Surface Brightness Fluctuations to constrain secondary stellar
  populations: Revealing very low-metallicity stars in massive galaxies
  The aim of this work is to explore the potential of Surface Brightness
Fluctuations (SBF) for studying composite stellar populations (CSP). To do so,
we have computed the standard (mean) and SBF spectra with E-MILES stellar
population synthesis code. We have created a set of models composed by
different mass fractions of two single stellar populations (SSP), as a first
approximation of a CSP scenario. With these models we present an ensemble of
SBF colour-colour diagnostic diagrams that reveal different secondary
populations depending on the bands used. For this work we focus on those
colours capable of unveiling small fractions of metal-poor components in
elliptical galaxies, which are dominated by old metal-rich stellar populations.
We fit a set of synthetic models and a selection of nearby elliptical galaxies
to our CSP models using both mean and SBF colours. We find that the results are
highly improved and return small secondary components when mean and SBF values
are applied simultaneously, instead of employing them separately or as a
constraint. Finally, we explore the possibility of tracking chemical enrichment
histories by including in the analysis a variety of SBF colours. For this
purpose we present an example where, with two different SBF colour-colour
diagrams, we untangle a small contribution of a young solar population and an
old metal-poor component from an old solar principal population. The results we
have found are promising, but limited by the available data. We highlight the
urgent need for new, better and more consistent SBF observations.
