How do galaxies form? This is one of those big questions in astronomy, no one really knows. There are plenty of theories, and many observations providing evidence for different ideas, but the truth is that we just still aren't sure.
One particular model of the universe suggests that large galaxies, like our own Milky Way, formed as an amalgamation of smaller galaxies, proto-galaxies if you like. One of the consequences of this theory is that these larger galaxies should be surrounded by a halo containing the dispersed debris left over from the mergers of these smaller dwarf galaxies. Evidence for this theory is seen within our own Galaxy.
The first dwarf galaxy remnant discovered in the Milky Way is known as the Sagittarius dwarf because it is seen in the direction of the constellation of Sagittarius. In 1994, while investigating the motions of stars in the Galactic halo, researchers discovered a large group of comoving stars, stars moving with the same speed in the same direction. It seems that this poor little galaxy is slowly being ripped apart as it passes through our Galaxy again and again. Each pass through the plane of the Galaxy causes the dwarf to become more stretched out until, eventually, it becomes absorbed into the much larger Milky Way.
Credit: Dr R. Ibata et al
More recently, a group of researchers have found another dwarf galaxy being torn apart by a close encounter with the Milky Way. The Canis Major dwarf is the closest known dwarf galaxy to the centre of the Milky Way, there could be others there that we haven't seen yet, and is only about 2500 light years away from our solar system. If it is so close, why hasn't it been seen before? Well, the trouble with the centre of the Galaxy is that there is a lot of stuff in the way. The bulge is full of gas, dust and stars that block out any light coming from the far side. This new galaxy was discovered by looking at infra-red data from the 2-Micron All Sky Survey which is able to penetrate this junk and reach us. Stars which appear bright at infra-red wavelengths are cooler, older stars, implying that the Canis Minor dwarf is very old. As well as finding the core of the interloper, the astronomers also found streamers of associated stars that were pulled off by tidal interactions as the galaxies got close to each other. These contribute to the disk of the Milky Way, so it seems that smaller galaxies do indeed contribute to the formation of larger ones.
Today, B. Conn and colleagues report observations of a stream of stars known as the Monoceros Ring which is tidal debris that is possibly associated with the Canis Major dwarf. This ring is behind the Canis Major dwarf and is thought to be a tidal stream from the Canis Major dwarf itself, a scenario which these new observations support. This all shows that, far from being evolved, our Galaxy is still forming as these interactions continue.
It is worth downloading the paper from astro-ph just to read the rather bizarre acknowledgements, if nothing else.