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Cosmology - Galaxy Clusters - Galaxy Mergers

Interacting galaxies were pretty common in the early Universe. Gravity influence of tight clusters of galaxies resulting in several different types of merging:
  • Interacting galaxies - close proximity of two or more galaxies results in tidal distortions
  • Merging galaxies - sometimes called colliding galaxies, where two galaxies are in the process of joining together
  • Starburst galaxies - these are merging or colliding galaxies that involve close proximity of the nucleus of each galaxy sparking sudden star births
Click on the image to view a video of a typical merger.

In this case, the two interacting galaxies will merge into one larger galaxy (video care of Swinburne Astronomy Online).

It is suggested that elliptical galaxies are the result of two merging spiral galaxies. The evidence of this is the distribution of elliptical galaxies in present day clusters. It is also suggested that the fuel required for an AGN is from galaxy mergers.

It seems strange that in an expanding Universe that colliding galaxies would be rare, but as we have seen already galaxies exist in clusters. If the cluster is compact enough, influence by gravity is enough to initiate a collision. An example is our own local group: all galaxies are moving away from us save one - the Andromeda galaxy (M31) is moving toward us at about 300 km/s. In about 2 to 3 billion years, the two will finally interact.

An interacting galaxy, like this image of Arp273, demonstrates how the gravity if the lower galaxy is affecting the spiral structure of the upper galaxy.

Such interactions cause what is called tidal distortions. It seems strange that the smaller galaxy would cause such distortions in a larger galaxy, but in this case the lower galaxy is an AGN (not seen in this image).

63 million light years away, the poster child of galaxy mergers is the Antennae, or NGC4038 and 4039:

These two galaxies will eventually coalesce into a single, larger galaxy.

Sometimes if a galaxy merger involves the nucleus of a galaxy, violent star formation results. An example is the nearby galaxy M82:

This irregular shaped galaxy is the result of a smaller galaxy that has merged near the core of the larger galaxy that has not only created the single galaxy we see, but intense star formation in the heart of the galaxy.


(Image credit: Brooks/Cole Thomson Learning)

What has happened here is that massive dust clouds help to create massive stars that "live fast and die young" that eventually supernova. The supernova stimulates nearby dust clouds to form more large stars and the cycle continues - rapidly.


(Image credit: Brooks/Cole Thomson Learning)
Nearby galaxies have also been observed undergoing merging. An example is the Stephan's Quintet - a nearby compact cluster.

In reality, the galaxy in the lower left of the image is a part of the observed cluster but is not a physical member of the cluster - as measure by redshift.

The top three galaxies are a part of the compact cluster and are interacting.

Some of the current research involving the merging of galaxies include:

  • Correlation between galaxy merging and Seyfert galaxies
  • Interacting galaxies provide fuel for AGN - AGN then stimulates starburst formation
  • Correlation between far infra-red emission and starburst galaxies

By continuing to study merging and interacting galaxies Astronomers hope to understand the process of galactic evolution.

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