Thick disk

Source: Wikipedia, the free encyclopedia.
Edge on view of the Milky Way with several structures indicated (not to scale). The thick disk is shown in light yellow.

The thick disk is one of the structural components of about 2/3 of all disk

solar neighborhood, is composed almost exclusively of older stars. Its stellar chemistry and stellar kinematics (composition and motion of it stars) are also said to set it apart from the thin disk.[3][4] Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.[5]

The thick disk is a source of early kinematic and chemical evidence for a galaxy's composition and thus is regarded as a very significant component for understanding galaxy formation.

With the availability of observations at larger distances away from the Sun, more recently it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all galactic radii. It was found instead that it is metal poor inside the solar radius, but becomes more metal rich outside it.[6] Additionally, recent observations have revealed that the average stellar age of thick disk stars quickly decreases as one moves from the inner to the outer disk.[7]

Origin

It was shown that there is a diversity of thick disc formation scenarios.[8] In general, various scenarios for the formation of this structure have been proposed, including:

  • Thick disks come from the heating of the thin disk[9][10]
  • It is a result of a merger event between the Milky Way and a massive dwarf galaxy[3]
  • More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii[11][12]
  • The disk forms thick at high redshift with the thin disk forming later[13][14]
  • Disk flaring combined with inside-out disk formation[15][16]
  • Scattering by massive clumps: stars born in massive gas clumps tend to be scattered to a thick disc and to be enriched in alpha-elements, while those formed out of these clumps form a thin disc and are alpha-poor[17][18][19]

Dispute

Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and it's even thought to be a common component of disk galaxies in general,[20] its nature is still under dispute.

The view of the thick disk as a single separate component has been questioned by a series of papers that describe the galactic disk with a continuous spectrum of components with different thicknesses.[21][22]

See also

Galaxy parts

References

  1. ISSN 0004-637X
    .
  2. ^ a b Gilmore, G.; Reid, N. (1983). "New light on faint stars. III - Galactic structure towards the South Pole and the Galactic thick disc".
    doi:10.1093/mnras/202.4.1025
    .
  3. ^ a b Bensby, T.;
    S2CID 18562648
    .
  4. ^ Kordopatis, G.; et al. (2011). "A spectroscopic survey of thick disc stars outside the solar neighbourhood".
    S2CID 118616147
    .
  5. ^ Freeman, K. C. (2010). "The HERMES Project: Reconstructing Galaxy Formation". In Block, D. L.; Freeman, K. C.; Puerari, I. (eds.). Galaxies and their Masks.
    ISBN 978-1-4419-7316-0
    .
  6. S2CID 119266731
    .
  7. S2CID 54652169
    .
  8. ^ Kasparova, A. (2016). "The Diversity of Thick Galactic Discs".
    doi:10.1093/mnrasl/slw083
    .
  9. S2CID 14881474
    .
  10. ^ Steinmetz, M. (2012). "The Galactic thin and thick disk".
    S2CID 119112515
    .
  11. ^ Schoenrich, R.; Binney, J. (2009). "Chemical Evolution with Radial Migration" (PDF).
    S2CID 14749516
    .
  12. ^ Loebman, S.; et al. (2011). "The Genesis of the Milky Way's Thick Disk via Stellar Migration".
    S2CID 21097464
    .
  13. S2CID 54633942
    .
  14. S2CID 118528894
    .
  15. S2CID 59389858
    .
  16. ^ Fohlmeister, J. (24 April 2015). "The riddle of galactic thin–thick disk solved". Phys.org. Retrieved 24 May 2015.
  17. S2CID 118528894
    .
  18. ISSN 0004-637X
    .
  19. ISSN 0004-637X
    .
  20. ^ Yoachim, P.; Dalcanton, J. (2012). "Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies".
    S2CID 2219155
    .
  21. S2CID 118530729
    .
  22. ^ Bovy, J.; Rix, H.- W.; Hogg, D. W. (2012). "The Milky Way Has No Distinct Thick Disk".
    S2CID 119299930
    .

External links
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