Merging Galaxies

We observe many instances (like this! and this!) where galaxies appear to be interacting with each other enough to cause obvious distortions. These interactions may have a significant connection with the way that galaxies evolve over time. It appears that most galaxies have a long history of numerous interactions with other galaxies. Spirals consume smaller galaxies and cannibalize their fuel, and ellipticals may be the product of large spiral galaxy mergers. There are strong tidal distortions, and bursts of new stars are formed as the gas clouds are compressed and shock waves rush through the galaxies.

Almost all galaxies in the Universe are moving away from us, due to the overall expansion of the Universe. A few nearby galaxies are actually moving toward us, however, because of local gravitational attractions. An example of this is the our neighbor Andromeda, which will collide with the Milky Way in about 3 billion years, as well as the many smaller (dwarf) galaxies which surround the two spirals. This merger looks like an amazingly destructive process, but the great plumes are all composed of gas clouds, while the small, dense stars of the disk will be far less affected. Nonetheless, disaster insurance wouldn't be a terrible idea ...

Diagram of the local group of galaxies, with a few scattered spiral galaxies like the Milky Way and Andromeda (M31) surrounded by devoted entourages of smaller redder galaxies. The Milky Way and Andromeda galaxies are projected to interact in the far future; a linked animation shows a simulation of two disk galaxies interacting, with giant plumes of gas being thrown out wildly as the galaxies meet and dance, their cores coming together and separating in an energetic courtship.
The Local Group of galaxies [NMSU, N. Vogt] Will M31 and the Milky Way merge?
[J. Dubinsky, J. Barnes]

Stephan's Quintet NGC 2207 & IC 2163 Arp 273 NGC 3314
Gorgeous group of interacting galaxies of all colors and shapes, showing clear disorder. Telescope image of two interacting spiral galaxies. The smaller one shows more distortion in structure, and will eventuially be subsumed into the larger. Telescope image of two interacting galaxies; the warps in the galaxy disks are clear signs of distortion. Be careful with this image - rather than interacting galaxies, we have here two galaxies which by chance align along a single line of sight when viewed from Earth! Because the foreground galaxy is viewed face-on, this gives us a wonderful opportunity to examine the spiral arm patterns in its disk which surround the bright nucleus.
The arc of purple-white and blue stars which jets across the disordered galaxy shows the path of a smaller one which passed through it. The Cartwheel Galaxy has a distinctive blue ring of stars in its outer regions, a relic from a collision 400 million years ago with a smaller galaxy. The greatly extended spiral arms of this galaxy are the result of interactions with a much lower mass dwarf galaxy. A mosaic of nine galaxies showing extreme disorder, thought to be derived from significant interaction events.
NGC 6745 Cartwheel NGC 6872 Collisions
How many of these galaxies are interacting? [NASA/HST, NSF/NOAO/WIYN, AAO]