The ACS Survey of Galactic Globular Clusters. VII. Relative Ages

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 associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases or processes. The first one would be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clusters in the old group were formed. Also these clusters could have been formed before reionization in dwarf galaxies that would later merge to build the Milky Way halo as predicted by Lambda-CDM cosmology. However, the galactocentric metallicity gradient shown by these clusters seems difficult to reconcile with the latter. As for the younger clusters, it is very tempting to argue that their origin is related to their formation within Milky Way satellite galaxies that were later accreted, but the origin of the age-metallicity relation remains unclear.

50 pages, 14 figures. Accepted for publication in ApJ

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• A combined study of thermohaline mixing and envelope overshooting with PARSEC: Calibration to NGC 6397 and M4
• Trojan globular clusters: radial migration via trapping in bar resonances
• Globular Cluster Intrinsic Iron Abundance Spreads: II. Protocluster Metallicities and the Age-Metallicity Relations of Milky Way Progenitors
• The $Hubble$ Missing Globular Cluster Survey. I. Survey overview and the first precise age estimate for ESO452-11 and 2MASS-GC01
• Globular cluster ages and their relation to high-redshift stellar cluster formation times from different globular cluster models
• Evolution of long-lived globular cluster stars IV. Initial helium content and white-dwarf properties
• The Bottom-Light Present Day Mass Function of the Peculiar Globular Cluster NGC 6535
• The Blue Straggler Population in Dwarf Galaxies
• Evolutionary history of four binary blue stragglers from the globular clusters ωCen, M55, 47 Tuc and NGC 6752
• Searching for new observational signatures of the dynamical evolution of star clusters
• Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. III. Grids of Isochrones for $-2.5\le$ [Fe/H] $\le -0.5$ and Helium Abundances $Y=0.25$ and $0.29$ at Each Metallicity
• Unveiling the Universe with Emerging Cosmological Probes
• Core collapse and horizontal-branch morphology in galactic globular clusters
• To Grow Old and Peculiar: A Survey of Anomalous Variable Stars in M80 and Age Determination using K2 and Gaia
• Investigating a predicted metallicity [Fe/H] variation in the Type II Globular Cluster NGC 362
• Inferring the Helium abundance of extragalactic Globular Clusters using Integrated Spectra
• 2MASS photometry and age estimate of globular clusters in the outer halo of M31
• On the original composition of the gas forming first-generation stars in clusters: insights from HST and JWST
• The kinematics of globular clusters systems in the outer halos of the Aquarius simulations
• Stellar photometry with Multi Conjugate Adaptive Optics
• Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud V: Multiple Populations in ancient Globular Clusters
• Kinematics of outer halo globular clusters: M 75 and NGC 6426
• Chemical composition of the outer halo globular cluster Palomar 15
• The formation and disruption of globular cluster populations in simulations of present-day $L^{\ast }$ galaxies with controlled assembly histories
• The Stellar Population and Orbit of the Galactic Globular Cluster Palomar 3
• Effect of Binary Fraction on Horizontal Branch Morphology under Tidally Enhanced Stellar Wind
• The GALAH survey: Improving chemical abundances using star clusters
• The bulge globular cluster Terzan 6 as seen from multi-conjugate adaptive optics and HST
• Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)
• The ACS survey of globular clusters. XIII. Photometric calibration in comparison with Stetson standards
• The Effects of $\Lambda$CDM Dark Matter Substructure on the Orbital Evolution of Star Clusters
• Simulating Eclipsing Binary Yields of the Rubin Observatory in the Galactic Field and Star Clusters
• Spectroscopy of Red Giants in the globular cluster Terzan 8: kinematics and evidence for the surrounding Sagittarius stream
• On the use of asymmetric PSF on NIR images of crowded stellar fields
• The GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) II: Characterization of 47 Tuc with Bayesian Statistics
• On the impossible NGC 4372 V1 and V2: an extended AGB to the [Fe/H] = -2.2 cluster
• On the Fundamental Line of Galactic and Extragalactic Globular Clusters
• On the relationship between metallicity distributions of globular clusters and of circumgalactic gas
• Globular clusters in \textsc{OrbIT}: complete dynamical characterisation of the globular cluster population of the Milky Way through updated orbital reconstruction
• The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios
• Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud: III. Horizontal Branch Morphology
• Stellar Ages: A Code to Infer Properties of Stellar Populations
• On the origin of globular cluster bimodality
• Quantifying chemical and kinematical properties of Galactic disks
• Statistical analysis of Galactic globular cluster type properties
• New Way to Date Globular Clusters: Brown Dwarf Cooling Sequences
• Exploration about the origin of galactic and extragalactic star clusters through simulated H-R diagrams
• Not all nitrogen-rich field stars originate from globular clusters
• Is Palomar 1 really associated with the Canis Major overdensity?
• Exotic populations in Globular Clusters: Blue Stragglers as tracers of the internal dynamical evolution of stellar systems
• Impact of the low solar abundance on the ages of globular clusters
• The Escape of Globular Clusters from the Satellite Dwarf Galaxies of the Milky Way
• Constraining the original composition of the gas forming first-generation stars in globular clusters
• Infrared photometry and CaT spectroscopy of the most metal-poor in-situ globular cluster VVV-CL001
• Revealing the stellar population of the ultra-obscured Galactic globular cluster Glimpse-C02
• Tidal stripping as a mechanism for placing globular clusters on wide orbits: the case of MGC1 in M31