Thick disk
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.[1] It is supposed to dominate the stellar number density between 1 to 5 kiloparsecs (3.3 to 16.3 kly) above the Galactic plane[1] 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.[2][3] 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.[4]
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.[5]
- More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii.[6][7]
- It is a result of a merger event between the Milky Way and another dwarf galaxy.[2]
- 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.[8]
- The disc forms thick at high redshift with the thin disc forming later [9]
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,[10] its nature is still under dispute.
The view of the thick disc as a single separate component has been questioned by a series of papers that describe the Galactic disc with a continuous spectrum of components with different thicknesses.[11][7][12]
See also
- Thin Disk
- Galaxy formation and evolution
- Galactic coordinate system
- Galactic bulge
- Disc galaxy
- Spiral arm
- Galactic halo
References
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- Thin and Thick Galactic Disks
- Structure and Evolution of the Milky Way
- Populations & Components of the Milky Way
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