The Acs Survey of Galactic Globular Clusters. Vii. Relative Ages

Abstract

The ACS Survey of Galactic globular clusters is a Hubble Space Telescope 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 (MS) turnoffs, using MS 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 re-ionization in dwarf galaxies that would later merge to build the Milky Way halo as predicted by ΛCDM 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.

The kinematics of globular clusters systems in the outer halos of the Aquarius simulations

“…Further supporting evidence for the external origin of the halo GCs in the Milky Way is found by analysing their ages, metallicities, kinematics, horizontal branch morphologies, luminosities, and sizes; they are all consistent with the predictions of hierarchical formation models (e.g. Mackey & Gilmore 2004;Mackey & van den Bergh 2005;Marín-Franch et al 2009;Forbes et al 2010;Dotter et al 2011;Keller et al 2012).…”

Massive star clusters in galaxies

“…Published work (e.g. Gratton et al 2003;De Angeli et al 2005;Hansen et al 2007;Marín-Franch et al 2009; among many others) indicates that the absolute age of the metal-poor clusters is near 13 Gyr and shows a cluster-to-cluster dispersion that may be as low as ±0.5 Gyr, confirming their classic status as among the first stellar structures to have formed in the galaxy. The mean age for the metal-richer subsystem is perhaps 2 Gyr younger than the extremely old, metal-poor subsystem, and exhibits a higher cluster-to-cluster age scatter, near ±2 Gyr rms.…”

Clues on the missing sources of reionization from self-consistent modelling of Milky Way and dwarf galaxy globular clusters

“…Past observations have shown that galactic metal-poor GCs tend to be older than metal-rich GCs and the age spread in metal-poor GCs is ∼ 1 Gyr compared to the ∼ 6 Gyr dispersion in metal-rich GCs (Rosenberg et al 1999;Salaris & Weiss 2002;Marín-Franch et al 2009). There is some evidence of self enrichment in GCs; however, the age gap between metal-poor and metal-rich GCs is greater than the range present within each population suggesting that these are two distinct populations (Marín-Franch et al 2009).…”

“…There is some evidence of self enrichment in GCs; however, the age gap between metal-poor and metal-rich GCs is greater than the range present within each population suggesting that these are two distinct populations (Marín-Franch et al 2009). However, more recent observations have shown a gradual trend of increasing age spread with increasing metallicity which weakens this age gap described in the previous works (Dotter et al 2011;VandenBerg et al 2013).…”