Galaxies show a huge diversity of structure and stellar populations, reflecting a complex and unknown mixture of star formation episodes and hierarchical merging events in their past that are impossible to observe directly. Promising new methods to recover the distribution of stellar orbits within a galaxy can reveal its dynamical ‘skeleton’, which in turn sets the spatial distribution of its baryons. However, to fully reconstruct the assembly history of the galaxy we must simultaneously infer ages for the recovered orbits of stars and star clusters. Age dating these stellar tracers provides a dynamical memory for the galaxy, as the orbits are imprinted by discrete events (galaxy mergers, starbursts) in the galaxy’s history. 
We have begun the first steps towards this novel combination of stellar population and orbit-based dynamical modelling. With the support of the ISSI team framework, we will apply this revolutionary tool to existing data available from the astronomical community. The dynamical records this method identifies will pinpoint the epoch of major mergers which grew a galaxy’s bulge and halo, or timescales over which its disk(s) formed. These focused meetings are necessary, as a particularly diverse set of expertise (spectroscopic data analysis, chemical evolution, dynamical modeling, algorithm optimization, model validation via comparison with simulations) are required to solve this problem. The timing of this funding proposal is crucial to fully exploit the wealth of space and ground based imaging and wide field spectroscopic data that are being released for thousands of galaxies – and for the first time, age-date the invisible past assembly history of galaxies
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