With the Gaia launch hopefully in early 2014, we proposed a workshop – Gaia Challenge – aimed at improving our mass modelling methods from Jeans modelling and distribution function modelling to Schwarzschild modelling, M2M and others. We had some 59 participants from all over the world spanning a wide range of astrophysics communities from observers working on dense stellar systems to cosmological simulators. All were brought together by a shared interest in exploiting the forthcoming Gaia data to understand our Galaxy and its constituents.

The workshop revolved around a series of test problems (mock data) posted online in advance, divided into four main working groups:

• Spherical & Triaxial (P.I. Read)
• Discs (P.I. Kawata)
• Streams & Halo Stars (P.I. A. Font)
• Collisional/Star Clusters (P. I. M. Gieles)

Attendees were invited to apply their favourite methods to these mock data to recover the underlying gravitational potential and/or phase space distribution function. Building dynamically realistic mocks meant bringing together N-body simulators, dynamicists and the stellar population synthesis and dust extinction community. A key goal of this workshop – in addition to improving mass modelling techniques – was to facilitate the exchange of expertise between these communities and to foster longer term collaborations.

Several exciting new results already came out of the meeting. Key highlights include: (i) standard mass modelling techniques that assume spherical symmetry can be reasonably applied to triaxial models provided that we are not `staring down the barrel'; (ii) for 1D disc models, it is sampling that is all important; (iii) a single globular cluster stream – given good enough data – can give tight constraints on the halo mass and shape (and several very different techniques look capable of doing exactly this); and (iv) simple distribution functions give an excellent fit to collisional star clusters, but which one depends very much on how tidally limited the cluster is. Each of these key results has potentially far-reaching implications: for measuring the dark matter distribution in dwarfs; for measuring the local dark matter density near the Sun; for probing alternative gravity models and cosmology; and for hunting for intermediate mass black holes.

The workshop was extremely well received by all involved and we hope to run Gaia Challenge-II in Heidelberg next year. The goal is to have a meeting ~every year up to final Gaia data release building mocks of ever increasing realism and sophistication. Only by honing our methods on these mock data, will we truly become ready for the `Gaia Challenge'.

A list of participants is here. The final programme can be downloaded also: gaia_challenge_programme.pdf.

You can access talk .pdf files on this page: talks.