Hickson Compact Group 40, an ensemble of galaxies in Hydra

Hickson Compact Group 40, an ensemble of galaxies in Hydra

Hickson Compact Group 40 (also known as Arp 321 or VV 116) is an ensemble of seven galaxies — five of which are clearly seen — located about 300 million light-years away in the constellation of Hydra.

From top to bottom, the 5 prominent galaxies in Hickson 40 are a spiral, an elliptical, two more spirals, and a lenticular. They clearly appear to be touching each other. The elliptical galaxy and two of the spiral galaxies show some levels of nuclear activity.

Single, isolated galaxies are rather rare in the Universe. They tend to form groups or clusters. A system with two galaxies is called a binary galaxy, a system containing more than two but less than several dozen galaxies is called a group (like the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds). A big system containing more than this is called a cluster.

There are groups of galaxies in which the members are in so small a space that they appear to be touching each other. These are called compact groups of galaxies. Hickson Compact Group 40 is one of 100 compact galaxy groups catalogued by the Canadian astronomer Paul Hickson.

Many galaxies which are located so close to each other are gravitational interacting and either slowly merging to form 1 or 2 giant galaxies, or pulling each other apart. Evidence of tidal interaction as a result of mutual gravitational attraction is actually seen in all 3 spiral galaxies in this group. The lenticular galaxy also shows evidence of interaction at its nucleus. We may be observing such a merger here.

Compact groups offer a window into what commonly happened in the Universe’s formative years when large galaxies were created from smaller building blocks. This isolated group of galaxies provides an interesting laboratory for studying the effects of close proximity and interaction on the evolution of galaxies. For instance, how these factors influence the presence of active galactic nuclei, and the relationship between galaxy interaction, activity, and morphology.

Two blueish white dots in the image are stars in our own Galaxy. Small reddish objects are galaxies located billions of light years away. They appear redder than the members of Hickson Compact Group 40 because of the Doppler effect caused by the expansion of the Universe.

Image Credit: CISCO, Subaru 8.3-m Telescope, NAOJ

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