NGC 6946

NGC 6946, also known as the Fireworks Galaxy or Caldwell 12, 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.[3] Both were once considered to be part of the Local Group.[4] but are now known to be among the dozen bright spiral galaxies near the Milky Way but beyond the confines of the Local Group.[5] NGC 6946 lies within the Virgo Supercluster.[6]

NGC 6946
An image of NGC 6946 as taken by the Chandra X-ray Observatory
Observation data (J2000 epoch)
ConstellationCepheus & Cygnus
Right ascension20h 34m 52.3s[1]
Declination+60° 09′ 14″[1]
Redshift0.000133[1]
Helio 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]
Apparent size (V)11.5 x 9.8 arcmin[1]
Other designations
UGC 11597, PGC 65001, Arp 29,[1] Caldwell 12

Discovered by William Herschel on 9 September 1798, this well-studied galaxy has a diameter of approximately 40,000 light-years[dubious ] , about one-third of the Milky Way's size,[7] and it contains roughly half the number of stars as the Milky Way. It is heavily obscured by interstellar matter due to its location close to the galactic plane of the Milky Way, with a dimming of ~1.5 magnitudes.[8] Due to its prodigious star formation it has been classified as an active starburst galaxy.[5]

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.[5] A third peculiar object, discovered in 1967, is now known as "Hodge's Complex".[9] 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.[5]

Supernovae

Fireworks Galaxy (NGC 6946)
RGB image (Liverpool Telescope)
(2014 and 2016)
Mysterious X-ray spots
(blue; green/post-14d, May 2017)

Ten supernovae have been observed in NGC 6946 in the last century: SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et,[10] SN 2008S, and SN 2017eaw.[11][12][13] For this reason NGC 6946 in 2005 was dubbed the "Fireworks Galaxy",[14][15] a name becoming increasingly popular. NGC 6946 has an unusually high rate of supernovae production compared to our Milky Way galaxy, whose rate averages just one supernova event per century.[16] This is the more remarkable as our Galaxy comprises twice as many stars.

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 several days earlier revealed no such star, indicating destruction of the star occurred on the 22 September. The progenitor of SN 2004et has been 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.[10]

During 2009, a bright star within the galaxy NGC 6946 flared up over several months to become over one million times as bright as the Sun. Shortly thereafter it appeared to vanish. New observations with the Hubble Space Telescope strongly suggest that the star did not survive, although a faint trickle of infrared light emanates from where the star used to be. The remnant glow probably comes from debris falling onto a black hole that formed when the star died. This potential black hole-forming star is designated N6946-BH1.[17] Based on UBVR photometry, the star was determined to most likely be an early G-type supergiant star.[18]

In May 2017, supernova SN 2017eaw was first observed to be 12.8 magnitude in the northwest region of the galaxy and light curves obtained over the next 600 days showed that it was a Type II-P.[19] The progenitor was determined to have been a ~ 15${\displaystyle M_{\odot }}$  red supergiant before going nova.[20]

In X-rays, observations using the Chandra Space Telescope have revealed three of the oldest supernovae so far detected. The attached composite image also includes optical data from the Gemini Observatory in red, yellow, and cyan.[21]

• IC 342 - similar galaxy heavily obscured by Milky Way stars and dust.

References

1. "NASA/IPAC Extragalactic Database". Results for NGC 6946. Retrieved 2006-11-18.
2. ^ a b 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. ^ Sandage, A.; Bedke, J. (1994). The Carnegie Atlas of Galaxies. Volume I. Carnegie Institution of Washington. Bibcode:1994cag..book.....S.
4. ^ "NGC 6946". SEDS. Retrieved 2017-11-20.
5. ^ a b c d 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.
6. ^ "Nearby Groups of Galaxies". ned.ipac.caltech.edu. Retrieved 2018-05-03.
7. ^ Nemiroff, R.; Bonnell, J., eds. (1 January 2011). "NGC 6946 : the Fireworks Galaxy". Astronomy Picture of the Day. NASA. Retrieved 2 November 2017.
8. ^ http://www.messier.seds.org/xtra/supp/o_NED.html#n6946
9. ^ 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.
10. ^ a b 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.