NGC 6946

NGC 6946, sometimes referred to as the Fireworks Galaxy, is a face-on intermediate spiral galaxy with a small bright nucleus, whose location in the sky straddles the boundary between the northern constellations of Cepheus and Cygnus. Its distance from Earth is about 25.2 million light-years or 7.72 megaparsecs,[2] similar to the distance of M101 (NGC 5457) in the constellation Ursa Major.[5] Both were once considered to be part of the Local Group,[6] but are now known to be among the dozen bright spiral galaxies near the Milky Way but beyond the confines of the Local Group.[7] NGC 6946 lies within the Virgo Supercluster.[8]

NGC 6946
Observation data (J2000 epoch)
ConstellationCepheus & Cygnus
Right ascension20h 34m 52.3s[1]
Declination+60° 09′ 14″[1]
Redshift0.000133[1]
Heliocentric radial velocity48 ± 2 km/s[1]
Distance25.2 ± 1.0 Mly
7.72± 0.32 Mpc)[2]
Apparent magnitude (V)+9.6[1]
Characteristics
TypeSAB(rs)cd[1]
Size26.77 kpc (87,300 ly)
(diameter; 25.0 mag/arcsec2 B-band isophote)[1][3]
Apparent size (V)16.0 x 11.2 arcmin[4]
Other designations
UGC 11597, PGC 65001, Arp 29,[1] Caldwell 12

The galaxy was discovered by William Herschel on 9 September 1798. Based on an estimation by the Third Reference Catalogue of Bright Galaxies (RC3) in 1991, the galaxy has a D25 B-band isophotal diameter of 26.77 kiloparsecs (87,300 light-years).[1][3] It is heavily obscured by interstellar matter due to its location close to the galactic plane of the Milky Way.[9] Due to its prodigious star formation it has been classified as an active starburst galaxy.[7] NGC 6946 has also been classified as a double-barred spiral galaxy, with the inner, smaller bar presumably responsible for funneling gas into its center.[10][11]

Various unusual celestial objects have been observed within NGC 6946. This includes the so-called 'Red Ellipse' along one of the northern arms that looks like a super-bubble or very large supernova remnant, and which may have been formed by an open cluster containing massive stars. There are also two regions of unusual dark lanes of nebulosity, while within the spiral arms several regions appear devoid of stars and gaseous hydrogen, some spanning up to two kiloparsecs across.[7] A third peculiar object, discovered in 1967, is now known as "Hodge's Complex".[12] This was once thought to be a young supergiant cluster, but in 2017 it was conjectured to be an interacting dwarf galaxy superimposed on NGC 6946.[7]

Supernovae

Ten supernovae have been observed in NGC 6946 in the 20th and early 21st century: SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et,[13] SN 2008S, and SN 2017eaw.[14][15][16][17] For this reason, NGC 6946 has sometimes been referred to as the "Fireworks Galaxy".[18][19] This is about ten times the rate observed in our Milky Way galaxy,[20] even though the Milky Way has twice as many stars as NGC 6946.

On 27 September 2004, the Type II supernova SN 2004et was observed at magnitude 15.2 and rose to a maximum visual magnitude of 12.7. Images taken during the preceding days revealed that the supernova explosion occurred on 22 September. The progenitor of the supernova was identified on earlier images –– only the seventh time that such an event was directly identified with its host star. The red supergiant progenitor had an initial mass of about 15M☉ in an interacting binary system shared with a blue supergiant.[13]

During 2009, a bright star within NGC 6946 flared up over several months to become over one million times as bright as the Sun. Shortly thereafter it faded rapidly. Observations with the Hubble Space Telescope suggest that the star did not survive, although there remains some infrared emission from its position. This is thought to come from debris falling onto a black hole that formed when the star died. This potential black hole-forming star is designated N6946-BH1.[21] The progenitor is believed to have been a yellow hypergiant star.[22]

In May 2017, supernova SN 2017eaw was detected in the northwest region of the galaxy, and light curves obtained over the next 600 days showed that it was a Type II-P.[23] The progenitor was determined to have been a red supergiant, with a mass of around 15M☉.[24][25]

