2018MNRAS.480.1587S

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 turn-off is 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.