Credit: ESA

Compact Group Controversy

"Compact groups" of galaxies are isolated regions of space where the apparent density of galaxies is especially high. The word "apparent" is important, since it's not clear if the galaxies in these groups are physically related or if they simply represent chance superpositions of unrelated galaxies which happen to lie along the same line of sight. X-ray observations can help resolve this issue by looking for telltale traces of hot gas existing between the galaxies. If the galaxies in the group are physically related, there should be a significant amount of diffuse emission from this hot gas; if they are unrelated, there should be relatively little hot gas. This X-ray test can only be accomplished with an X-ray telescope which is sensitive to faint, low surface brightness emission; which is sensitive to low-energy X-ray emission; and which has good spatial resolution (to distinguish the emission from the member galaxies from the diffuse emission). The XMM-Newton observatory provides astronomers with such a telescope. The image above was taken by the EPIC-PN CCD camera on XMM-Newton, and shows a first look at one important compact group called HCG 16 (for "Hickson compact group"). The image above shows a "true color" image, in which the color of the emission represents the temperature of the X-ray emitting gas. Additional observations like this one will probably resolve the compact group controversy.

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Page Author: Dr. Michael F. Corcoran
Last modified September 15, 2000