Milky Way and Local Group
The Milky Way with its cohort of small satellite galaxies are almost the only galaxies that can be studied star-by-star and that can be mapped in 3D: an ideal testbed for understanding galaxy assembly. Along with the Milky Way's big neighbour, the Andromeda galaxy, and a few smaller galaxies they form the 'Local Group'.
The unique detail in which we can study the Local Group makes those most nearby galaxies model organism for understanding how galaxies were assembled:
- we can see where the stars that formed early live and on what orbits they move
- we can see how many shreds from earlier mergers of galactic sub-units are still left over
- only in the Milky Way do we have a chance to see the faintest galaxies in the Universe
- only there can we which fraction of the gravitational mass is flat (e.g. a stellar disk) and or spheroidal (a dark matter halo).
Recent and ongoing systematic mappings of the night sky (SDSS, Pan-STARRS) have 100-folded the observational information in the last years, and have e.g. unveiled the presence of numerous, almost completely dark dwarf galaxies in their outer regions. Stellar streams, produced by their destructive journey near the Milky Way or Andromeda, are now seen and interpreted as the signposts of the ongoing formation history of these two massive spiral galaxies. The Gaia mission is measuring the phase-space coordinates (positions and velocities) of over a billion stars, and spectrophotometry of hundreds of millions of sources, and is bringing about a revolution in our understanding of the formation of our galaxy and the nature of its stellar and dark matter content.