Stephan's Quintet

Stephan's Quintet


Credit: HST
Observation data (Epoch J2000)
Constellation(s) Pegasus
Right ascension 22h 35m 57.5s[1]
Declination +33° 57′ 36″[1]
Brightest member NGC 7318B[1]
Number of galaxies 5[1]
Other designations
HCG 92, Arp 319, VV 288,[1] SQ[2]

Stephan's Quintet is a visual grouping of five galaxies of which four form the first compact galaxy group ever discovered.[2] The group, visible in the constellation Pegasus, was discovered by Édouard Stephan in 1877 at the Marseille Observatory.[3] The group is the most studied of all the compact galaxy groups.[2] The brightest member of the visual grouping is NGC 7320 that is shown to have extensive H II regions, identified as red blobs, where active star formation is occurring.

Description

Four of the five galaxies in Stephan's Quintet form a physical association, Hickson Compact Group 92, and will likely merge with each other. Radio observations in the early 1970s revealed a mysterious filament of emission which lies in inter-galactic space between the galaxies in the group. This same region is also detected in the faint glow of ionized atomic hydrogen seen in the visible part of the spectrum as a green arc.

Two space telescopes have recently provided new insight into the nature of the filament, which is now believed to be a giant intergalactic shock-wave (similar to a sonic boom but traveling in intergalactic gas rather than air) caused by one galaxy (NGC 7318B) falling into the center of the group at several millions of kilometers per hour.

X-ray source

Stephan's Quintet. Image Credits: X-ray (blue): NASA/CXC/CfA/E. O'Sullivan Optical (brown): Canada-France-Hawaii-Telescope/Coelum.

As NGC 7318B collides with gas in the group, a huge shock wave bigger than the Milky Way spreads throughout the medium between the galaxies, heating some of the gas to temperatures of millions of degrees where they emit X-rays detectable with the NASA Chandra X-ray Observatory.

Molecular hydrogen emission

The NASA Spitzer Space Telescope, which detects infrared radiation, discovered a very powerful molecular hydrogen signal from the shock wave between the galaxies. This emission is one of the most turbulent formations of molecular hydrogen ever seen, and the strongest emission originates near the center of the green area in the visible light picture discussed earlier. This phenomenon was discovered by an international team led by scientists at the California Institute of Technology and including scientists from Australia, Germany and China. The detection of molecular hydrogen from the collision was initially unexpected because the hydrogen molecule is very fragile and is easily destroyed in shock waves of the kind expected in Stephan's Quintet. However, one solution is that when a shock front moves through a cloudy medium like the center of the group, millions of smaller shocks are produced in a turbulent layer, and this can allow molecular hydrogen to survive.

Redshift

NGC 7320 indicates a small redshift (790 km/s) while the other four exhibit large redshifts (near 6600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground projection[2] and is ~39 million ly[2] from Earth versus the 210-340 million ly of the other five.

NGC 7319 has a type 2 Seyfert nucleus.

Photo of the area of the Hubble photo, with the various galaxies labeled by NGC number
The galaxies in the vicinity of Stephan's Quintet. The rectangle indicates the area covered by the 1998–99 Hubble Space Telescope image below.

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

Members of the Hickson Compact Group 92
Name Type[4] R.A. (J2000)[4] Dec. (J2000)[4] Redshift (km/s)[4] Apparent Magnitude[4]
NGC 7317 E4 22h 35m 51.9s +33° 56′ 42″ 6599 ± 26 +14.6
NGC 7318a
(UGC 12099)		 E2 pec 22h 35m 56.7s +33° 57′ 56″ 6630 ± 23 +14.3
NGC 7318b
(UGC 12100)		 SB(s)bc pec 22h 35m 58.4s +33° 57′ 57″ 5774 ± 24 +13.9
NGC 7319 SB(s)bc pec 22h 36m 03.5s +33° 58′ 33″ 6747 ± 7 +14.1
NGC 7320c (R)SAB(s)0/a 22h 36m 20.4s +33° 59′ 06″ 5985 ± 9 +16.7
Detail of the quintet in a photo by Hubble Space Telescope, 1998–99. Credits: NASA/ESA.

See also

Notes

  1. 1 2 3 4 5 "NASA/IPAC Extragalactic Database". Results for HCG 92. Retrieved 2006-09-18.
  2. 1 2 3 4 5 Moles, M.; Marquez, I.; Sulentic, J. W. (1998). "The observational status of Stephan's Quintet". Astronomy and Astrophysics. 334: 473–481. Bibcode:1998A&A...334..473M. arXiv:astro-ph/9802328 Freely accessible.
  3. ↑ Stephan, M. E. (1877). "Nebulæ (new) discovered and observed at the observatory of Marseille, 1876 and 1877, M. Stephan". Monthly Notices of the Royal Astronomical Society. 37: 334–339. Bibcode:1877MNRAS..37..334S. doi:10.1093/mnras/37.6.334.
  4. 1 2 3 4 5 "NASA/IPAC Extragalactic Database". Results for various galaxies. Retrieved 2006-10-20.

References

Coordinates: 22h 35m 57.5s, +33° 57′ 36″

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