Monoceros Ring

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The Milky Way galaxy.

The Monoceros Ring(monoceros: Greek for 'unicorn') is a long, complex, ring of stars that wraps around the Milky Way three times. This is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy by tidal forces as part of the process of merging with the Milky Way over a period of billions of years, although this view has long been disputed. [1] The ring contains 100 million solar masses and is 200,000 light years long. [2]

Contents

The stream of stars was first reported in 2002 by astronomers conducting the Sloan Digital Sky Survey. In the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, they discovered the Canis Major Dwarf Galaxy. [3]

Dispute

In 2006, a study using 2MASS data cast doubts on the nature of the "Ring", arguing that the data suggests that the ring is actually part of the warped galactic disc of the Milky Way. [1] However, observations using the Anglo-Australian Telescope published in 2007 suggest that a warped disc cannot create the observed structure, which must therefore be formed either by a flare of the galactic disc or have an extra-galactic origin. [4]

Several members of the scientific community restated their position in 2012, affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way. [5]

In 2015, building on the 2002 studies which revealed the Monoceros Ring, M.L. Martialay and colleagues sorted through galactic data from the Sloan Digital Sky Survey suggested that the Milky Way is actually 50 percent larger than previously thought, [6] showing that disk of the Milky Way is not just a disk of stars in a flat plane, but is instead corrugated. As it radiates outward from the Sun, there appear to be at least four ripples in the disk of the Milky Way. Scientists assume that this pattern is going to be found throughout the disk. [7] However, a more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of their constituent RR Lyrae variable stars found to be higher and consistent with halo membership. [8]

Using the distance of the Monoceros Ring, the diameter of the Milky Way has been claimed to extend as much as 150,000 to 180,000 light years across. In this revised paradigm, the Solar System lies about halfway between the core and the edge. [9] However, it is more common in the astronomical literature to define the sizes of galaxies using other methods, notably by the D25 isophote and variations of the half-light radius. An earlier study in 1998 using the D25 isophote gave a diameter for the Milky Way at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,590 ly). [10] [11] [12]

N-body simulations have been used to investigate the possible location of the progenitor of this structure and these calculations show that, if the Ring has a dwarf galaxy progenitor, it might be found in the background of one out of eight specific areas in the sky. [13] A subsequent analysis, that used Gaia DR2 data, found a bimodal Gaussian distribution towards Galactic coordinates (271, +2) degrees in Vela, which is one of the locations of the progenitor proposed in the previous study. [14] This finding may signal the presence of the progenitor of the Monoceros Ring, but the authors indicate that it might also be compatible with the existence of an unrelated kinematically coherent structure.

See also

Related Research Articles

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Local Group</span> Group of galaxies that includes the Milky Way

The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×1019 kilometres), and a total mass of the order of 2×1012 solar masses (4×1042 kg). It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellites form one lobe, and the Andromeda Galaxy and its satellites constitute the other. The two collections are separated by about 800 kiloparsecs (3×10^6 ly; 2×1019 km) and are moving toward one another with a velocity of 123 km/s. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.

<span class="mw-page-title-main">Large Magellanic Cloud</span> Magellanic spiral galaxy that is a satellite of the Milky Way in the constellation Dorado

The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (c. 16 kiloparsecs (52,000 light-years) away) and the possible dwarf irregular galaxy called the Canis Major Overdensity. Based on the D25 isophote at the B-band (445 nm wavelength of light), the Large Magellanic Cloud is about 9.86 kiloparsecs (32,200 light-years) across. It is roughly one-hundredth the mass of the Milky Way and is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33).

<span class="mw-page-title-main">Andromeda Galaxy</span> Barred spiral galaxy in the Local Group

The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a diameter of about 46.56 kiloparsecs and is approximately 765 kpc from Earth. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

<span class="mw-page-title-main">Galactic bulge</span> Tightly packed group of stars within a larger formation

In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

<span class="mw-page-title-main">Sagittarius Dwarf Spheroidal Galaxy</span> Satellite galaxy of the Milky Way

The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy, is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in its main body, with the brightest of them—NGC 6715 (M54)—being known well before the discovery of the galaxy itself in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way. In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it moved past the Milky Way between 300 and 900 million years ago.

<span class="mw-page-title-main">Messier 32</span> Dwarf elliptical galaxy in the constellation Andromeda

Messier 32 is a dwarf "early-type" galaxy about 2,650,000 light-years (810,000 pc) from the Solar System, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil in 1749.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

<span class="mw-page-title-main">Messier 79</span> Globular cluster in constellation Lepus

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The Canis Major Overdensity or Canis Major Dwarf Galaxy is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

<span class="mw-page-title-main">Galactic disc</span> Component of disc galaxies comprising gas and stars

A galactic disc is a component of disc galaxies, such as spiral galaxies, lenticular galaxies, and the Milky Way. Galactic discs consist of a stellar component and a gaseous component. The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc. The Milky Way's disc, for example, is approximately 1 kly thick, but thickness can vary for discs in other galaxies.

