A prediction of standard cosmology is that dwarf protogalaxies are the first to born as individual systems in the universe. Afterward, many of these merge to form larger galaxies such as the Milky Way. The way in which this process takes place has consequences for the present-day structure of the Milky Way. The significant issues are how the merging efficiency compares with the star formation efficiency in the protogalactic fragments and how the fragment merging and disruption  compare with the age of the Milky Way. If fragments are able to form stars before merging, they will collapse nondissipatively. If disruption was not complete, Galactic precursors should be visible today as dwarf galaxy satellites or as stellar streams within the Galactic halo. 

The Sagittarius dwarf galaxy, the closest Milky Way satellite in an advanced state of tidal disruption, provides a "living" test for tidal interaction models and for galaxy formation theories. It was soon apparent that its extent was larger than at first assumed, and dynamical models predict that the stream associated with the galaxy should envelop the whole Milky Way in an almost polar orbit.

Using the Wide Field Camera on the Isaac Newton Telescope, astronomers detected a very low density stellar system at 50 ± 10 kpc from the Galactic centre that could be related to a merger process.

Color-magnitude diagrams of the (panel a) control and (panels b and c) target fields. Panel a the distribution of the foreground Milky Way stars. The overdense strip at ( provides B-R) 0.8, 22 "V" 23.5 in panel b CMD is interpreted as produced by a stellar system at a distance of 51 ± 12 kpc from us, which could make it part of the Sagittarius northern stream or, alternatively, could be the trace of a hitherto unknown tidally disrupted dwarf galaxy. Squares represent variable star candidates. Panel c shows the CMD of the target field with an old, low-metallicity (age: 12 Gyr; metallicity: 1/20 solar) isochrone from the Padua library superposed. The isochrone MS shape shows good agreement with the hypothetical target field MS. Also, the variable star candidates (squares) fall in the predicted region of the horizontal branch. [ JPEG | TIFF ]