Scientific Rationale

This ESO workshop focuses on three interrelated topics: Satellites, Streams, and the Star Cluster-Dwarf Galaxy interface.


Near-field cosmology has become increasingly important over the last decades. While the current concordance cosmological model (ΛCDM) has been very successful in reproducing and predicting the properties of the universe on large scales, several possible tensions have been identified on small scales (≤1 Mpc). Issues like the 'missing satellite problem’, the ‘core/cusp problem’, ‘too big to fail’, and detections of satellite disks around the Milky Way (MW) and M31 pose challenges to our understanding of structure and star formation in the early universe, and the feedback between baryons and dark matter.

But how well do we understand what it means to be a satellite of the MW or M31? Even in the era of high-precision cosmology, we are yet uncertain about the total masses of the two dominant galaxies in the Local Group, their assembly histories, and the shape and extent of their dark-matter halos — key aspects for gaining a consistent picture of these galaxies and their satellite systems in a ΛCDM context. On the contrary, the discovery of transition objects at the star cluster–dwarf galaxy interface has made things worse. It blurred the historical distinction between satellite classes, putting at question our understanding about tidal transformation and our census of small stellar systems.


While satellites give us an account of low-mass substructures at present, tidal streams are tracers of how these substructures disrupt in reaction to the gravitational field of their host, and contribute to their host's assembly. Therefore, satellites and streams should be regarded together. They offer a unique opportunity to test and improve our understanding of structure formation at small scales and the large scale shape of the gravitational potential. This workshop aims at bringing experts from both fields together to create this bigger picture.

We envisage the following topical structure for the meeting:

  1. Satellite systems
    Structure formation at small scales is traced by the satellite population of larger galaxies. Of particular interest for testing cosmological theories are the internal dynamics, luminosity function, and phase-space distribution of low-mass satellites. In this workshop we would like to discuss both the ensemble and internal properties of the nearby dwarf galaxies. Particular emphasis will be put on the dark matter content implied by internal dynamics data, and how typical the satellites of MW and M31 are in their ensemble properties compared to cosmological predictions and observations in other environments.

    Itemized topic list:

    Ensemble properties
         - Dwarf galaxy luminosity & mass function vs. LCDM predictions
         - Substructure in phase space (e.g. planes / disks) vs. LCDM predictions
    Internal properties
         - kinematics, is there an unambiguous need for dark matter?
         - stellar populations
    Ultra-faint dwarf galaxies vs. dwarf spheroidals

  2. Tidal streams
    A continuously growing number of discovered tidal streams is giving us important constraints on the gravitational potential of the Milky Way and M31. With the increasing precision of observational and numerical studies, these streams may even allow tests for granularity in the dark matter distribution on small scales. This workshop gives us the opportunity to review this ever growing number of observations and detections of streams in the Local Group. A major focus will be on the modelling of these streams, and the constraints they put on the dark matter halos of the Milky Way and M31, the assembly histories of these galaxies, and on the disruption and formation processes of low-mass satellites.

    Itemized topic list:

        - Stream detections in the Milky Way, Andromeda, and nearby galaxies
        - Theoretical interpretation of the observations and predictions:
                 - Constraints on the Galactic potential & DM halo shape of the Milky Way
                 - Constraints on dark matter substructures
                 - Constraints on built-up of galaxies
                 - Constraints on disruption and formation of low-mass satellites

  3. The star cluster-dwarf galaxy interface
    Detections of objects below a few hundred pc in size have filled up almost the entire mass-size parameter space between star clusters and dwarf galaxies. This gives us for the first time a full account of small stellar systems in the nearby universe and a valuable opportunity to study how and to which extent tidal transformations affect satellite properties. Of particular interest is the star cluster-dwarf galaxy interface: what defines a galaxy?, what is the nature of ultra-compact dwarfs (UCDs)?, how can we distinguish between dissolving star clusters and ultra-faint dwarf galaxies? Understanding the channels that lead to such fundamentally different objects will be an integral part of this workshop.

    Itemized topic list:

        - Filling up the mass-size plane of stellar systems below 1 kpc radius
        - What defines a galaxy?
                 - What is the nature of ω Centauri?
                 - How to distinguish between dissolving star clusters and ultra-faint dwarf galaxies
                 - Galaxy behaviour in star clusters: metal enrichment, multiple populations
        - The nature of UCDs