Thethick disk is one of the structural components of about 2/3 of all diskgalaxies, including theMilky 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 thethin disk and thehalo.^[2]It is thought to dominate the stellar number density between1 and 5kiloparsecs (3.3 and 16.3 kly) above the galactic plane^[2] and, in thesolar neighborhood, is composed almost exclusively of older stars. For theMilky Way, the thick disk has ascale height of around0.6–1.1kiloparsecs (2.0–3.6 kly) in the axis perpendicular to the disk, which is 3-4 times larger than thethin disk, and a scale length of around1.9–2.3kiloparsecs (6.2–7.5 kly) in the horizontal axis, in the direction of the radius.^[3]Itsstellar chemistry andstellar kinematics (composition and motion of its stars) also set it apart from thethin disk.^[4] Compared to the thin disk, thick disk stars typically have significantly lower levels ofmetals—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 understandinggalaxy formation.

With the availability of observations at larger distances away from the Sun, it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all distances from the galactic centre. Instead, it is metal-poor inside the solar radius, but becomes more metal-rich outside it.^[6] Additionally, 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]

Various scenarios for the formation of this structure have been proposed,^[8] including:

• Thick disks come from the heating of thethin disk^[9][10]
• It is a result of a merger event between the Milky Way and a massivedwarf galaxy^[11]
• More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii^[12][13]
• The disk forms thick at high redshift with the thin disk forming later^[14][15]

• Disk flaring combined with inside-out disk formation^[16][17]
• Scattering by massive clumps:

Stars born in massive gas clumps tend to be scattered into a thick disc, and to be enriched inalpha process elements

Stars formed outside of such clumps tend to settle into a thin disc, and are alpha-element poor^[15][18][19]

Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and is thought to be a common component ofdisk 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]

• Galactic coordinate system
• Galaxy formation and evolution

Galaxy parts

• Galactic bulge
• Spiral arm
• Thin disk

• Thin and Thick Galactic Disks
• Structure and Evolution of the Milky Way
• Populations & Components of the Milky Way

• Galaxies

• Articles with short description
• Short description is different from Wikidata