Scientific Rationale

Garching, Germany, 13 - 19 October, 2014


The study of stellar populations is one of the most relevant diagnostics to constrain galaxy formation and evolution. Quantitative analyses of the stellar content of stellar systems pave the way to `convert'  starlight into physical quantities like stellar masses, chemical abundances and star formation rates,  and trace back in time the evolution and the chemical enrichment history of galaxies.

Historically, stellar population studies have followed two parallel paths. Observations and analyses of individual stars belonging to "resolved stellar populations" have enabled to understand the fundamental physics of stellar evolution, through a detailed comparison between theory (isochrones, luminosity functions) and observations (photometry, spectroscopy). This has also enabled us to reconstruct the star formation history of the Milky Way and nearby Local Group galaxies. The knowledge of the evolutionary properties of nearby stellar populations has, in turn, allowed us to constrain the nature of unresolved stellar populations and galaxy evolution back to the dawn of time (Walcher et al. 2011, Ap&SS, 331, 1; Conroy 2013, ARAA, in press).

Curiously, despite their tight connection, the `resolved' and the `unresolved' approaches have made progress almost independently, with marginal overlap between the two communities. The time is now ripe for a new synergy between these two communities. The advent of the latest generation of ground- and space-based telescopes has had a relevant impact in the field of stellar populations. As an example, in globular clusters that were considered until a few years ago the template for simple stellar populations (same age, same chemical composition), multiple populations have been recently detected (Gratton et al. 2012, A&A, 20, 50). In nearby dwarf spheroidal galaxies that were considered relaxed stellar systems which experienced multiple star formation events with no evidence of rotation, secondary kinematic features have been recently identified (Battaglia et al. 2008, ApJ, 681, L13; Fabrizio et al. 2011, PASP, 123, 384).

Moreover, and even more importantly, the wealth of information on the metallicity (alpha elements, r- and s-elements, CNO) and kinematics of thousands of stars in the Milky Way are making possible to compare our own Galaxy with external systems, providing a unique opportunity for understanding the mechanisms of galaxy evolution. Finally, unresolved studies of nearby and high-redshift objects can now provide unprecedented constraints to resolved stellar population studies, as clearly indicated by the recent claims of a non-universal IMF in external galaxies (Boselli et al. 2009, ApJ, 706, 1527; Cappellari et al. 2012, Nature, 484, 485; Conroy & van Dokkum 2012, ApJ, 769, 71). Thus, with the era of the Extremely Large Telescopes approaching, it is more important than ever to make sure that both 'resolved' and 'unresolved' stellar population studies share similar ingredients, compare and validate homogeneously their specific tools .

It is therefore time to gather the communities involved in resolved and the unresolved stellar populations. The main goal of this workshop is to share observations, models, techniques and recent results. Detailed discussions on the methods will favour new and tighter exchanges of ideas concerning our view of galaxy evolution. Particular emphasis will be given to the current limitations of both resolved and unresolved observations in removing the intrinsic degeneracy of the multi-dimensional parameter space (age and metallicity distributions, IMF, chemical mixture, kinematics, reddening law, dust), and to the possible solutions to build a more unified and coherent picture of galaxy evolution.