{{short description|Structural component of some galaxies}} 260px|thumb|Edge on view of the Milky Way with several structures indicated (not to scale). The thick disk is shown in light yellow. A '''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.<ref>{{Cite journal|last=Burstein|first=D.|date=1979-12-01|title=Structure and origin of S0 galaxies. III - The luminosity distribution perpendicular to the plane of the disks in S0's|journal=The Astrophysical Journal|volume=234|pages=829–836|doi=10.1086/157563|issn=0004-637X|bibcode = 1979ApJ...234..829B }}</ref> Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo.<ref name="GILMORE"> {{cite journal |last1=Gilmore |first1=G. |last2=Reid |first2=N. |year=1983 |title=New light on faint stars. III - Galactic structure towards the South Pole and the Galactic thick disc |journal=Monthly Notices of the Royal Astronomical Society |volume=202 |issue=4 |pages=1025 |bibcode=1983MNRAS.202.1025G |doi=10.1093/mnras/202.4.1025 |doi-access=free }}</ref> It is thought to dominate the stellar number density between {{nowrap|{{convert|1|and|5|kpc|lk=on}}}} above the galactic plane<ref name="GILMORE"/> and, in the solar neighborhood, is composed almost exclusively of older stars. For the Milky Way, the thick disk has a scale height of around {{Nowrap|{{convert|0.6-1.1|kpc|lk=on}}}} in the axis perpendicular to the disk, which is 3-4 times larger than the thin disk, and a scale length of around {{Nowrap|{{convert|1.9-2.3|kpc|lk=on}}}} in the horizontal axis, in the direction of the radius.<ref name="Tkachenko"> {{cite journal |last1= Tkachenko |first1=R. |display-authors=etal |year=2025 |title=Determining the Scale Length and Height of the Milky Way's Thick Disc Using RR Lyrae |journal=Universe |volume=11 |issue=4 |pages=132 |doi=10.3390/universe11040132 |doi-access=free |arxiv=2505.00780 |bibcode=2025Univ...11..132T }}</ref> Its stellar chemistry and stellar kinematics (composition and motion of its stars) also set it apart from the thin disk.<ref> {{cite journal |last1=Kordopatis |first1=G. |display-authors=etal |year=2011 |title=A spectroscopic survey of thick disc stars outside the solar neighbourhood |journal=Astronomy & Astrophysics |volume=535 |pages=A107 |arxiv=1110.5221 |bibcode=2011A&A...535A.107K |doi=10.1051/0004-6361/201117373 |s2cid=118616147 }}</ref> 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.<ref name="freeman2010"> {{cite book | last1=Freeman | first1=K. C. | title=Galaxies and their Masks | year=2010 | chapter=The HERMES Project: Reconstructing Galaxy Formation | editor1-first=D. L. | editor1-last=Block | editor2-first=K. C. | editor2-last=Freeman | editor3-first=I. | editor3-last=Puerari | page=319 | publisher=Springer | bibcode=2010gama.conf..319F | doi=10.1007/978-1-4419-7317-7_27 | isbn=978-1-4419-7316-0 }}</ref>
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, 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.<ref>{{Cite journal|last1=Bensby|first1=T.|last2=Alves-Brito|first2=A.|last3=Oey|first3=M. S.|last4=Yong|first4=D.|last5=Meléndez|first5=J.|date=2011-07-01|title=A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc|journal=The Astrophysical Journal Letters|volume=735|issue=2|pages=L46|doi=10.1088/2041-8205/735/2/L46|issn=0004-637X|arxiv = 1106.1914 |bibcode = 2011ApJ...735L..46B |s2cid=119266731}}</ref> 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.<ref>{{Cite journal|last1=Martig|first1=Marie|last2=Minchev|first2=Ivan|last3=Ness|first3=Melissa|last4=Fouesneau|first4=Morgan|last5=Rix|first5=Hans-Walter|date=2016-11-01|title=A Radial Age Gradient in the Geometrically Thick Disk of the Milky Way|journal=The Astrophysical Journal|volume=831|issue=2|pages=139|doi=10.3847/0004-637X/831/2/139|issn=0004-637X|arxiv = 1609.01168 |bibcode = 2016ApJ...831..139M |s2cid=54652169 |doi-access=free }}</ref>
