GALAXY CLUSTERS



Hickson Compact Group 88, a cluster of four spiral galaxies that do not currently appear
to be disrupting each other: courtesy Sloan Digital Sky Survey

Instead of a random scattering through space, galaxies are distributed in gravitationally bound clusters. Our own Milky Way is no exception, as it travels in a loose cluster known as the Local Group along with two other large spiral galaxies and dozens of small elliptical and irregular galaxies. Members of such compact galaxy groups are often engaged in complex gravitational interactions, including fly-bys, pass-throughs, and mergers, which result in dramatic deformations and restructuring. Interactions between members of a group are believed to play a key role in galactic evolution.

The image at the top of the page represents Hickson 88, a group of four spiral galaxies in the direction of Aquarius. From right to left they are NGC 6975, NGC 6976, NGC 6977, and NGC 6978. In the latter three, the rate of new star formation increases from right to left, as indicated by the increasing luminosity of each successive galaxy. (See the SDSS Image of the Week Archive.) Hickson 88 illustrates the well-established observation that members of a compact group tend to share similar morphologies -- in this case, well-developed spiral arms.





Hickson Compact Group 87 photographed by the Hubble Space Telescope
Courtesy NASA/STScI/AURA

The astronomer Paul Hickson of the University of British Columbia is one of the leading investigators of galaxy groups. He has catalogued and described the three compact groups that appear on this page. Hickson 87, shown above, is located about 400 million light years away in the direction of Capricorn. The image is dominated by an oddly-shaped spiral galaxy presented edge-on (for comparison see Edge-On Galaxies). This galaxy is involved in a mutually disruptive interaction with the elliptical galaxy apparently perched on its rim. Without any visible internal structure, the elliptical galaxy looks like a gigantic star. Above the edge-on galaxy are two face-on spirals. Neither has been disrupted. The smaller of the two may be a background object unrelated to the cluster. The two bright stars are also unrelated, being foreground objects within our own Galaxy. (See APOD Archive.)

As Hickson states in "Compact Groups of Galaxies" in the 1997 Annual Review of Astronomy and Astrophysics:

Most compact groups contain a high fraction of galaxies having morphological or kinematical peculiarities, nuclear radio and infrared emission, and starburst or active galactic nuclei (AGN) activity. They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution.

(See this electronic version of Hickson's review.)


Stephan's Quintet in Pegasus. Counterclockwise from upper left: NGC 7317, NGC 7318A, NGC 7318B, NGC 7319, and NGC 7320. The last-named spiral galaxy is a foreground object unrelated to the others. NGC 7317 and NGC 7318A are elliptical galaxies. NGC 7318B and NGC 7319 are barred spiral galaxies that have suffered major disruption. Image courtesy Gemini Observatory/ Travis Rector/ University of Alaska.


By far the best known and most photogenic of the compact galaxy groups is Stephan's Quintet, which is located in the constellation Pegasus and was first identified in 1877 by Edouard Stephan. However, only four of the five visible galaxies are bona fide members of the compact group. The intricate spiral galaxy (NGC 7320) at the top of the Gemini Observatory photograph is a foreground object unrelated to the others.

The remaining quartet of interacting galaxies has evidently undergone one or more collisions in the past, creating enormous shock waves of hot hydrogen gas. Two of them (NGC 7317 and NGC 7318A) are elliptical galaxies that have remained relatively undisrupted, although the latter is involved in an interaction with NGC 7318B. That object and NGC 7319 are barred spiral galaxies that have undergone violent distortions. (See Moles et al. 1998 in References.)

The dramatic interplay visible within Stephan's Quintet provides further illustrations of Hickson's analysis of compact galaxy groups. Hickson notes that star-formation is triggered by the tidal effects of interacting galaxies, and that such interactions may be the mechanism responsible for the evolution of galactic disks. The outcome of these long-term perturbations may be the merger of member objects into one gigantic spherical or elliptical galaxy. Such may be the ultimate fate of the interacting quartet shown here.





Arp 256 North and South, photographed by the Hubble Space Telescope
Courtesy NASA/ESA/STScI/AURA/A. Evans

This pair of galaxies, known simply as Arp 256 North and South, reveal the early stages of gravitational interaction. Although the cores of these two galaxies are still widely separated, their disks are already disrupted.





NGC 4038 and NGC 4039, photographed by the Hubble Space Telescope
Courtesy NASA/ESA/STScI/AURA/B. Whitmore

NGC 4038 and 4039 exhibit a more advanced state of mutual disruption. Both galaxies are extremely distorted, and long streamers of luminous hydrogen extend out of frame from each one. Coalescence is well under way. Galactic mergers like this one trigger generations of new stars in a process known as starburst.









All text is copyright Raymond Harris 2006-2007. Credits for each image are listed in the accompanying caption.