 2-7% precision, independent of theoretical models, and identifies two main cluster populations.
Findings
Old clusters have ~5% age dispersion and no age-metallicity relation.
Younger clusters show an age-metallicity relation similar to those in dwarf galaxies.
Results support a two-phase formation scenario for the Milky Way halo.
Abstract
The ACS Survey of Galactic Globular Clusters is a Hubble Space Telescope (HST) Treasury program designed to provide a new large, deep and homogeneous photometric database. Based on observations from this program, we have measured precise relative ages for a sample of 64 Galactic globular clusters by comparing the relative position of the clusters' main sequence turn offs, using main-sequence fitting to cross-compare clusters within the sample. This method provides relative ages to a formal precision of 2-7%. We demonstrate that the calculated relative ages are independent of the choice of theoretical model. We find that the Galactic globular cluster sample can be divided into two groups -- a population of old clusters with an age dispersion of ~5% and no age-metallicity relation, and a group of younger clusters with an age-metallicity relation similar to that of the globular clusters…
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