==Origin== Various scenarios for the formation of this structure have been proposed,<ref> {{cite journal |last=Kasparova |first=A. |year=2016 |title=The Diversity of Thick Galactic Discs |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=460 |issue=1 |pages=89–93 |arxiv=1604.07624 |bibcode=2016MNRAS.460L..89K |doi=10.1093/mnrasl/slw083|doi-access=free}}</ref> including: * Thick disks come from the heating of the thin disk<ref>{{Cite journal|last1=Villalobos|first1=Álvaro|last2=Helmi|first2=Amina|date=2008-12-01|title=Simulations of minor mergers - I. General properties of thick discs|journal=Monthly Notices of the Royal Astronomical Society|volume=391|issue=4|pages=1806–1827|doi=10.1111/j.1365-2966.2008.13979.x|doi-access=free |issn=0035-8711|bibcode = 2008MNRAS.391.1806V |arxiv=0803.2323|s2cid=14881474}}</ref><ref>{{cite journal |last=Steinmetz |first=M. |year=2012 |title=The Galactic thin and thick disk |journal=Astronomische Nachrichten |volume=333 |issue=5–6 |pages=523–529 |arxiv=1205.6098 |bibcode=2012AN....333..523S |doi=10.1002/asna.201211698 |s2cid=119112515}}</ref> * It is a result of a merger event between the Milky Way and a dwarf galaxy<ref name=Bensby> {{cite conference |last1=Bensby |first1=T. |last2=Feltzing |first2=F.|author2-link= Sofia Feltzing |year=2009 |title=The galactic thin and thick discs in the context of galaxy formation |conference=IAU Symposium |volume=265 |pages=300–303 |arxiv=0908.3807 |bibcode=2010IAUS..265..300B |doi=10.1017/S1743921310000773 |s2cid=18562648 }} </ref> * More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii<ref> {{cite journal |last1=Schoenrich |first1=R. |last2=Binney |first2=J. |year=2009 |title=Chemical evolution with radial migration |journal=Monthly Notices of the Royal Astronomical Society |volume=396 |issue=1 |pages=203–222 |arxiv=0809.3006 |bibcode=2009MNRAS.396..203S |doi=10.1111/j.1365-2966.2009.14750.x |doi-access=free |s2cid=14749516 |url=http://mnras.oxfordjournals.org/content/396/1/203.full.pdf }} </ref><ref name=Loebman> {{cite journal |first1=S.R. |last1=Loebman |first2=R. |last2=Roškar |first3=V.P. |last3=Debattista |first4=Ž. |last4=Ivezić |first5=T.R. |last5=Quinn |first6=J. |last6=Wadsley |year=2011 |title=The genesis of the Milky Way's thick disk via stellar migration |journal=The Astrophysical Journal |volume=737 |issue=1 |pages=8 |arxiv=1009.5997 |bibcode=2011ApJ...737....8L |doi=10.1088/0004-637X/737/1/8 |s2cid=21097464 }} </ref> * The disk forms thick at high redshift with the thin disk forming later<ref> {{cite journal |last1=Brook |first1=C.B. |last2=Kawata |first2=D. |last3=Gibson |first3=B.K. |last4=Freeman |first4=K.C. |year=2004 |title=The emergence of the thick disk in a CDM universe |journal=The Astrophysical Journal |volume=612 |issue=2 |pages=894–899 |arxiv=astro-ph/0405306 |bibcode=2004ApJ...612..894B |doi=10.1086/422709 |s2cid=54633942 }} </ref><ref name=Bournd-Elmgrn-Martg-2009> {{cite journal |last1=Bournaud |first1=Frédéric |last2=Elmegreen |first2=Bruce G. |last3=Martig |first3=Marie |date=December 2009 |title=The thick disks of spiral galaxies as relics from gas-rich, turbulent, clumpy disks at high redshift |journal=The Astrophysical Journal Letters |volume=707 |issue=1 |pages=L1–L5 |issn=0004-637X |arxiv=0910.3677 |bibcode=2009ApJ...707L...1B |doi=10.1088/0004-637X/707/1/L1 |s2cid=118528894 }} </ref>
