Whirlpool Galaxy

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Whirlpool Galaxy
Messier51 sRGB.jpg
Whirlpool Galaxy (M51A or NGC 5194). The smaller object in the upper right is M51B or NGC 5195. Credit: NASA/ESA
Observation data (J2000 epoch)
Constellation Canes Venatici[1]
Right ascension 13h 29m 52.7s[2]
Declination +47° 11′ 43″[2]
Redshift 463 ± 3 km/s[2]
Distance 23 ± 4 Mly (7.1 ± 1.2 Mpc)[3][4]
Type SA(s)bc pec[2]
Apparent dimensions (V) 11′.2 × 6′.9[2]
Apparent magnitude (V) 8.4[5]
Notable features Interacting with NGC 5195[6]
Other designations
Question Mark Galaxy,[2] Rosse's Galaxy,[2] M51a,[2] NGC 5194,[2] UGC 8493,[2] PGC 47404,[2] VV 001a,[2] VV 403,[2] Arp 85,[2] GC 3572[2]
See also: Galaxy, List of galaxies

The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting[6] grand-design[7] spiral galaxy with a Seyfert 2 active galactic nucleus[8] in the constellation Canes Venatici. Recently it was estimated to be 23 ± 4 million light-years from the Milky Way,[3] but different methods yield distances between 15 and 35 million ly. Messier 51 is one of the best known galaxies in the sky.[9] The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars.[10] The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.

Discovery[edit]

Sketch of M51 by Lord Rosse in 1845.

What later became known as the Whirlpool Galaxy was discovered on October 13, 1773 by Charles Messier while hunting for objects that could confuse comet hunters, and was designated in Messier's catalogue as M51.[11] Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain, although it was not known whether it was interacting or merely another galaxy passing at a distance. It was, however, not until 1845 that William Parsons, 3rd Earl of Rosse, employing a 72 inch (1.8 m) reflecting telescope at Birr Castle, Ireland, found the Whirlpool possessed a spiral structure, the first known "nebula" to be known to have one. These "spiral nebulae" were not recognized as galaxies until Edwin Hubble was able to observe Cepheid variables in some of these spiral nebulae, which provided evidence that these objects were indeed separate galaxies.[12]

The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated that the Whirlpool and its companion galaxy are indeed interacting. Sometimes the designation M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).

Visual appearance[edit]

The image of the Whirlpool Galaxy in visual light (left) and infrared light (right) show two dramatically different face-on views.

Located within the constellation Canes Venatici, M51 is found by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, making it a circumpolar for observers located above 43°N latitude; it reaches high altitudes throughout the northern hemisphere making it an accessible object from the early hours in winter through the end of spring season, after which observation is hindered in lower latitudes.[citation needed]

M51 is visible through binoculars under dark sky conditions and can be resolved in detail with modern amateur telescopes.[10] When seen through a 100 mm telescope the basic outlines of M51 (limited to 5x6') and its companion are visible. Under dark skies, and with a moderate eyepiece through a 150 mm telescope, M51's intrinsic spiral structure can be detected. With larger (>300 mm) instruments under dark sky conditions, the various spiral bands are apparent with HII regions visible, and M51 can be seen to be attached to M51B.

As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance.

In January 2005 the Hubble Heritage Project constructed a 11477x7965 pixel composite image (shown in the info box above) of M51 using Hubble's ACS instrument. The image highlights the galaxy's spiral arms, and shows detail into some of the structures inside the arms.[13]

Properties[edit]

The cross within the nucleus of M51 indicating two dust rings around the black hole at the centre of the nebula.

With the recent SN 2005cs derived estimate of 23 Mly distance, and an angular diameter of roughly 11.2′, it can be inferred that M51's bright circular disk has a radius of about 43,000 light-years. Its mass is estimated to be 160 billion solar masses.

A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral nebula. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring.[14]

Spiral structure[edit]

The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy NGC 5195; specifically, its passing through the main disk of M51 about 500 to 600 million years ago. In this model,[15] NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 million years ago until it is where we observe it to be now, slightly behind M51.

