Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The ACS Survey of Galactic Globular Clusters. VII. Relative Ages   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.

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The categories of this paper are: q-alg
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The categories of this paper are: q-fin
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The categories of this paper are: cmp-lg
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The categories of this paper are: atom-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: q-bio
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The categories of this paper are: math-ph
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The categories of this paper are: math
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The categories of this paper are: econ
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The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: chem-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-lat