Surface Brightness Fluctuations to constrain secondary stellar populations: Revealing very low-metallicity stars in massive galaxies

TL;DR

This study demonstrates that combining Surface Brightness Fluctuations (SBF) with mean colours enhances the detection of minor secondary stellar populations, such as very low-metallicity stars, in massive galaxies.

Contribution

It introduces a new method using combined SBF and mean colours to better identify small fractions of secondary stellar populations in elliptical galaxies.

Findings

Combined SBF and mean colours improve detection of secondary populations.

SBF colour-colour diagrams can reveal low-metallicity and young populations.

Results are promising but limited by current data quality.

Abstract

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.