Galaxy Mergers and Starbursts

Key points: Galaxy cannibalism; collisions and starbursts; mergers; implications of these processes for the evolution of galaxies

Because galaxies are relatively large compared to their separations, there is a non-negligible probability that they collide.

	And, as these spectacular examples show, they do!

Some collisions can result in the merging of the two colliding galaxies into one galaxy. This process is important early in the lives of many galaxies, including the Milky Way, and continues to this day.

It is likely that the Milky Way is in the early stages of such an event with the Magellanic Clouds! This radio image of atomic hydrogen shows a streamer of gas (="Leading Arm") being stripped from the Magellanic Clouds (LMC & SMC) and falling into the Milky Way (Galactic Plane). (From M. Putnam, CSIRO, http://www.csiro.au/news/mediarel/mr1998/mr98194.html)

To the left is an artist's concept of how the Milky Way is stripping gas from the Large and Small Magellanic Clouds. (from D. Parr, CSIRO, see http://antwrp.gsfc.nasa.gov/apod/ap980826.html) This process is probably the beginning of the Milky Way absorbing the Magellanic Clouds in a future merger. A similar fate may await many other small members of the Local Group, merging either with the Andromeda Galaxy or with the Milky Way. Here is an animation of how it will happen (American Museum of Natural History,  http://haydenplanetarium.org/resources/ava/)

In addition, the Milky Way and the Andromeda galaxy are approaching each other with a speed of 300,000 miles per hour. It's not certain yet whether we're in store for a head-on collision or a simple sideswipe. Is this what the sky will look like at some future time? (From T. & D. Hallas, http://www.astrophoto.com/index.htm)

Modeling Galaxy Collisions

We can model the collisions using computer simulations -- the laws of motion and gravity are all that's required..

This simulation shows what happens when a small "green" galaxy collides with a large "red" one. The contours show the overall distribution of starlight, since in reality we have no convenient way to tell the stars that belonged to one from those that belonged to the other. In this case, both galaxies are ellipticals (without spiral disks). The smaller galaxy loses energy as a whole, while some of its stars are given high velocities and escape. As a result, it gets captured and spirals inward.

We see evidence of such collisions and mergers between elliptical galaxies. Some very large ellipticals at the centers of galaxy clusters have multiple nuclei, which can only result from the merging of galaxies. (central galaxy of cluster Abell 3827, from M. J. West)

Starburst Galaxies

A number of galaxies have nuclei that are very bright in the far-infrared.

==> This  behavior cannot be due to the direct output of stars which are too hot to have spectra peaking in the far-infrared --- instead, the stars have heated the interstellar dust to about 60K.

• In some cases, the luminosity from the central 1000 pcs is as much - or even a lot more - as the entire Milky Way produces.
• The dust shrouds and reddens the nucleus making study of the luminous region difficult.

The heated dust is widely distributed and has a relatively uniform temperature which suggests that a cluster of stars does the heating.

By modeling a starburst as the summation of the outputs of many newly formed stars, we have been able to match the properties of these far-infrared bright galaxies so we are confident that stars heat the dust.

• Interactions and mergers: When galaxies collide, the resulting compression of the interstellar medium and the changing gravitational field can induce large amounts of star formation.
• Bars rather than round bulges: Gas clouds orbiting in a bar are brought close to the nucleus, where they can collect and collide to produce the star formation.

Similar interactions may trigger active nuclei

	Here is the result of a head-on collision, leaving a large star forming ring and various pieces of the two galaxies.
	In this case (called, for obvious reasons, "the antennae"), stars have been stripped out of the galaxies and are streaming into space, while in the center there are huge HII regions, many enshrouded in dusty molecular clouds (that look red because of interstellar reddening). To the far left is a large scale view, with the region of the color image outlined.

	This simulation shows the collision of two disk galaxies, with results similar to the antennae above. (reload page to restart animations)(by C. Mihos and L. Hernquist: more at http://research.amnh.org/~summers/mihos/mihos.html, adapted by G. Rieke) In collisions of this type, the bulk of both galaxies tends to merge into a single new galaxy
	Here is a 'still' of theoretical merging disk galaxies. The gas is colored red and the stars blue. The stars are distributed roughly as in the antennae, and the gas has been collected into dense concentrations that become the sites for vigorous star formation, also as seen in the antennae.

Smash galaxies together yourself. Its fun! (your computer must be java applet capable, if so go to http://burro.astr.cwru.edu/JavaLab/GalCrashWeb/main.html)

Test your understanding before going on

Ultraviolet view of M81 enhances spiral arms, from APOD, NASA, UIT: http://antwrp.gsfc.nasa.gov/apod/ap960713.html		The Birth of Venus, by Sandro Botticelli
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