Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). With only a few member galaxies per group, HCGs are much smaller than the immense clusters of galaxies which lurk in the cosmos, but like the large galaxy clusters, some HCGs seem to be filled with hot, x-ray emitting gas.
In the image, black and green colors represent low intensities while red and purple hues indicate high x-ray intensities. Striking features of the x-ray image are the low brightness blobs at the upper left and lower right which symmetrically flank the intense central x-ray region. HCG 62 lies in Virgo, and near the group's center resides elliptical galaxy NGC 4761.
At optical wavelengths, some HCGs make for rewarding viewing, even with modest sized telescopes.
The Hickson Compact Group 87 (HCG 87) shown below, are interesting partly because they slowly self-destruct because the galaxies of are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers.
HCG 87 is composed of a large edge-on spiral galaxy visible on the lower left, an elliptical galaxy visible on the lower right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. The image was taken in 1999 July by the Hubble Space Telescope's Wide Field Planetary Camera 2. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.
The Daily Galaxy via NASA, Chandra Observatory and Via http://totherow.tripod.com/index-21.html