Validation of optimized population synthesis through mock spectra and Galactic globular clusters

Abstract

Optimized population synthesis provides an empirical method to extract the relative mix of stellar evolutionary stages and the distribution of atmospheric parameters within unresolved stellar systems, yet a robust validation of this method is still lacking. We here provide a calibration of population synthesis via non-linear bound-constrained optimization of stellar populations based upon optical spectra of mock stellar systems and observed Galactic globular clusters (GGCs). The MILES stellar library is used as a basis for mock spectra as well as templates for the synthesis of deep GGC spectra from Schiavon et al. Optimized population synthesis applied to mock spectra recovers mean light-weighted stellar atmospheric parameters within a mean uncertainty of 240 K, 0.04 dex, and 0.03 dex for T_eff, log g, and [Fe/H], respectively. We use additional information from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) deep colour-magnitude diagrams (CMDs) from Sarajedini et al. and literature metallicities to validate our optimization results on GGCs. Decompositions of both mock and GGC spectra confirm the method's ability to recover the expected mean light-weighted metallicity in dust-free conditions (E(B - V) ≲ 0.15) with uncertainties comparable to evolutionary population synthesis methods. Dustier conditions require either appropriate dust modelling when fitting to the full spectrum, or fitting only to select spectral features. We derive light-weighted fractions of stellar evolutionary stages from our population synthesis fits to GGCs, yielding on average a combined 25 ± 6 per cent from main-sequence and turn-off dwarfs, 64 ± 7 per cent from subgiant, red giant, and asymptotic giant branch stars, and 15 ± 7 per cent from horizontal branch stars and blue stragglers. Excellent agreement is found between these fractions and those estimated from deep HST/ACS CMDs. Overall, optimized population synthesis remains a powerful tool for understanding the stellar populations within the integrated light of galaxies and globular clusters.