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Globular Clusters and Galaxy Halos
Scientific Report: Globular Clusters and Galaxy Halos
The aim of our workshop was to bring together experts on the stellar halos around galaxies and the globular clusters that are associated with these halos. We wanted to contrast and compare the properties of GCs with other halo tracers in nearby galaxies, in order to assess the limitations of using GCs as primary halo tracers in more distant systems. In particular, we discussed 1) whether one gets a consistent picture of halo properties from different tracers (different types of individual stars, GCs, planetary nebulae), 2) How universal various properties (such as metallicity distributions, age-metallicity relations and detailed chemical abundances) of stellar halos and GCs are in different galaxies, and 3) how such information may be used to constrain galaxy formation scenarios. We also discussed the puzzling phenomenon of multiple stellar populations within globular clusters, and whether it is possible to put together a consistent picture for their origin.
Prior to the workshop, we had hoped to identify types of investigations where it is “safe” to use GCs as tracers of halos, as well as areas where further research is needed. At the end of the week, a consensus seemed to be emerging that GCs are excellent tracers of halo substructure, and may indeed be very useful ways to identify streams and other remnants of past accretion events. However, as pointed out by Bill Harris in the conference summary, it is also clear that the biggest question remains unanswered: How do GCs, along with the halos, actually form? There are various scenarios, but a real theory is sorely lacking. This may come from more sophisticated computer simulations, which are now becoming feasible.
Much of our scheduled discussion on Wednesday afternoon revolved around multiple populations in GCs, and many participants were keen on continuing this discussion in a splinter session on Thursday. A possible new formation scenario was discussed, but it was not clear whether this new scenario is feasible. Some of the participants will take a closer look and perhaps this will result in a publication.
The discussion of future instrumentation on Tuesday afternoon was very useful. It tied in well with the discussion of GCs in the outskirts of galaxies, and it seems clear that many of the upcoming facilities (such as Euclid, LSST) that will image large parts of the sky at high resolution and/or in multiple colours will enable a much more complete census of such remote GCs. This discussion certainly motivated the SOC chair to join the Euclid consortium.
Many participants stated that they enjoyed the facilities and the support provided by the Lorentz Center very much. This is also true for the organisers: we have been very happy with the support, and we had a very useful discussion during the intake meeting that helped us plan the workshop. Some participants (particularly the session chairs) initially thought that the large amount of discussion time after each talk seemed somewhat excessive. But it was actually very good; there was lots of good discussion and the session chairs did an excellent job managing the discussion in the “wrap-up” time at the end of each session.
For the Wednesday afternoon discussion, we had asked the participants to submit questions that they would like to see addressed. We then collected these questions into a few overall themes. For each theme, we started by asking the participants who had submitted questions to elaborate a bit on why they asked those questions, and then the discussion was allowed to flow freely (with a minimum of steering from the moderators). This went very well and we had a very lively discussion.