HCG 87, a group of galaxies in Capricornus

HCG 87, a group of galaxies in Capricornus

Image Credit: The Hubble Heritage Team (STScI/AURA/NASA)

Hickson Compact Group 87 is a small group of gravitational bound galaxies, located about 400 million light-years away in the constellation of Capricornus.

A Hickson Compact Group (HCG for short) is a collection of four or five gravitational bound galaxies in close physical proximity to one another, published in a list of 100 objects (462 galaxies) by the Canadian astronomer Paul Hickson in 1982. These groups usually contain large quantities of diffuse gas and are dominated by dark matter. Strong galaxy interactions result and merging is expected to lead to the formation of one, most likely elliptical, larger galaxy.

They are among the densest concentrations of galaxies known, comparable to the centers of rich galaxy clusters. Compact groups are worthy of intense study as they provide a rich opportunity to study galaxy interactions and mergers.

Compact groups are relatively short lived entities, they slowly self-destruct. Simulations predict that merging of the group members should proceed rapidly within one billion years. Hickson groups are therefore snapshots at various stages in this merging process. They may represent an intermediate stage between loose groups and individual galaxies. A better understanding of the nature of HCGs could help explain galaxy formation on a larger scale in the early Universe. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution.

HCG 87 is composed of a large edge-on spiral galaxy (PGC 65415 or HCG 87a) of about 175 thousand light-years across and 375 million light-years away, visible on the lower left; a fuzzy elliptical galaxy (PGC 65409 or HCG 87b) of about 80 thousand light-years across and some 390 million light-years away, visible on the lower right; and a spiral galaxy (PGC 65412 or HCG 87c) of about 80 thousand light-years across and some 400 million light years away, visible near the top.

The small spiral (PGC 65414) near the center of the image, of about 35 thousand light-years across and some 455 million light-years away, is likely a more distant background galaxy. The deep image reveals other galaxies which certainly lie far beyond HCG 87. Several stars from our Galaxy are also visible in the foreground.

The complex dark bands across PGC 65415 are due to interstellar dust silhouetted against the galaxy’s background starlight. A faint tidal bridge of stars can be seen between the edge-on and elliptical galaxies.

The HCG 87 galaxies are interacting gravitationally, influencing their fellow group members’ structure and evolution. They are gravitationally stretching each other during their 100-million year orbits around a common center, and will most likely eventually merge to form a single bright elliptical galaxy.

The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. Two of the galaxies — PGC 65415 and PGC 65409 — contain active galactic nuclei, which are thought to harbor massive black holes that are consuming gas. Another galaxy, PGC 65412, is undergoing a burst of intense star formation.

The image was taken in 1999 by the Hubble Space Telescope’s Wide Field Planetary Camera 2, made by combining images taken in four different color filters in order to create a three-color picture. Regions of active star formation are blue (hot stars) and also pinkish if hot hydrogen gas is present.

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