Hickson Compact Group 40
January 28, 1999
Low Res. (86 KB)
High Res. (555 KB)
Object Name: Hickson Compact Group 40
Telescope: Subaru Telescope / Cassegrain Focus
Filter: J (1.25 micron), K' (2.15 micron)
Color: Blue (J), Green ([J+K']/2), Red (K')
Date: UT 1999 Jan 14 (J), Jan 12 (K')
Exposure: 480 sec (J), 480 sec (K')
Field of View: 1.9 arcmin by 2.9 arcmin
Orientation: North up, east left
Position: RA (J2000.0)=9h36m24.1s, DEC (J2000.0)=-4d37m39s
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, and 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.
The image shown here is a compact group of galaxies at a distance of 300 million light years in the constellation of Hydra. It is the 40th group in Hickson's catalog and is therefore called Hickson Compact Group (HCG) 40. From top to bottom, the 5 galaxies in the group are a spiral, an elliptical, two more spirals, and a lenticular (S0). They clearly appear to be touching each other.
Interactions often occur in compact groups where galaxies are located so close to each other. Evidence of tidal interaction as a result of mutual gravitational attraction is actually seen in all 3 spiral galaxies in this group. The S0 galaxy at the bottom also show evidence of interaction at its nucleus. Computer simulations show that, as a result of gravitational interaction, galaxies merge to form 1 or 2 giant galaxies in a time scale short compared to the age of the Universe. We may be observing such a merger here. 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 HCG40 because of the Doppler effect caused by the expansion of the Universe.