When Galaxies Collide!

NGC 2623: Galaxy Merger from Hubble
Credit: NASA, ESA and A. Evans (Stony Brook).

Where do stars form when galaxies collide? To help find out, astronomers imaged the nearby galaxy merger NGC 2623 in high resolution with the Hubble Space Telescope in 2007. Analysis of this Hubble image and images of NGC 2623 in infrared light by the Spitzer Space Telescope, in X-ray light by XMM-Newton, and in ultraviolet light by GALEX, indicate that two originally spiral galaxies appear now to be greatly convolved and that their cores have unified into one active galactic nucleus (AGN). Star formation continues around this core near the above image center, along the stretched out tidal tails visible on either side, and perhaps surprisingly, in an off-nuclear region on the upper left where clusters of bright blue stars appear. Galaxy collisions can take hundreds of millions of years and take several gravitationally destructive passes. NGC 2623, also known as Arp 243, spans about 50,000 light years and lies about 250 million light years away toward the constellation of the Crab (Cancer). Reconstructing the original galaxies and how galaxy mergers happen is often challenging, sometimes impossible, but generally important to understanding how our universe evolved.

Date:14th November,2010

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. There are some generally accepted results, however:

1. When one of the galaxies is significantly larger than the other, the larger will often "eat" the smaller, absorbing most of its gas and stars with little other major effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.

2. If two spiral galaxies that are approximately the same size collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.

Note that the Milky Way and the Andromeda Galaxy will probably collide in about 4.5 billion years. If these galaxies merged, the result would quite possibly be an elliptical galaxy as described above.

One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae.