The body of photometric and astrometric data on stars in the Milky Way Galaxy has been growing very fast in recent years (Hipparcos/Tycho, OGLE-3, 2-Mass, DENIS, UCAC2, SDSS, RAVE, Pan Starrs, Hermes, ...) and at the time of the General Assembly the launch of ESAòÀÙs Gaia mission, which will measure astrometric data of unprecedented precision for ~ 10⁹ stars, will be less than three years away. On account of our position within the Galaxy and the complex observational biases that are built into most catalogues, dynamical models of the Galaxy are a prerequisite full exploitation of these catalogues. On account of the enormous detail in which we can observe the Galaxy, models of great sophistication are required. Moreover, in addition to models we require algorithms for observing them with the same errors and biases as occur in real observational programs, and statistical algorithms for determining the extent to which a model is compatible with a given body of data.

Progress in our understanding of the Milky Way will be unnecessarily delayed if techniques for constructing dynamical models are not developed to the level required for modelling Gaia data before a preliminary version of the Gaia catalogue is released. Producing models and fitting them to the available data will involve complex software. It is clearly in the interests of the astronomical community that these costly tools be developed collaboratively in modular form. In particular it is desirable that most tasks can be accomplished by independently developed modules whose performance can be critically compared at di erent institutions. For this to be possible, standard interfaces between modules will have to be defined.

JD5 will review status of the different modelling techniques that might be used, and what will be required to extract our science goals from the data that will be on hand when the Gaia Catalogue becomes available. It would also consider the steps that should be taken to identify tasks and define interfaces.

Producing models of the Milky Way that combine dynamics and stellar evolution theory with the immense catalogues that will be available by 2015 is an enormous undertaking that will require the participation of astronomers from all over the World with expertise in almost every area of astronomy. The General Assembly of the IAU provides an opportunity for enthusiasts to get together and think about what can be done, and how to do it. We already have a formidable body of observational data to work with, and just six years before a preliminary version of the Gaia Catalogue should appear. So at JD5 we should consider what needs to be done, and set up international collaborations that have the expertise and technical resources to get the job done.

Contacts:
E-mail: binney@thphys.ox.ac.uk