MORGOTH: Incorporating horizontal branch modelling into star formation history determinations

Abstract

We present a new method that incorporates the horizontal branch morphology into synthetic colour-magnitude diagram-based star formation history determinations. This method, we call MORGOTH, self-consistently takes into account all the stellar evolution phases up to the early asymptothic giant branch, flexibly modelling red giant branch mass loss. We test MORGOTH on a range of synthetic populations, and find that the inclusion of the horizontal branch significantly increases the precision of the resulting star formation histories. When the main sequence turnoffis detected, MORGOTH can fit the star formation history and the red giant branch mass-loss at the same time, efficiently breaking this degeneracy. As part of testing MORGOTH, we also model the observed colour-magnitude diagram of the well-studied Sculptor dwarf spheroidal galaxy. We recover a new more detailed star formation history for this galaxy. Both the new star formation history and the red giant branch mass-loss we determined for Sculptor with MORGOTH are in good agreement with previous analyses, thus demonstrating the power of this new approach.