Galaxy population

The MW has a couple of small galaxies gravitationally bound to it, for example the Large and Small Magellanic Clouds. The Magellanic stream is a stream of gas which originates from the Large Magellanic Cloud, which can be traced over a significant fraction of the whole sky. It may result from the tidal forces exerted by the MW.

The Sagittarius Dwarf galaxy is the neighbouring galaxy closest to the MW. It was discovered as recently as 1994, having eluded detection since it happens to be at the other side of the MW bulge. And so it is hiding behind a large amount of foreground stars. Sagittarius has ventured so close to the MW that the tidal forces will probably tear it apart in the near future. Its discovery was actually hampered by its close proximity, since the large area on the sky ( $\sim 8^o\times 2^o$) made it difficult to pick out against the bright MW foreground. Even away from the galactic plane (where dust obscuration limits our view), new members of the LG continue to be identified. The study of these small galaxies is exciting, since they allow us to constrain models for galaxy formation and evolution.

The Andromeda galaxy M31 is about twice as bright as the MW, and is the brightest member of the Local Group (LG). It is very similar to the MW, and so also has its own set of small galaxies orbiting around it. These galaxies, together with another 20 small galaxies, form the LG. M31 and the MW are by far the most massive galaxies in the LG.

The LG is almost certainly gravitationally bound to other nearby groups, and so does not really have a well defined edge. Galaxies in the outskirts of the LG may in fact belong to other groups.

The distribution of galaxies in the LG is rather untidy and lacks any obvious symmetry. Most of the galaxies are found either close to the MW, or close to M31. There is a third, smaller condensation of galaxies, hovering around NGC 3109.

The motion M31 can be used to estimate the mass of the Local Group, using the Local Group timing argument.