<span class="mw-page-title-main">Milky Way</span> Galaxy containing the Solar System

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<span class="mw-page-title-main">Satellite galaxy</span> Galaxy that orbits a larger galaxy due to gravitational attraction

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<span class="mw-page-title-main">Virgo Stellar Stream</span> Stellar stream in the constellatiion Virgo discovered in 2005

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<span class="mw-page-title-main">Stellar kinematics</span> Study of the movement of stars

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References

  1. 1 2 Y. Momany; et al. (2006). "Outer structure of the Galactic warp and flare: explaining the Canis Major over-density" (PDF). Astronomy and Astrophysics . 451 (2): 515–38. arXiv: astro-ph/0603385 . Bibcode:2006A&A...451..515M. doi:10.1051/0004-6361:20054081. S2CID   18547813. Archived from the original (PDF) on 2016-03-04. Retrieved 2013-12-21.
  2. ↑ Ibata, Rodrigo; Gibson, Brad (April 2007). "The Ghosts of Galaxies Past". Scientific American. 296 (4): 40–45. Bibcode:2007SciAm.296d..40I. doi:10.1038/scientificamerican0407-40. PMID   17479629.
  3. ↑ Newberg; et al. (1 April 2002). "The Ghost of Sagittarius and Lumps in the Halo of the Milky Way". The Astrophysical Journal. 569 (1): 245–274. arXiv: astro-ph/0111095 . Bibcode:2002ApJ...569..245N. doi:10.1086/338983. S2CID   16909562.
  4. ↑ Conn, Blair C.; Lane, Richard R.; Lewis, Geraint F.; Gil-Merino, Rodrigo; Irwin, Mike J.; Ibata, Rodrigo A.; Martin, Nicolas F.; Bellazzini, Michele; Sharp, Robert; Tuntsov, Artem V.; Ferguson, Annette M. N. (April 1, 2007). "The AAT/WFI survey of the Monoceros Ring and Canis Major dwarf galaxy". Monthly Notices of the Royal Astronomical Society. 376 (3): 939–959. arXiv: astro-ph/0701664 . Bibcode:2007MNRAS.376..939C. doi:10.1111/j.1365-2966.2007.11503.x. S2CID   17927154.
  5. ↑ M. Lopez-Corredoira, A; et al. (July 2012). "Comments on the "Monoceros" affair". arXiv: 1207.2749 . Bibcode:2012arXiv1207.2749L.{{cite journal}}: Cite journal requires |journal= (help)
  6. ↑ Mary L. Martialay (March 11, 2015). "The Corrugated Galaxy—Milky Way May Be Much Larger Than Previously Estimated" (Press release). Rensselaer Polytechnic Institute. Archived from the original on March 13, 2015.
  7. ↑ Scott Sutherland (March 16, 2015). "This 'corrugated' view of the Milky Way just made our home galaxy a LOT bigger". The Weather Network. Archived from the original on March 19, 2015.
  8. ↑ Sheffield, Allyson A.; Price-Whelan, Adrian M.; Tzanidakis, Anastasios; Johnston, Kathryn V.; Laporte, Chervin F. P.; Sesar, Branimir (2018). "A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities". The Astrophysical Journal. 854 (1): 47. arXiv: 1801.01171 . Bibcode:2018ApJ...854...47S. doi: 10.3847/1538-4357/aaa4b6 . S2CID   118932403.
  9. ↑ Yan Xu; et al. (March 1, 2015). "Rings and Radial Waves in the Disk of the Milky Way". The Astrophysical Journal . 801 (2): 105. arXiv: 1503.00257 . Bibcode:2015ApJ...801..105X. doi:10.1088/0004-637X/801/2/105. S2CID   119124338.
  10. ↑ Goodwin, S. P.; Gribbin, J.; Hendry, M. A. (22 April 1997). "The Milky Way is just an average spiral". arXiv: astro-ph/9704216 .
  11. ↑ Goodwin, S. P.; Gribbin, J.; Hendry, M. A. (August 1998). "The relative size of the Milky Way". The Observatory. 118: 201–208. Bibcode:1998Obs...118..201G.
  12. ↑ Castro-Rodríguez, N.; López-Corredoira, M.; Sánchez-Saavedra, M. L.; Battaner, E. (2002). "Warps and correlations with intrinsic parameters of galaxies in the visible and radio". Astronomy & Astrophysics. 391 (2): 519–530. arXiv: astro-ph/0205553 . Bibcode:2002A&A...391..519C. doi:10.1051/0004-6361:20020895. S2CID   17813024.
  13. ↑ Magda Guglielmo; et al. (March 11, 2018). "On the origin of the Monoceros Ring - I. Kinematics, proper motions, and the nature of the progenitor". Monthly Notices of the Royal Astronomical Society . 474 (4): 4584–4593. arXiv: 1711.06682 . Bibcode:2018MNRAS.474.4584G. doi:10.1093/mnras/stx3048.
  14. ↑ de la Fuente Marcos, Raúl; de la Fuente Marcos, Carlos (21 November 2018). "Searching for the lost Unicorn: a prominent feature in the radial velocity distribution of stars in Vela from Gaia DR2 data". Monthly Notices of the Royal Astronomical Society Letters. 481 (1): L64–L68. arXiv: 1808.09921 . Bibcode:2018MNRAS.481L..64D. doi:10.1093/mnrasl/sly163.
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