Thick disk

The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies.[1] Soon after, it was proposed as a unique galactic structure in the Milky Way, different from the thin disk and the halo in the 1983 article by Gilmore & Reid.[2] It is supposed to dominate the stellar number density between 1 and 5 kiloparsecs (3.3 and 16.3 kly) above the galactic plane[2] and, in the 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]

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 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]

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,[17] 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.[18][19]

See also
Galaxy parts

References
  1. Burstein, D. (1979-12-01). "Structure and origin of S0 galaxies. III - The luminosity distribution perpendicular to the plane of the disks in S0's". The Astrophysical Journal. 234: 829–836. Bibcode:1979ApJ...234..829B. doi:10.1086/157563. ISSN 0004-637X.
  2. Gilmore, G.; Reid, N. (1983). "New light on faint stars. III - Galactic structure towards the South Pole and the Galactic thick disc". Monthly Notices of the Royal Astronomical Society. 202 (4): 1025. Bibcode:1983MNRAS.202.1025G. doi:10.1093/mnras/202.4.1025.
  3. Bensby, T.; Feltzing, F. (2009). "The Galactic thin and thick discs in the context of galaxy formation". Proceedings of the IAU Symposium. 265: 300–303. arXiv:0908.3807. Bibcode:2010IAUS..265..300B. doi:10.1017/S1743921310000773.
  4. Kordopatis, G.; et al. (2011). "A spectroscopic survey of thick disc stars outside the solar neighbourhood". Astronomy & Astrophysics. 535: A107. arXiv:1110.5221. Bibcode:2011A&A...535A.107K. doi:10.1051/0004-6361/201117373.
  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: A Conference in Honour of K.C. Freeman, FRS. Galaxies and Their Masks. Springer. p. 319. Bibcode:2010gama.conf..319F. doi:10.1007/978-1-4419-7317-7_27. ISBN 978-1-4419-7316-0.
  6. Bensby, T.; Alves-Brito, A.; Oey, M. S.; Yong, D.; Meléndez, J. (2011-07-01). "A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc". The Astrophysical Journal Letters. 735 (2): L46. arXiv:1106.1914. Bibcode:2011ApJ...735L..46B. doi:10.1088/2041-8205/735/2/L46. ISSN 0004-637X.
  7. Martig, Marie; Minchev, Ivan; Ness, Melissa; Fouesneau, Morgan; Rix, Hans-Walter (2016-11-01). "A Radial Age Gradient in the Geometrically Thick Disk of the Milky Way". The Astrophysical Journal. 831 (2): 139. arXiv:1609.01168. Bibcode:2016ApJ...831..139M. doi:10.3847/0004-637X/831/2/139. ISSN 0004-637X.
  8. Kasparova, A. (2016). "The Diversity of Thick Galactic Discs". Monthly Notices of the Royal Astronomical Society: Letters. 460 (1): 89–93. arXiv:1604.07624. Bibcode:2016MNRAS.460L..89K. doi:10.1093/mnrasl/slw083.
  9. Villalobos, Álvaro; Helmi, Amina (2008-12-01). "Simulations of minor mergers - I. General properties of thick discs". Monthly Notices of the Royal Astronomical Society. 391 (4): 1806–1827. arXiv:0803.2323. Bibcode:2008MNRAS.391.1806V. doi:10.1111/j.1365-2966.2008.13979.x. ISSN 0035-8711.
  10. Steinmetz, M. (2012). "The Galactic thin and thick disk". Astronomische Nachrichten. 333 (5–6): 523–529. arXiv:1205.6098. Bibcode:2012AN....333..523S. doi:10.1002/asna.201211698.
  11. Schoenrich, R.; Binney, J. (2009). "Chemical Evolution with Radial Migration" (PDF). Monthly Notices of the Royal Astronomical Society. 396 (1): 203–222. arXiv:0809.3006. Bibcode:2009MNRAS.396..203S. doi:10.1111/j.1365-2966.2009.14750.x.
  12. Loebman, S.; et al. (2011). "The Genesis of the Milky Way's Thick Disk via Stellar Migration". The Astrophysical Journal. 737 (1): 8. arXiv:1009.5997. Bibcode:2011ApJ...737....8L. doi:10.1088/0004-637X/737/1/8.
  13. Brook, C. B; Kawata, D.; Gibson, B. K.; Freeman, K. C. (2004). "The Emergence of the Thick Disk in a CDM Universe". The Astrophysical Journal. 612 (2): 894–899. arXiv:astro-ph/0405306. Bibcode:2004ApJ...612..894B. doi:10.1086/422709.
  14. Bournaud, Frédéric; Elmegreen, Bruce G.; Martig, Marie (2009-12-01). "The Thick Disks of Spiral Galaxies as Relics from Gas-rich, Turbulent, Clumpy Disks at High Redshift". The Astrophysical Journal Letters. 707 (1): L1–L5. arXiv:0910.3677. Bibcode:2009ApJ...707L...1B. doi:10.1088/0004-637X/707/1/L1. ISSN 0004-637X.
  15. Minchev, I.; Martig, M.; Streich, D.; Scannapieco, C.; de Jong, R. S.; Steinmetz, M. (2015-04-24). "On the Formation of Galactic Thick Disks". The Astrophysical Journal. 804 (1): L9. arXiv:1502.06606. Bibcode:2015ApJ...804L...9M. doi:10.1088/2041-8205/804/1/L9. ISSN 2041-8213.
  16. Fohlmeister, J. (24 April 2015). "The riddle of galactic thin–thick disk solved". Phys.org. Retrieved 24 May 2015.
  17. Yoachim, P.; Dalcanton, J. (2012). "Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies". The Astronomical Journal. 131 (1): 226–249. arXiv:astro-ph/0508460. Bibcode:2006AJ....131..226Y. doi:10.1086/497970.
  18. Bovy, Jo; Rix, Hans-Walter; Liu, Chao; Hogg, David W.; Beers, Timothy C.; Lee, Young Sun (2012-07-01). "The Spatial Structure of Mono-abundance Sub-populations of the Milky Way Disk". The Astrophysical Journal. 753 (2): 148. arXiv:1111.1724. Bibcode:2012ApJ...753..148B. doi:10.1088/0004-637X/753/2/148. ISSN 0004-637X.
  19. Bovy, J.; Rix, H.- W.; Hogg, D. W. (2012). "The Milky Way Has No Distinct Thick Disk". The Astrophysical Journal. 751 (2): 131. arXiv:1111.6585. Bibcode:2012ApJ...751..131B. doi:10.1088/0004-637X/751/2/131.
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