Spiral galaxies

The earliest modern theory of the formation of our galaxy (known by astronomers as ELS, after the initials of the authors of that paper, Olin Eggen, Donald Lynden-Bell, and Allan Sandage[3]) describes a single (relatively) rapid monolithic collapse, with the halo forming first, followed by the disk. Another view published in 1978 (known as SZ for its authors, Leonard Searle and Robert Zinn[4]) describes a more gradual process, with smaller units collapsing first, then later merging to form the larger components. An even more recent idea is that significant portions of the stellar halo could be stellar debris from destroyed dwarf galaxies and globular clusters that once orbited the Milky Way. The halo would then be a "newer" component made of "recycled" old parts.

In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies. Rapid technological progress in computers have allowed much better simulations of galaxies, and improved observational technologies have provided much more data about distant galaxies undergoing merger events. After the discovery in 1994 that our own Milky Way has a satellite galaxy (the Sagittarius Dwarf Elliptical Galaxy, or SagDEG) which is currently gradually being ripped up and "eaten" by the Milky Way, it is thought these kinds of events may be quite common in the evolution of large galaxies. The Magellanic Clouds are satellite galaxies of the Milky Way that will almost certainly share the same fate as the SagDEG. A merger with a fairly large satellite galaxy could explain why M31 (the Andromeda Galaxy) appears to have a double core.

The SagDEG is orbiting our galaxy at almost a right angle to the disk. It is currently passing through the disk; stars are being stripped off of it with each pass and joining the halo of our galaxy. Eventually, only the core of SagDEG will exist. Although it will have the same mass as a large globular cluster like Omega Centauri and G1, it will appear rather different, as it has far lower surface density due to the presence of substantial amounts of dark matter, while globular clusters appear, mysteriously, to contain very little dark matter.

Further examples of satellite dwarf galaxies that are in the process of merging with the Milky Way are the Canis Major Dwarf Galaxy, discovered in 2003 and thought to be responsible for the Monoceros Ring, and the Virgo Stellar Stream, discovered in 2005.