Globular Clusters and Galaxy Halos

Description and aim

Stellar halos usually account for only a very small fraction of the total stellar mass of their parent galaxies, but they nevertheless carry crucial information about hierarchical galaxy assembly histories. They are chemically the most pristine components of galaxies, and offer unique insight into nucleosynthesis and chemical enrichment at low metallicities. Identification of spatial, chemical and temporal substructure (streams, etc.) helps us understand the properties of fragments that contributed to the early build-up of galaxies.

The studies of globular clusters (GCs) and the stellar halos they inhabit are inextricably linked. Globular clusters provide distinct sampling points in space and time that can be observed and studied in detail at much greater distances -- far beyond the Local Group -- than individual halo stars. Indeed, the classical Searle-Zinn picture of a chaotic early phase of galaxy formation was, to a large extent, based on observations of globular clusters. More recently, observations of GCs have suggested that halo metallicities correlate with host galaxy properties such as mass and/or luminosity. Such relations provide important clues to the early assembly- and chemical enrichment histories of galaxies.

While GCs are classical tracers of metal-poor stellar halos, much remains to be learned about the link between GCs and the general halo populations. In particular, a large fraction of the stars in GCs have abundances of the light elements (e.g., He, C, N, O, Na, Mg, Al) in combinations that are largely unique to GC stars. Only a small fraction of the stars in the Galactic halo show similar abundance patterns, suggesting these stars were lost from GCs. It is thus important to understand what fraction of halo stars may come from fully disrupted GCs and from clusters that are still "alive". Besides the implication for the origin of halos, this provides an important constraint on models for the origin of abundance anomalies in GCs.

The use of globular clusters (and other distinct tracers, such as planetary nebulae, blue horizontal branch stars, RR Lyraes, etc) is motivated by the low surface brightness of halos, which makes it very challenging to study the general halo populations directly in most external galaxies. However, recent years have seen significant progress in studying individual halo stars, both from large-scale surveys of our own Milky Way (e.g. SEGUE, Gaia-ESO) as well as other nearby galaxies (e.g. PAndAS). We are also gradually starting to build up a more complete and detailed picture of the detailed chemical composition of halos in nearby dwarf galaxies, both from GCs and individual stars. Time is therefore ripe for a critical assessment of how well GCs trace the general halo populations in their parent galaxies.

This workshop will bring together experts on halo populations and globular clusters and examine the current status of the GC-halo connection. Specifically, we aim to contrast and compare the properties of GCs with other halo tracers in nearby galaxies. Such a comparison is a crucial step in order to set the stage for using GCs as primary halo tracers in more distant systems. Among the specific questions we will address are the following:

-     How complete is our census of globular clusters (and other halo tracers) in Local Group galaxies (and beyond)?

-     Do we get a consistent picture of halo properties from different tracers (RR Lyrae / BHB stars, red giants, globular clusters, PNe)?

-     How universal are the properties (such as metallicity distributions, age-metallicity relations and detailed chemical abundances) of (GCs in) stellar halos?

-     Are there systematic differences between large and small galaxies? How can this information be used to constrain hierarchical assembly models for galaxy formation?

-     What fraction of halo stars belong to GCs presently, and what fraction may have initially formed within GCs? How do these fractions depend on host galaxy properties? How do they vary within galaxies (e.g., as a function of position or metallicity)?

-     Can we constrain models for dynamical evolution of GCs by identifying halo stars lost from individual GCs (e.g., via chemical tagging or orbit reconstruction)?

-     What fraction of GCs can be associated with discrete accretion events? Are there systematic differences between properties of GCs formed “in situ” and those that were accreted? To what extent is such a distinction even meaningful?

-     To what extent are the detailed properties of galaxy halos accounted for by theoretical models for hierarchical galaxy assembly?

We hope to stimulate lively discussion around these questions and foster new collaborations among experts from the different fields.