Mass of the Galaxy to Large Radii

The nature of the dark matter in the Milky Way is not well understood. There are several conflicting theories that predict very different distributions of mass and dark matter in galaxies. With SIM, however, we will be able to create a detailed, three-dimensional model of the mass distribution in our Galaxy, out to a radius of 270 kpc. We will use two tests to measure the Galactic potential at large radii:

1. Using halo stars to study Galactic dynamics: Stars in the halo offer the possibility of determining the overall mass distribution in the Galaxy, the shape and extent of the "dark halo", and the contribution of the halo and disk to the gravitational potential. The gravitational potential is the gravitational force exerted by the entire Galaxy, determined by combining the contributions of the masses of stars and gas in the bulge, disk, and halo, as well as the dark matter.

2. Proper motions of Milky Way globular clusters and dwarf satellites: We will use SIM to measure the proper motions for most or all of the Galactic globular clusters (about 150 clusters) and dwarf spheroidal galaxies within 250 kpc. In addition to serving as valuable test particles for determining the halo potential, these cluster data will play an essential role in understanding the formation and evolution of these objects.
   
   Modern distribution of Milky Way Globular Clusters. The X marks the Sun. From Majewski (1999, in Globular Clusters , eds. C. Martinez-Roger, I. Perez Fournon & F. Sanchez) based on data from Harris (1996).

3. Motions of stars in tidal streams: More than one of the Milky Way's satellites has streams of stars associated with it, aligned ahead and behind along its orbital path. These stars are thought to have been pulled off from the satellite over time by the Milky Way. The exact path that the stars take after leaving the satellite depends on how much mass there is in the Milky Way and how it is distributed.

   Using SIM we can measure the motions of these stars and ask whether these motions are consistent with them once being part of the satellite with which they are associated. If our model for the Milky Way is good we should find that most of the stars in a stream could plausibly have once been part of the satellite.

   The following movies represent this idea. In each we have measured the motions of the stars in the stream and then followed their orbital paths back in time and asked whether the stars ever realign with the satellite's position and motion. In only one of the movies have we used the right Milky Way mass distribution to calculate these paths. Can you figure out which one?

   Movie 1

   Movie 2

   Movie 3

   Movie 4

   Movie 5

   The tidal streams analysis is also a very sensitive probe of the SHAPE of the Galactic halo. Current models predict a flattened model, yet the work of Majewski et al. on the Sagittarius tidal debris suggests that the Milky Way seems to have a spherical potential (and therefore a fairly circular dark matter distribution). ******

   Flat halo

   Spherical halo

We will use SIM to measure the motions of stars in the tidal streams of Milky Way satellites. These measurements will enable us to determine accurately both the mass of the satellites and the mass distribution of the Milky Way to a distance of 250 kpc from the Galactic Center.

There is already evidence for tidal tail populations in the 110° long Magellanic Stream and the Sagittarius dwarf galaxy. With SIM, we will observe target stars chosen from the satellite galaxies Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), Sagittarius, Sextans, Draco, Sculptor, Ursa Minor, Carina, Fornax, Leo I, Leo II, and many globular clusters. The locations of these satellites are shown below.