Star formation[edit]

Stars are usually formed in the center of the galaxy. The center part of M51 appears to be undergoing a period of enhanced star formation. The present efficiency of star formation, defined as the ratio of mass of new stars to the mass of star-forming gas, is only ~1%, quite comparable to the global value for the Milky Way and other galaxies. It is estimated that the current high rate of star formation can last no more than another 100 million years or so. [16]

Induced spiral structure in the larger galaxy is not the only effect of the interaction. Significant compression of hydrogen gas occurs that leads to the development of starbirth regions. In pictures of M51 these show up as the bright blue 'knots' throughout the spiral arms.

Generally speaking, hydrogen gas is the most common component of the interstellar medium (the vast space between stars and planetary systems in galaxies). It exists primarily in its atomic structure and molecular form, and forms huge clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression (density) waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas; these are dust lanes one so often sees in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further collapse under its own gravitational pull occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.[17]

When this happens, these new-born stars consume huge amounts of gas causing them to expand, shine even hotter, and finally sweep away the surrounding layers of dust and gas by increasing efflux of the stellar wind. The gigantic proportions of the clouds out of which they are born means stars seldom, if ever, are created in isolation. Thus regions of several hot young stars emit sufficient light energy that they can be seen in the high resolution pictures of M51 across millions of lightyears distance.

For an example of such a formation in our own galaxy, see M16, the Eagle Nebula.

Transient events[edit]

In 2005 a supernova (SN 2005cs) was observed in the Whirlpool Galaxy, peaking at apparent magnitude14.[18][19]

On 31 May 2011 a type II supernova, was detected in the Whirlpool Galaxy, peaking at magnitude 12.1.[20] This supernova, designated SN 2011dh, showed a spectrum much bluer than average, with P Cygni like characteristics in its hydrogen-Balmer lines.[21] Interestingly, the progenitor was probably a yellow supergiant[22] and not a red or blue supergiant, which is unusual.

Companion[edit]

Main article: NGC 5195
Wide field photograph of M51 photographed with amateur astrophotography equipment.

Galaxy group information[edit]

Main article: M51 Group

The Whirlpool Galaxy is the brightest galaxy in the M51 Group, a small group of galaxies that also includes M63 (the Sunflower Galaxy), NGC 5023, and NGC 5229.[23][24][25][26] This small group may actually be a subclump at the southeast end of a large, elongated group that includes the M101 Group and the NGC 5866 Group, although most group identification methods and catalogs identify the three groups as separate entities.[27]

See also[edit]

References[edit]