* Disk flaring combined with inside-out disk formation<ref> {{cite journal |last1=Minchev |first1=I. |last2=Martig |first2=M. |last3=Streich |first3=D. |last4=Scannapieco |first4=C. |last5=de Jong |first5=R.S. |last6=Steinmetz |first6=M. |date=2015-04-24 |title=On the formation of galactic thick disks |journal=The Astrophysical Journal |volume=804 |issue=1 |page=L9 |doi=10.1088/2041-8205/804/1/L9 |issn=2041-8213 |arxiv=1502.06606 |bibcode=2015ApJ...804L...9M |s2cid=59389858 }} </ref><ref name=JF15> {{cite news |last=Fohlmeister |first=J. |date=24 April 2015 |title=The riddle of galactic thin–thick disk solved |department=News |website=Phys.org |url=http://phys.org/news/2015-04-riddle-galactic-thinthick-disk.html |access-date=24 May 2015 }}</ref> * Scattering by massive clumps: :: Stars born in massive gas clumps tend to be scattered into a thick disc, and to be enriched in alpha process elements :: Stars formed outside of such clumps tend to settle into a thin disc, and are alpha-element poor<ref name=Bournd-Elmgrn-Martg-2009/><ref> {{cite journal |last1=Clarke |first1=A. |last2=Debattista |first2=V.P. |last3=Nidever |first3=D. |date=April 2019 |title=The imprint of clump formation at high redshift - I. A disc alpha-abundance dichotomy |journal=Monthly Notices of the Royal Astronomical Society |volume=484 |issue=3 |pages=3476–3490 |doi=10.1093/mnras/stz104 |doi-access=free |issn=0004-637X |arxiv=1901.00931 |bibcode=2019MNRAS.484.3476C }} </ref><ref> {{cite journal |last1=Beraldo e Silva |first1=L. |last2=Debattista |first2=V.P. |last3=Khachaturyants |first3=T. |date=March 2020 |title=Geometric properties of galactic discs with clumpy episodes |journal=Monthly Notices of the Royal Astronomical Society |volume=492 |issue=4 |pages=4716–4726 |doi= 10.1093/mnras/staa065 |doi-access=free |issn=0004-637X |arxiv = 1911.03717 |bibcode = 2020MNRAS.492.4716B }} </ref>
==Dispute== Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and is thought to be a common component of disk galaxies in general,<ref> {{cite journal |last1=Yoachim |first1=P. |last2=Dalcanton |first2=J. |year=2012 |title=Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies |journal=The Astronomical Journal |volume=131 |issue=1 |pages=226–249 |arxiv=astro-ph/0508460 |bibcode=2006AJ....131..226Y |doi=10.1086/497970 |s2cid=2219155 }}</ref> 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.<ref>{{Cite journal|last1=Bovy|first1=Jo|last2=Rix|first2=Hans-Walter|last3=Liu|first3=Chao|last4=Hogg|first4=David W.|last5=Beers|first5=Timothy C.|last6=Lee|first6=Young Sun|date=2012-07-01|title=The Spatial Structure of Mono-abundance Sub-populations of the Milky Way Disk|journal=The Astrophysical Journal|volume=753|issue=2|pages=148|doi=10.1088/0004-637X/753/2/148|issn=0004-637X|arxiv = 1111.1724 |bibcode = 2012ApJ...753..148B |s2cid=118530729}}</ref><ref>{{cite journal |last1=Bovy |first1=J. |last2=Rix |first2=H.- W. |last3=Hogg |first3=D. W. |year=2012 |title=The Milky Way Has No Distinct Thick Disk |journal=The Astrophysical Journal |volume=751 |issue=2 |pages=131 |arxiv=1111.6585 |bibcode=2012ApJ...751..131B |doi=10.1088/0004-637X/751/2/131|s2cid=119299930}}</ref>
==See also== * Galactic coordinate system * Galaxy formation and evolution
; Galaxy parts * Galactic bulge * Spiral arm * Thin disk
== References == {{Reflist}}
== External links == * [https://arxiv.org/abs/astro-ph/0310326 Thin and Thick Galactic Disks] * [https://arxiv.org/abs/1108.5028 Structure and Evolution of the Milky Way] * [http://www.ifa.hawaii.edu/~barnes/ast626_95/pcmw.html Populations & Components of the Milky Way]
{{Galaxy}}
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