Stephan’s Quintet

Stephan’s Quintet in the constellation Pegasus is a visual grouping of five galaxies which four form the first compact galaxy group ever discovered. The group was discovered by Édouard Stephan in 1877 at Marseilles Observatory and is the most studied of all the compact galaxy groups. NGC 7320, which has extensive H II regions, is the brightest member of the visual grouping and is where active star formation is occurring.

Hickson Compact Group 92, which contains four of the five galaxies in Stephan’s Quintet, is a cosmic dance of galaxies that will most likely end with the galaxies merging.

In the early 1970’s radio observations revealed a filament of emission amongst the galaxies that lies in inter-galactic space. Space telescopes have shown that the filament is a giant intergalactice show-wave which was caused by one galaxy falling into the center of Stephan’s Quintet at several million miles per hour. This same region is also detected in the faint glow of ionized atomic hydrogen seen in the visible part of the spectrum as the magnificent green arc.

NASA’s Chandra X-ray Observatory detects the X-rays that are emitted by gas heated to millions of degrees. This gas comes from NGC 7318B colliding with the gas creating a shock wave that heats the gas.

Molecular Hydrogen signals, an unexpected discovery by NASA’s Spritzer Space Telescope, were created by the shockwave. The hydrogen emission, seen through infrared spectral analysis, is one of the most turbulent formations of molecular hydrogen ever seen. Near the center of the green area in the visible light is where the strongest emissions originate. The discovery of the hydrogen was made by a team of scientists at the California Institute of Technology composed of scientists from Australia, Germany, and China. Normally hydrogen molecules are very fragile and would not be able to survive a large shockwave and because of that the discovery of the hydrogen from the collision was unexpected. One possible explanation is that as a shock front moves through a cloudy medium millions of smaller shocks are produced in a turbulent layer which could allow the molecular hydrogen to survive. Since molecular hydrogen was likely to be present in the early universe the collision can help provide information on what happened in the postulated beginning of the universe some 14 billion years ago.

NGC 7320 indicates a small redshift (790 km/s) while the other four exhibit large redshifts (near 6600 km/s). NGC 7320 is a lot closer to earth than the other five. It lies 39 million light years from earth as opposed to the 210-340 million light years that the other five are at. NGC 7319 has a type 2 Seyfert nucleus.

NGC 7320C, a sixth galaxy, probably belongs to the association, with a redshift similar to the Hickson galaxies. A tidal tail appears to connect it with NGC 7319.