Stephan's Quintet is a visual grouping of five galaxies of which four form the first compact galaxy group ever discovered. The group, visible in the constellation Pegasus, was discovered by Édouard Stephan in 1877 at the Marseille Observatory. The group is the most studied of all the compact galaxy groups. The brightest member of the visual grouping (and the only non-member of the true group) is NGC 7320, which has extensive H II regions, identified as red blobs, where active star formation is occurring.
Four of the five galaxies in Stephan's Quintet form a physical association, a true galaxy group, Hickson Compact Group 92, and will likely merge with each other. Radio observations in the early 1970s revealed a filament of emission between the galaxies in the group. This same region is also detected in the faint glow of ionized atoms seen in the visible part of the spectrum as a green arc. Out of 4 galaxies, 3 of which are spiral galaxies, the Spiral Fraction of HCG 92 is 0.75 or 75%.
Space telescopes have provided new insight into the nature of the filament, which is now believed to be a shock-wave in the intergalactic gas, caused by one galaxy (NGC 7318B) falling into the center of the group at several million kilometres per hour. The shock wave is believed to have created multiple gaseous stellar filaments, and the formation of tidal dwarfs.
Stephan's Quintet was selected as one of the five cosmic objects observed by the James Webb Space Telescope as part of the release of its first official science images.
Properties NGC 7320 has a Major Axis(") of 69.8 and a Minor Axis(") of 44.20, indicating it is moderately elongated large galaxy. By contrast, NGC 7317 has a Major Axis(") of 22.9 and a Minor Axis(") of 21.2, suggesting it is a nearly circular small Galaxy. HCG 92 has a median galaxy separation of 28.2 kpc, indicating it is a very compact galaxy group. It also has a Crossing Time () of 0.0054 and a Mass to Light Ratio M⊙/L⊙ of 43.7 , indicating it is a dynamically young group and dominated by matter that isn't emitting light - Dark matter.
In the environment surrounding HCG 92, there are 21 background galaxies. Of which they are, 4 Elliptical galaxies, 1 Lenticular galaxy and 16 Spiral galaxies. This indicates that the environment of HCG 92 is an evolutionary young environment as Elliptical galaxies form by intense gravitational interactions and ram-pressure stripping. The projected axial ratio of the fitted spheroid of HCG 92 (including nearby galaxies) q=0.53, indicating a moderately elongated structure in the sky that is consistent with the tidal tails.
NGC 7319 has a Blue minus Red (B-R) of 1.62 and is the highest in the group, indicating it is likelihood a dusty galaxy or it is dominated by old stars. In contrast, NGC 7320 has the lowest of B-R (1.09) of the entire group, indicating it is likely undergoing active star formation and is dominated by young stars.
Emissions
X-rays
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. HCG 92 has a X-ray luminosity of log = 42.16 ± 0.04 erg s⁻¹ and has a hot intragroup gas temperature of 0.75 ± 0.08 keV (~9 million K).
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 6,600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground projection and is ~39 million light-years from Earth, making it a member of the NGC 7331 Group or LGG 459 Group, versus the 210–340 million light-years of the other four.
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.
Infrared Using its Mid Infrared Instrument (MIRI), the James Webb Space Telescope shows details shrouded by dust in visible light including large shock waves and tidal tails in four of the five galaxies, and previously hidden areas of star formation. These new details will contribute to insights on galaxy evolution. 1996 IRAS observations at 12, 25, 60, and 100 μm detected only one member of HCG 92, which is HCG 92C, with fluxes of 0.20, 0.24, 0.58, and 3.01 Jy respectively.
Using Spitzer Space Telescope's Infrared Array Camera (IRAC) and Multiband Imaging Photometer (MIPS), Akari's Infrared Camera (IRC) and William Herschel Telescope's Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE), NGC 7318B is detected at Near, Mid, Far-Infrared and Submillimeter (3.2-500 μm), while NGC 7320 is not detected at 250 and 350 μm, NGC 7319 is not detected at 350 μm, NGC 7318A is not detected at 250 μm and NGC 7317 is not detected at 24 and 100 μm. This indicates that NGC 7318B is actively forming stars, while NGC 7317 is an early-type quiescent galaxy and is not actively formation stars. NGC 7317 has the lowest dust mass and it has lowest polycyclic aromatic hydrocarbon (PAH).
There is an infrared bright region within the intragroup medium of Stephan's Quintet, known as SQ-A. It's star formation rate (SFR) is about 1.45 M⊙ yr⁻¹, and has an infrared luminosity of (5−1000 μm) ≈ 7 × 1042 erg s⁻¹ with a dust temperature of 22–24 K. It is classified as a collision-triggered starburst in the intragroup medium, its classification of Tidal dwarf galaxy remains uncertain. A tidal star-forming knot known as SQ-B has been found, a Mid-IR detections found prominent PAH emission bands at 6.2, 7.7, 8.6, 11.2 μm and 12.81 μm feature attributed to [Ne II] blended with the 12.7 μm, confirming active star-formation in this region. It is also found that SQ-B has the highest Polycyclic Aromatic Hydrocarbon (PAH) in the Stephen Quintet.
