POPulation SYnthesis for CLusters and galaxies - matching the accuracy of modern spectrophotometric surveys via a fresh look at stellar physics – POPSYCLE

Large astronomical surveys are reaching exquisit precisions, and with projects such as Euclid or LSST a jump in accuracy in extragalactic observations is also to be expected. The impact will be major, reminiscent of the changes brought about by the Gaia mission in studies of the Milky Way. This should in principle allow us to reconstruct the chronology of the assembly of galaxies and their substructures. The bottlenecks in these studies are the shortcomings of the models for the emission of stellar populations. They currently are incomplete and suffer from internal inconsistencies responsible for biases that may prevent us from achieving those goals. Details of stellar physics neglected until now must be included, and the consistency of the spectrophotometric predictions across wavelengths obtained. Among the components of galaxies, globular clusters are of particular interest because they are thought to trace eventful episodes in galaxy lives, but also because they have long been used as references for the calibration of population synthesis models, that were then applied to the analysis of other galaxy light.
The POPSYCLE project focuses on globular clusters and old stellar populations, and endeavours to provide self-consistent panchromatic models that include more of the physical processes relevant to the evolution of the spectra of stars: initial abundances specific of first and second generation stars of globular clusters, classical and non-classical mixing processes that occur on the main sequence or the red giant branch. To this end, we bring together expertise in stellar physics on one hand, and in the modeling and analysis of stellar populations on the other. We will quantify the effects of a variety of stellar model ingredients on the colors and spectra of old populations. Iterating between models and reference observations, that we participate in collecting, we will identify which processes help reduce discrepancies, and how the changes in the models affect age or composition estimates. We will use these results to re-discuss the assembly scenarii for globular cluster systems and early-type galaxies.