Hickson Compact Groups (HCG)
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2010 July 61999 September 6
Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 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 above picture 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.
2010 February 22
Will the result of these galactic collisions be one big elliptical galaxy? Quite possibly, but not for another billion years. Pictured above, several of the dwarf galaxies of in the Hickson Compact Group 31 are seen slowly merging. Two of the brighter galaxies are colliding on the far left, while an elongated galaxy above is connected to them by an unusual bridge of stars. Inspection of the above image further indicates that the bright duo trail a rope of stars pointing to the spiral galaxy on the far right. Most assuredly, the pictured galaxies of Hickson Compact Group 31 will pass through and destroy each other, millions of stars will form and explode, and thousands of nebula will form and dissipate before the dust settles and the final galaxy emerges about one billion years from now. The above image is a composite of images taken in infrared light by the Spitzer Space Telescope, ultraviolet light by the GALEX space telescope, and visible light by the Hubble Space Telescope. Hickson Compact Group 31 spans about 150 thousand light years and lies about 150 million light years away toward the constellation of Eridanus.
2009 March 13
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). This sharp Hubble image shows one such galaxy group, HCG 90, in startling detail. Three galaxies are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted between a pair of large elliptical galaxies. The close encounter will trigger furious star formation. On a cosmic timescale, the gravitational tug of war will eventually result in the merger of the trio into a large single galaxy. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 90 lies about 100 million light-years away in the constellation Piscis Austrinus. This Hubble view spans about 80,000 light-years at that estimated distance. Of course, Hickson Compact Groups also make for rewarding viewing for Earth-bound astronomers with more modest sized telescopes.
2007 March 19
Galaxies, like stars, frequently form groups. A group of galaxies is a system containing more than two galaxies but less than the tens or hundreds typically found in a cluster of galaxies. A most notable example is the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds. Pictured above is nearby compact group Hickson 44. This group is located about 60 million light-years away toward the constellation of Leo. Also known as the NGC 3190 Group, Hickson 44 contains several bright spiral galaxies and one bright elliptical galaxy on the upper left. The bright source on the upper right is a foreground star. Many galaxies in Hickson 44 and other compact groups are either slowly merging or gravitationally pulling each other apart.
2005 July 162003 July 31
Posing for this cosmic family photo are the galaxies of HCG (Hickson Compact Group) 87, about four hundred million light-years distant toward the amphibious constellation Capricornus. The large edge-on spiral near picture center, the fuzzy elliptical galaxy immediately to its right, and the spiral near the top of the image are identified members of the group, while the small spiral galaxy in the middle is likely a more distant background galaxy. In any event, a careful examination of the deep image reveals other galaxies which certainly lie far beyond HCG 87. While not exactly locked in a group hug, the HCG 87 galaxies are interacting gravitationally, influencing their fellow group members' structure and evolution. This new image is from an instrument undergoing commissioning on the Gemini Observatory's South Telescope at Cerro Pachon, Chile. It compares favorably with views of this photogenic galaxy group recorded by the Hubble Space Telescope.
2001 March 9
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 fact, groups of galaxies like HCGs may be the building blocks of the large clusters. This false-color x-ray image from the orbiting Chandra Observatory reveals x-ray emission from the gas in one such group, HCG 62, in startling detail. 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.
1999 February 17
Galaxies, like stars, frequently form groups. A group of galaxies is a system containing more than two galaxies but less than the tens or hundreds typically found in a cluster of galaxies. A most notable example is the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds. Pictured above is nearby compact group Hickson 40. This group is located about 300 million light-years away toward the constellation of Hydra. Of the five prominent galaxies in Hickson 40, three are spirals, one is an elliptical and one is a lenticular. Many galaxies in compact groups are either slowly merging or gravitationally pulling each other apart.