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On the Incorporation of Metallicity Data into Star Formation History Measurements from Resolved Stellar Populations

The combination of spectroscopic stellar metallicities and resolved star color-magnitude diagrams (CMDs) has the potential to constrain the entire star formation and chemical enrichment history (SFH) of a galaxy better than fitting CMDs alone (as is most common in SFH studies using resolved stellar populations). In this paper, two approaches for incorporating external metallicity information into color-magnitude diagram fitting techniques are presented. Overall, the joint fitting of metallicity and CMD information can increase the precision on measured age-metallicity relationships and star formation rates by ~10% over CMD fitting alone. However, systematics in stellar isochrones and mismatches between spectroscopic and photometric metallicity determinations can reduce the accuracy of the recovered SFHs. I present a simple mitigation of these systematics that can reduce the amplitude of these systematics to the level obtained from CMD fitting alone, while ensuring the age-metallicity relationship is consistent with spectroscopic metallicities. As is the case in CMD-fitting analysis, improved stellar models and calibrations between spectroscopic and photometric metallicities are currently the primary impediment to gains in SFH precision from jointly fitting stellar metallicities and CMDs.