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The thick disk is one of the structural components of several galaxies, including the Milky Way. It was first proposed as a unique galactic structure, different from the thin disk and the halo in an 1983 article by Gilmore & Reid.〔Gilmore & Reid, 1983, "New light on faint stars. III - Galactic structure towards the South Pole and the Galactic thick disc", ()〕 It is supposed to dominate the stellar number density between above the Galactic plane〔 and is composed almost exclusively of older stars. Its chemical composition and kinematics (those of the stars comprising it) are also said to set it apart from the thin disk.〔Bensby & Feltzing, "The Galactic thin and thick discs in the context of galaxy formation", ()〕〔Kordopatis et al., "A spectroscopic survey of thick disc stars outside the solar neighbourhood", ()〕 Stars within the thick disk may be called old-disk stars. Compared to stars in the thin disk, these typically have a significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium. 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. ==Origin== Various scenarios for the formation of this structure have been proposed, including: * Thick disks come from the heating of the thin disk.〔Matthias Steinmetz, ''The Galactic thin and thick disk'' (2012), (p. 4 )"〕 * More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii.〔 R. Schoenrich & J. Binney, ''Chemical Evolution with Radial Migration'' (2009), ()"〕 〔 S. Loebman et al. ''The Genesis of the Milky Way's Thick Disk via Stellar Migration'' (2011), ()"〕 * It is a result of a merger event between the Milky Way and another dwarf galaxy. *Multiple small galactic mergers disturb stars from the thin disk so that progressively older stars are scattered out further, and even more so, far from the galactic center. * The disc forms thick at high redshift with the thin disc forming later 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thick disk」の詳細全文を読む スポンサード リンク
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