Thick disk

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 galaxies, including the Milky Way. It was discovered first in external edge-on galaxies.[1] Soon after, it was proposed as a distinct 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]

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]

  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. ^ a b 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. S2CID 18562648.
  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. 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. 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. S2CID 119266731.
  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. S2CID 54652169.