  1. ^ Dreyer, J. L. E. (1988). Sinnott, R. W., ed. The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters. Sky Publishing Corporation/Cambridge University Press. ISBN 0-933346-51-4. 
  2. ^ a b c d e f g h i j k l m n o "NASA/IPAC Extragalactic Database". Results for NGC 5194. Retrieved 2006-12-06. 
  3. ^ a b Takáts, K.; Vinkó, J. (2006). "Distance estimate and progenitor characteristics of SN 2005cs in M51". Monthly Notices of the Royal Astronomical Society 372 (4): 1735. arXiv:astro-ph/0608430. Bibcode:2006MNRAS.372.1735T. doi:10.1111/j.1365-2966.2006.10974.x. 
  4. ^ "Distance Results for MESSIER 051". NASA/IPAC Extragalactic Database. Retrieved 2011-06-06. 
  5. ^ "M51". SEDS.org. 
  6. ^ a b Arp, H. (1966). "Atlas of Peculiar Galaxies". Astrophysical Journal Supplement 14: 1–20. Bibcode:1966ApJS...14....1A. doi:10.1086/190147. 
  7. ^ Elmegreen, D. M.; Elmegreen, B. G. (1987). "Arm classifications for spiral galaxies". Astrophysical Journal 314: 3–9. Bibcode:1987ApJ...314....3E. doi:10.1086/165034. 
  8. ^ Matsushita, Satoki; Muller, Sebastien; Lim, Jeremy (9 April 2007). Jet-disturbed molecular gas near the Seyfert 2 nucleus in M51 (A&A Letters Special Issue). arXiv:0704.0947. Bibcode:2007A&A...468L..49M. doi:10.1051/0004-6361:20067039. Retrieved 16 February 2014. 
  9. ^ Scalzi, John (2003). The Rough Guide to the Universe. Rough Guides. p. 250. ISBN 1-85828-939-4. 
  10. ^ a b Nemiroff, Robert; Bonnell, Jerry (2000-07-24). "M51: The Whirlpool Galaxy". Astronomy Picture of the Day. NASA. Retrieved 2007-04-22. 
  11. ^ Messier, Charles (1781). "Catalogue des Nébuleuses & des amas d'Étoiles". Connaissance des Temps [1784]. pp. 227–267 [246]. 
  12. ^ Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31". Astrophysical Journal 69: 103–158. Bibcode:1929ApJ....69..103H. doi:10.1086/143167. 
  13. ^ "Out of This Whirl: the Whirlpool Galaxy (M51) and Companion Galaxy". News Center. HubbleSite. April 25, 2005. Retrieved August 7, 2006. 
  14. ^ "NASA's Hubble Space Telescope Resolves a Dark "x" Across the Nucleus of M51". News Center. HubbleSite. June 8, 1992. Retrieved August 7, 2006. 
  15. ^ Salo, Heikki; Laurikainen, Eija (1999). "A Multiple Encounter Model of M51". Astrophysics and Space Science 269: 663–664. Bibcode:1999Ap&SS.269..663S. doi:10.1023/A:1017002909665. 
  16. ^ Thronson, Harley A.; Greenhouse, Matthew A. (1988). "Near-Infrared Mass-to-light ratios in Galaxies: Stellar Mass and Star Formation in the Heart of the Whirlpool". The Astrophysical Journal 327: 671–679. Bibcode:1988ApJ...327..671T. doi:10.1086/166224. 
  17. ^ Kaler, James B (2001). The Little Book of Stars. New York, NY: Copernicus Books. p. 21. ISBN 0-387-95005-2. 
  18. ^ MacRobert, Alan M. (August 24, 2005). "Supernova in M51". Sky Tonight. Sky and Telescope. Retrieved August 7, 2006. 
  19. ^ Bishop, David. "Supernova 2005cs in M51". supernovae.net. Retrieved August 7, 2006. 
  20. ^ Bishop, David. "Supernovae 2011dh in M51". supernovae.net (International Supernovae Network). Retrieved 2011-06-06. 
  21. ^ Kinne (kqr), Richard (2011-06-03). "AAVSO Special Notice #241: New Supernova in M51". AAVSO. Retrieved 2011-06-06. 
  22. ^ "ATEL 3401: Properties of the Candidate Progenitor of SN 2011dh in M51". Astronomers Telegram. 2011-06-03. Retrieved 2011-06-06. 
  23. ^ Tully, R. B. (1988). Nearby Galaxies Catalog. Cambridge University Press. ISBN 0-521-35299-1. 
  24. ^ Fouque, P.; Gourgoulhon, E.; Chamaraux, P.; Paturel, G. (1992). "Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members". Astronomy and Astrophysics Supplement 93: 211–233. Bibcode:1992A&AS...93..211F. 
  25. ^ Garcia, A. (1993). "General study of group membership. II - Determination of nearby groups". Astronomy and Astrophysics Supplement 100: 47–90. Bibcode:1993A&AS..100...47G. 
  26. ^ Giuricin, G.; Marinoni, C.; Ceriani, L.; Pisani, A. (2000). "Nearby Optical Galaxies: Selection of the Sample and Identification of Groups". Astrophysical Journal 543 (1): 178–194. arXiv:astro-ph/0001140. Bibcode:2000ApJ...543..178G. doi:10.1086/317070. 
  27. ^ Ferrarese, L.; Ford, H. C.; Huchra, J.; Kennicutt Jr., R. C.; Mould, J. R.; Sakai, S.; Freedman, W. L.; Stetson, P. B.; Madore, B. F.; Gibson, B. K.; Graham, J. A.; Hughes, S. M.; Illingworth, G. D.; Kelson, D. D.; Macri, L.; Sebo, K.; Silbermann, N. A. (2000). "A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations". Astrophysical Journal Supplement 128 (2): 431–459. arXiv:astro-ph/9910501. Bibcode:2000ApJS..128..431F. doi:10.1086/313391. 

External links[edit]

Coordinates: Sky map 13h 29m 52.7s, +47° 11′ 43″