As of 2017, more supernovae had been seen in NGC 6946 than in any other galaxy,[14] a record that has since been surpassed by NGC 3690.[26]

Supernovae in NGC 6946[14]
Supernova apmag type
2017eaw12.6IIP
2008S16.5IIn-pec/LBV
2004et12.3II
2002hh15.0II
1980K11.4IIL
1969P13.9 ?
1968D13.5II
1948B14.9II
1939C13.0 ?
1917A14.6II
  • This image from the Kitt Peak National Observatory contextualizes the locations of supernovae 2004et and 2017eaw within the galaxy.
    This image from the Kitt Peak National Observatory contextualizes the locations of supernovae 2004et and 2017eaw within the galaxy.
  • Images of 2017eaw in NGC 6946, taken between May and November 2017, as the supernova was slowly fading
    Images of 2017eaw in NGC 6946, taken between May and November 2017, as the supernova was slowly fading
  • Pair of visible-light and near-infrared photos from the Hubble Space Telescope showing the giant star N6946-BH1 before and after it vanished out of sight by imploding to form a black hole
    Pair of visible-light and near-infrared photos from the Hubble Space Telescope showing the giant star N6946-BH1 before and after it vanished out of sight by imploding to form a black hole
  • NASA/ESA/CSA James Webb Space Telescope image of NGC 6946, revealing large amounts of dust from the two supernovae SN 2004et and SN 2017eaw
    NASA/ESA/CSA James Webb Space Telescope image of NGC 6946, revealing large amounts of dust from the two supernovae SN 2004et and SN 2017eaw

