Stephan’s Quintet in HST-Subaru Composite

Source: Hubble Legacy Archive and 8.2 Meter Subaru Telescope (NAOJ)

Image Assembly and Processing: Robert Gendler and Judy Schmidt

Distance: 260 million light years

This small group of spiral galaxies was first discovered by the French astronomer, Edouard Stephan in 1877 and is often considered the prototype of small compact galactic groups.

It is the first of the compact groups found and probably the most investigated at all wavelengths. Initially Stephan included five relatively bright members (NGC 7317, 7318A/B, 7319, and 7320). In 1961 red shift measurements of the group revealed that NGC7320, the largest member of the group, had a discordant redshift and is receding at a velocity 5000 kilometers per second slower than the other four members.

A redshift is considered discordant if it differs from the median redshift of the group by more than 1000 kilometers per second. Although the case of NGC 7320 still fuels controversy it is generally agreed that all members of the group are gravitationally interacting with the exception of the interloper NGC 7320 which is a foreground galaxy.

QuintetCredit: © Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)Thursday, December 8, 2011: Stephan’s Quintet, in the constellation Pegasus, consists of five galaxies, although one lies closer than the others. Japan’s Subaru Telescope shows two views of the grouping using different filters to distinguish the recession velocity of each object, the speed at which the object movies away from the observer. The image on the left shows the galaxies filtered for a recession velocity of 0, while the one on the right shows them filtered for a recession velocity of 4,200 miles per second. The contrasting images show that NGC7320 (lower left) is closer than the other galaxies.

— Tom Chao