The analysis of red giant branch photometry in galaxies

Abstract

We present an analysis of the many possible methods for simulating and analysing colour-magnitude diagrams, with application to studies of the field halo stellar populations of resolved galaxies. Special consideration is given to the analysis of stars on the red giant branch, and methods for obtaining metallicity distributions for old populations based on analysis of evolved sections of the colour-magnitude diagram. These tools are designed to provide a reliable and accurate method for the analysis of resolved Population II stars in the haloes of nearby galaxies. In particular, we introduce a perpendicular distance method for calculating most likely source isochrones for stars in our input data set. This method is shown to have several advantages over a more traditional approach considering the isochrone as a set of finely spaced Gaussian probability distributions. We also consider methods by which the obtained metallicity distribution might be most efficiently optimized and especially evaluate the difficulties involved in avoiding sub-optimal local maxima in the likelihood maximization procedure.