See also

References
  1. "NASA/IPAC Extragalactic Database". Results for NGC 6946. Retrieved 2022-10-29.
  2. Eldridge, J J; Xiao, Lin (2 March 2019). "The distance, supernova rate, and supernova progenitors of NGC 6946". Monthly Notices of the Royal Astronomical Society: Letters. 485 (1): L58–L61. arXiv:1903.00173. Bibcode:2019MNRAS.485L..58E. doi:10.1093/mnrasl/slz030. S2CID 118831398.
  3. De Vaucouleurs, Gerard; De Vaucouleurs, Antoinette; Corwin, Herold G.; Buta, Ronald J.; Paturel, Georges; Fouque, Pascal (1991). Third Reference Catalogue of Bright Galaxies. Bibcode:1991rc3..book.....D.
  4. Jarrett, T. H.; Chester, T.; Cutri, R.; Schneider, S. E.; Huchra, J. P. (2003). "The 2MASS Large Galaxy Atlas". The Astronomical Journal. 125 (2): 525–554. Bibcode:2003AJ....125..525J. doi:10.1086/345794. S2CID 117784410.
  5. Sandage, A.; Bedke, J. (1994). The Carnegie Atlas of Galaxies. Volume I. Carnegie Institution of Washington. Bibcode:1994cag..book.....S.
  6. "NGC 6946". SEDS. Retrieved 2017-11-20.
  7. Efremov, Yu. N. (2016). "Unusual Objects in the Spiral Galaxy NGC 6946". Open Astronomy. 25 (4): 365–376. Bibcode:2016BaltA..25..369E. doi:10.1515/astro-2017-0255.
  8. "Nearby Groups of Galaxies". ned.ipac.caltech.edu. Retrieved 2018-05-03.
  9. "NED data for some important Non-Messier Galaxies".
  10. Schinnerer, Eva; et, al (2006). "Molecular Gas Dynamics in NGC 6946: A Bar-driven Nuclear Starburst Caught in the Act". The Astrophysical Journal. 649 (1): 181–200. arXiv:astro-ph/0605702. Bibcode:2006ApJ...649..181S. doi:10.1086/506265. S2CID 14717998.
  11. Eibensteiner, Cosima; et, al (2022). "A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946". Astronomy & Astrophysics. 659 (A173): 37. arXiv:2201.02209. Bibcode:2022A&A...659A.173E. doi:10.1051/0004-6361/202142624.
  12. Hodge, P.W. (1967). "A Possible "Super-Supernova" Remnant in NGC 6946". Open Astronomy. 79 (466): 466–470. Bibcode:1967PASP...79...29H. doi:10.1515/astro-2017-0255.
  13. Li, W.; Van Dyk, S.D.; Filippenko, A.V; Cuillandre, J.C. (2005). "On the Progenitor of the Type II Supernova 2004et in NGC 6946". Publications of the Astronomical Society of the Pacific. 117 (828): 121–131. arXiv:astro-ph/0412487. Bibcode:2005PASP..117..121L. doi:10.1086/428278. S2CID 17585230.
  14. Bishop, David (2017-05-14). "Supernova 2017eaw in NGC 6946". Retrieved 2023-09-29. This is our most prolific galaxy with now 10 confirmed supernovae discovered.
  15. "ATel #10372: Confirmation of AT 2017eaw, a Probable Supernova in NGC 6946".
  16. "ATel #10376: Spectroscopic Classification of SN 2017eaw in NGC 6946 as a Young Type IIP Supernova".
  17. "List of Supernovae". Harvard-Smithsonian Center for Astrophysics (IAU). Retrieved 2010-07-12.
  18. Michaud, Peter (1 January 2015). "Gemini Observatory Welcomes 2005 with Release of Galactic Fireworks Image". NASA. Retrieved 2017-10-15.
  19. Boen, Brooke (20 May 2015). "NGC 6946: The 'Fireworks Galaxy'". NASA. Retrieved 2016-12-08.
  20. "Gemini Observatory Welcomes 2005 with Release of Galactic Fireworks Image", Gemini Observatory, 1 January 2005, retrieved 2016-01-04
  21. Adams, S. M.; Kochanek, C. S.; Gerke, J. R.; Stanek, K. Z.; Dai, X. (2017). "The search for failed supernovae with the Large Binocular Telescope: conformation of a disappearing star". Monthly Notices of the Royal Astronomical Society. 468 (4): 4968–4981. arXiv:1609.01283v1. Bibcode:2017MNRAS.468.4968A. doi:10.1093/mnras/stx816. S2CID 12919545.
  22. Humphreys, Roberta M. (2019-10-31). "Comments on the Progenitor of NGC 6946-BH1". Research Notes of the AAS. 3 (10): 164. arXiv:1911.02037. Bibcode:2019RNAAS...3..164H. doi:10.3847/2515-5172/ab5191. ISSN 2515-5172. S2CID 207880492.
  23. Buta, Ronald J; Keel, William C (2019-07-21). "BVRI photometry of the classic Type II-P supernova 2017eaw in NGC 6946: d 3 to d 594". Monthly Notices of the Royal Astronomical Society. 487 (1): 832–844. arXiv:1905.03084. Bibcode:2019MNRAS.487..832B. doi:10.1093/mnras/stz1291. ISSN 0035-8711. S2CID 147703978.
  24. Szalai, Tamás; Vinkó, József; Könyves-Tóth, Réka; Nagy, Andrea P.; Bostroem, K. Azalee; Sárneczky, Krisztián; Brown, Peter J.; Pejcha, Ondrej; Bódi, Attila; Cseh, Borbála; Csörnyei, Géza (2019-04-29). "The Type II-P Supernova 2017eaw: From Explosion to the Nebular Phase". The Astrophysical Journal. 876 (1): 19. arXiv:1903.09048. Bibcode:2019ApJ...876...19S. doi:10.3847/1538-4357/ab12d0. ISSN 1538-4357. S2CID 84842034.
  25. Kilpatrick, Charles; Foley, Ryan (2018-09-05). "The Dusty Progenitor Star of the Type II Supernova 2017eaw". Monthly Notices of the Royal Astronomical Society. 481 (2): 2536–2547. arXiv:1806.00348. doi:10.1093/mnras/sty2435.
  26. Bishop, David (2023-08-27). "The most prolific galaxies". Retrieved 2023-09-29.
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