News Release Archive:

News Release 650 of 1051

July 19, 2001 09:00 AM (EDT)

News Release Number: STScI-2001-22

Star Clusters Born in the Wreckage of Cosmic Collisions

Background information useful for exploring this news release:

Compact Groups of Galaxies

Galaxies, Groups, and Clusters

Galaxies are titanic swarms of many millions, billions, or even trillions of stars. Laced between the stars is a patchwork of gas and dust, both glowing and dark. These components— stars, gas, and dust— combine to create the often exquisite structure of a galaxy.

From a bigger point of view, galaxies themselves can be collected together into larger structures by their mutual gravitational pull. Two galaxies can circle each other in a binary system. Several galaxies can be clumped together in a group. Clusters of galaxies, the largest gravitationally bound structures in the universe, may have hundreds or thousands of galaxies in orbit around one another. These collections of galaxies are similar to the collections of stars found within galaxies, but on a much larger scale.

Globular Star Cluster M80

A cluster of stars — Globular Star Cluster Messier

[HST/Hubble Heritage]
A Cluster of Galaxies — Coma Cluster [NOAO]

A cluster of galaxies — Coma Cluster


One intriguing class of galaxy collection is called a compact group. These compact groups of galaxies contain just a few galaxies very close together in the sky. The prototype of this category is Stephan's Quintet, a visual grouping of five galaxies that was discovered by the French astronomer Edouard Stephan in 1877. Since then many more have been found. Notably, in 1982 the Canadian astronomer Paul Hickson created a catalog of 100 compact groups that serves as a standard reference. The image below shows a group known as Hickson Compact Group 87. The intriguing question about these compact groups is whether or not they are held together by gravity.

Hickson Compact Group 87: A Group of Four Galaxies

Hickson Compact Group 87

[HST/Hubble Heritage]

Compact Groups: Clumps or Alignments?

A major problem in understanding the nature of compact groups is that galaxies have a wide range of sizes. It is often hard to tell if a galaxy is a small one nearby, or a large one farther away. It is even harder to tell if all the galaxies in a compact group are clumped at roughly the same distance, or if they are just a chance alignment of galaxies strung out along our line of sight into the universe.

Stephan's Quintet

Stephan's Quintet Illustration

[A. Feild (STScI) & F. Summers (STScI)]
The top panel of this diagram illustrates an alignment of galaxies streched out across space. The bottom panel shows a clump of galaxies at approximately the same distance.

One important clue is redshift. The redshift of a galaxy measures the stretching of the galaxy's light due to the expansion of the universe. Light from more distant galaxies has been travelling longer through expanding space, and thus shows a larger redshift. In theory, galaxies at the same distance will have the same redshift. In practice, astronomers' measurements include both the expansion of space and the individual motion of the galaxy (Doppler shift). The measured redshift does not provide an exact measure of distance. In compact groups, redshifts allow one to identify galaxies that are obviously not part of a clump. For example, the large spiral in the lower left of Stephan's Quintet is much closer than the other galaxies in the group.

Stephan's Quintet [NOAO]

Stephan's Quintet


On the other hand, the three central galaxies in Stephan's Quintet show telltale features which indicate they are close together. As galaxies pass near one another the tidal forces of gravity can distort their shapes. This is similar to way the gravitational tidal forces of the Moon creates the tides in the oceans on Earth. In galaxies, the tidal forces can pull away long streams of stars, gas, and dust called tidal tails. A computer simulation of this process is provided below. These long tails and other tidal features are sure signs that the galaxies are close enough to be interacting.

Galactic Collision Simulation

5.7 MB (Quicktime Video)

A computer simulation of two galaxies colliding.
[Simulation: Chris Mihos (Case Western) and Lars Hernquist (Harvard), Visualization: Frank Summers (STScI)]

Another sign that a compact group is a clump of galaxies comes from the matter between the galaxies. When a cluster of galaxies forms, the tenuous gas between the galaxies is heated to million-degree temperatures and begins to emit X-rays. A gravitationally bound compact group should have observable X-ray emission, while it is unlikely that one would see strong X-ray emission from an alignment of galaxies. A complication to this view is that some galaxies can have significant X-ray emission whether or not they are part of a cluster. Hence, the presence or absence of X-ray emission in a compact group is not the definitive test it was once hoped it would be.

Compact Groups and Galaxy Formation

By examining redshifts, tidal features, and X-ray emission, one can hope to learn if a compact group is bound together by gravity. The question of whether or not most compact groups are gravitationally bound has interesting implications for galaxy formation in general.

Compact groups that are gravitationally bound may not last long. The interactions between galaxies that produce tidal features will usually lead to the galaxies merging together. In 1989, the American astronomer Joshua Barnes performed a computer simulation of a gravitationally bound compact group. He found that all the galaxies merged together to form a single galaxy within a couple billion years. While a billion years sounds long, it is a rapid pace for galaxies. The hundreds of compact groups we see today would imply that many, many more had formed and merged previously over the history of the universe. Recent work has examined whether dark matter between the galaxies or fast motions of the galaxies can prevent rapid merging in compact groups.

A Computer Simulation of a Compact Group

9.8 MB (Quicktime Video)

A computer simulation of a compact group.
[Simulation and visualization: Joshua Barnes (Univ. of Hawaii)]

Another viewpoint suggests that compact groups should be mostly alignments. In the dominant theory of galaxy formation, called hierarchical structure formation, the arrangement of galaxies resembles a "cosmic web". Galaxies form along the strands of the web, and then gather together into clusters where the strands intersect. If one's line of sight looks along a strand, there would be an alignment of galaxies that would create a compact group. If hierarchical formation is correct, then many or most compact groups are just views aligned along a strand of the cosmic web.

Observations have shown that many compact groups are clumps of galaxies, and some are gravitationally bound. If most are gravitationally bound, merger arguments imply a high rate of galaxy formation. This may conflict with hierarchical formation ideas, which tend to favor alignments. The nature of compact groups is as yet unsolved, but it is progressing on a group-by-group basis. Its resolution will help define how galaxies and clusters of galaxies developed over the history of the universe.

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