Neutral hydrogen Neutral atomic hydrogen is the least gravitational bound component of galaxies and is the easiest (and hence first) to be stripped off and spread around during interactions between galaxies. Using the Five-hundred-meter Aperture Spherical Telescope (FAST), a compact cloud (northwest of NGC 7319) that is coincident with the intragroup-medium starburst of NGC 7318a, a inner and outer tail is optically detected by high-density HI gas traces. Detections found two new neutral atomic hydrogen (HI) sources near Stephen Quintet: NGC 7320a and Anon 7. These two new HI sources are found without any stellar counterparts. Using the Very Large Array's data, it is found that the neutral atomic hydrogen (HI gas) is not detected in any of the member galaxies of HCG 92, but is found in the tidal debris, outside of the member galaxies. A study using Arecibo Telescope and Green Bank Telescope found that HCG 92 has a HI integrated flux of 5.9 ± 0.3 Jy km s⁻¹.
Cold dust Based on William Herschel Telescope's data, detections found that NGC 7318B has a star formation rate (SFR) of 0.52 M⊙/yr, which is the highest in the group. NGC 7319 has a dust temperature of 43.1 K. Observations found that NGC 7318B and NGC 7319 show significant dust deficits of 70% and 56% respectively. NGC 7318B shows that 98% of its infrared flux originates from dust extending well-beyond the galaxy's own stellar disk (dust radius 55″ versus stellar radius 22″), indicating that its dust has been stripped and redistributed into the surrounding debris field.
Carbon monoxide Using Atacama Large Millimeter Array's Atacama Compact Array and Combined Array for Research in Millimeter-wave Astronomy (CARMA), SQ-A has a CO(2–1) flux and a CO(1–0) flux of 6.52 ± 0.32 Jy beam⁻¹ km s⁻¹ and 4.75 ± 0.41 Jy beam⁻¹ km s⁻¹ respectively, indicating it has a lot of molecular gas, but is cool and diffuse. The highest CO(2–1) flux and a CO(1–0) flux is E1, which is near the active galactic nucleus of NGC 7319, which has a CO(2–1) flux (Jy beam⁻¹ km s⁻¹) of 16.60 ± 0.85 and CO(1–0) flux of 5.74 ± 1.13, indicating it is undergoing active star-formation.
Ultraviolet and radio detections Using Gemini South Observatory and GALEX, it is found that there are 12 star-forming regions in the HI tail. Their ages are 2-100 million years, indicating they are young star clusters or dwarf galaxies. Radio continuum using data from Very Large Array detect HCG 92C as a strong source with a total flux of 24.74 mJy, which 19.99 mJy (81%) originates from the nucleus, indicating a AGN dominated radio source. HCG 92D is also detected, but is weak.
Active galactic nucleus NGC 7319 has a type 2 Seyfert nucleus. Stephen Quintet is one of the few systems that is able to exceed the L([C ii])/LFIR upper limits of 1% for star-forming galaxies, due to shock heating. HCG 92C (NGC 7319) is classified as an active galactic nucleus (AGN), with an emission-line diagnostic (log[NII]/Hα) of +0.16. HCG 92C also has a steeply rising red continuum and a prominent silicate absorption at 9.7 μm. This indicates that NGC 7319's AGN is buried within a thick screen of cold silicate dust. The AGN fraction () of NGC 7319 is 1.00, and the luminosity of the AGN is (5.68 ± 0.805) × 10¹⁰ L⊙, indicating that it is a moderately bright active galactic nucleus.
Members +Members of the Hickson Compact Group 92HCG Name Type R.A. (J2000) Dec. (J2000) Redshift (km/s) Apparent Magnitude92E NGC 7317 E4 6,599 ± 26 +14.692D NGC 7318a(UGC 12099) E2 pec 6,630 ± 23 +14.392B NGC 7318b(UGC 12100) SB(s)bc pec 5,774 ± 24 +13.992C NGC 7319 (PGC 69260) SB(s)bc pec 6,747 ± 7 +14.192A NGC 7320 SA(s)d 786 ± 20 +13.2NGC 7320C(R)SAB(s)0/a5985 ± 9 +16.7
In popular culture The angelic figures at the beginning of the 1946 holiday film It's a Wonderful Life are based on images of Stephan's Quintet.
See also Wild's Triplet Zwicky's Triplet NGC 7331 Group (Deer Lick Group, about half a degree northeast of Stephan's Quintet) Robert's Quartet Seyfert's Sextet Copeland's Septet
References
External links
GALEX: Stephan's Quintet and NGC 7331 Star Clusters Born in the Wreckage of Cosmic Collisions News Release at ESA/Hubble Stephan's Quintet NightSkyInfo.com : Stephan's Quintet What's Behind Stephan's Quintet? Peter Edmonds, Chandra Blog, 21 July 2009 Astronomy Picture of the Day on Stephan's Quintet: 13 Nov 2000 12 Aug 2003 11 Sep 2009 NASA’s James Webb Space Telescope reveals never-before-seen details of the galaxy group called “Stephan’s Quintet.”