The metallicity spread and the age-metallicity relation of Omega
  Centauri
  Omega Centauri is a peculiar Globular Cluster formed by a complex stellar
population. To shed light on this, we studied 172 stars belonging to the 5 SGBs
that we can identify in our photometry, in order to measure their [Fe/H]
content as well as estimate their age dispersion and the age-metallicity
relation. The first important result is that all of these SGBs has a
distribution in metallicity with a spread that exceeds the observational errors
and typically displays several peaks that indicate the presence of several
sub-populations. We were able to identified at least 6 of them based on their
mean [Fe/H] content. These metallicity-based sub-populations are seen to
varying extents in each of the 5 SGBs. Taking advantage of the age-sensitivity
of the SGB we showed that, first of all, at least half of the sub-populations
have an age spread of at least 2 Gyrs. Then we obtained an age-metallicity
relation that is the most complete up to date for this cluster. The
interpretation of the age-metallicity relation is not straightforward, but it
is possible that the cluster (or what we can call its progenitor) was initially
composed of two populations having different metallicities. Because of their
age, it is very unlikely that the most metal-rich derives from the most
metal-poor by some kind of chemical evolution process, so they must be assumed
as two independent primordial objects or perhaps two separate parts of a single
larger object, that merged in the past to form the present-day cluster.
