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HCG 7 (Hickson Compact Group 7)


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HCG 7 (Hickson Compact Group 7)

名前
(Name)
分類 明るさ
(Brightness)
別名
(Other names)
NGC 192 Gx SBa 12.6 PGC 2352, UGC 401, MCG 0-2-104
NGC 196 Gx SB0 13.6 PGC 2357, UGC 405, MCG 0-2-110
NGC 197 Gx S0 13.7 PGC 2365, UGC 406, MCG 0-2-107
NGC 201 Gx SBc 12.9 PGC 2388, UGC 419, MCG 0-2-115

距離 (Distance)
2億光年 (200 million light-years)


星座 (Constellation)
くじら座 (Cet) (Cetus)

HCG 7 (Hickson Compact Group 7) is an interacting galaxy located 200 million light years from the Earth in the constellation Cetus.

HCG 7 (Hickson Compact Group 7) : Picture

HCG 7 (Hickson Compact Group 7)
HCG 7 (Hickson Compact Group 7)
(C) NASA/ESA/Hubble
Hickson Compact Group 7, or HCG 7 for short, is a small group of galaxies consisting of 3 spiral galaxies and 1 elongated lenticular galaxy in close proximity and which are weakly gravitationally bound.

The 3 spiral galaxies are: NGC 192, the one that dominates the foreground; NGC 196 the one on the top right; and NGC 201 which is outside this image. The lenticular galaxy NGC 197 is the bright blue one in the middle. NGC 196 shows some merger activity and has probably experienced a closer encounter with NGC 197.

The NASA/ESA Hubble Space Telescope has imaged part of the Hickson Compact Group 7, or HCG 7 for short. This grouping is composed of one lenticular and three spiral galaxies in close proximity. In this image, one of the spirals dominates the foreground, with many more distant galaxies peppering the background. Observing tightly-knit galaxy groups like HCG 7 is important because they evolve in a different way from their more spaced-out counterparts in less crowded regions of the Universe. 

A recent study using Hubble data analysed the star clusters in HCG 7. Three hundred young clusters and 150 globular clusters were charted, and their ages and distributions measured. The results suggest that the rate of star formation has been fairly steady through time, although quite high in the central regions. Additional studies, including searches for material between the galaxies, hint that the stars in the HCG 7 galaxies formed by converting their gas without any gravitational influences caused by merging with other galaxies. This is puzzling, as the galaxies are depleting their supplies of gas at a rate that suggests that they have merged in the past. 

This raises the question of whether the group really has evolved serenely, or if there are mysterious processes at work that are yet to be understood. The currently known information is contradictory and an encouragement for further studies to discover the real story behind HCG 7.

NGC 201 (Spiral Galaxy)
NGC 201 (Spiral Galaxy)
(C) ESA/Hubble & NASA, Acknowledgement: Luca Limatola
Discovered by astronomer William Herschel in the late 1700s, NGC 201 is a barred spiral galaxy similar to our own galaxy, the Milky Way. It lies 200 million light-years from Earth in the constellation of Cetus, and is invisible to the naked eye.

This new NASA/ESA Hubble Space Telescope image of NGC 201 shows the galaxy in striking detail, capturing the bright centre and the barred spiral arms — arms that do not start directly from the galactic centre, but instead seem to be offset and stem from a "bar" of stars cutting through the middle of the galaxy.

Along with three of its closest galactic neighbours (outside the frame), NGC 201 belongs to a group known as the HCG 7 compact galactic group. Hickson Compact Groups (HCG) are relatively small and isolated systems containing a handful of bright, compact galaxies that lie close to one another. As the galaxies within these groups move closer together they interact strongly, dragging galactic material out into space and distorting the structure of the other group members.

Eventually, all the galaxies within one HCG will merge together. Simulations have shown that within a billion years, the galaxies within one HCG have merged to form a giant fossil galaxy. It is possible that this is the final fate of all galactic groups.

HCG 7 (Hickson Compact Group 7) NGC 201 (Spiral Galaxy)
HCG 7 (Hickson Compact Group 7)
(C) NASA/ESA/Hubble
NGC 201 (Spiral Galaxy)
(C) NASA/ESA/Hubble
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