Thick disk

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Edge on view of the Milky Way with several structures indicated (not to scale). The thick disk is shown in light yellow. Milky-way-edge-on.pdf
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]

Contents

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]

Origin

It was shown that there is a diversity of thick disc formation scenarios. [8] In general, various scenarios for the formation of this structure have been proposed, including:

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, [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]

See also

Galaxy parts

Related Research Articles

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.

<span class="mw-page-title-main">Star formation</span> Process by which dense regions of molecular clouds in interstellar space collapse to form stars

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

<span class="mw-page-title-main">Galactic Center</span> Rotational center of the Milky Way galaxy

The Galactic Center is the rotational center and the barycenter of the Milky Way. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.

<span class="mw-page-title-main">Galactic bulge</span> Tightly packed group of stars within a larger formation

In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

<span class="mw-page-title-main">Galactic disc</span> Component of disc galaxies comprising gas and stars

A galactic disc is a component of disc galaxies, such as spiral galaxies, lenticular galaxies, and the Milky Way. Galactic discs consist of a stellar component and a gaseous component. The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc. The Milky Way's disc, for example, is approximately 1 kly thick, but thickness can vary for discs in other galaxies.

<span class="mw-page-title-main">Satellite galaxy</span> Galaxy that orbits a larger galaxy due to gravitational attraction

A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group.

<span class="mw-page-title-main">Galaxy merger</span> Merger whereby at least two galaxies collide

Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.

Stacy McGaugh is an American astronomer and professor in the Department of Astronomy at Case Western Reserve University in Cleveland, Ohio. His fields of specialty include low surface brightness galaxies, galaxy formation and evolution, tests of dark matter and alternative hypotheses, and measurements of cosmological parameters.

In astronomy, the Sagittarius Stream is a long, complex structure made of stars that wrap around the Milky Way galaxy in an orbit that nearly crosses the galactic poles. It consists of tidally stripped stars from the Sagittarius Dwarf Elliptical Galaxy, resulting from the process of merging with the Milky Way over a period of billions of years.

The metallicity distribution function is an important concept in stellar and galactic evolution. It is a curve of what proportion of stars have a particular metallicity of a population of stars such as in a cluster or galaxy.

<span class="mw-page-title-main">Bahcall–Wolf cusp</span>

Bahcall–Wolf cusp refers to a particular distribution of stars around a massive black hole at the center of a galaxy or globular cluster. If the nucleus containing the black hole is sufficiently old, exchange of orbital energy between stars drives their distribution toward a characteristic form, such that the density of stars, ρ, varies with distance from the black hole, r, as

A stellar halo is the component of a galaxy's galactic halo that contains stars. The stellar halo extends far outside a galaxy's brightest regions and typically contains its oldest and most metal poor stars.

<span class="mw-page-title-main">NGC 708</span> Galaxy in the constellation Andromeda

NGC 708 is an elliptical galaxy located 240 million light-years away in the constellation Andromeda and was discovered by astronomer William Herschel on September 21, 1786. It is classified as a cD galaxy and is the brightest member of Abell 262. NGC 708 is a weak FR I radio galaxy and is also classified as a type 2 Seyfert galaxy.

<span class="mw-page-title-main">NGC 4318</span> Galaxy in the constellation Virgo

NGC 4318 is a small lenticular galaxy located about 72 million light-years away in the constellation Virgo. It was discovered by astronomer John Herschel on January 18, 1828. NGC 4318 is a member of the Virgo W′ group, a group of galaxies in the background of the Virgo Cluster that is centered on the giant elliptical galaxy NGC 4365.

<span class="mw-page-title-main">(α/Fe) versus (Fe/H) diagram</span> Graph used in astrophysics

The [α/Fe] versus [Fe/H] diagram refers to the graph, commonly used in stellar and galactic astrophysics. It shows the logarithmic ratio number densities of diagnostic elements in stellar atmospheres compared to the solar value. The x-axis represents the abundance of iron (Fe) vs. hydrogen (H), that is, [Fe/H]. The y-axis represents the combination of one or several of the alpha process elements compared to iron (Fe), denoted as [α/Fe].

CEERS-2112 is the most distant barred spiral galaxy observed as of 2023. The galaxy was observed when the universe was 2.1 billion years old and it has a mass similar to the Milky Way at the instant of the life of the universe.

References

  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. 1 2 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. 1 2 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.
  8. ↑ Kasparova, A. (2016). "The Diversity of Thick Galactic Discs". Monthly Notices of the Royal Astronomical Society: Letters . 460 (1): 89–93. arXiv: 1604.07624 . Bibcode:2016MNRAS.460L..89K. doi:10.1093/mnrasl/slw083.
  9. ↑ Villalobos, Álvaro; Helmi, Amina (2008-12-01). "Simulations of minor mergers - I. General properties of thick discs". Monthly Notices of the Royal Astronomical Society. 391 (4): 1806–1827. arXiv: 0803.2323 . Bibcode:2008MNRAS.391.1806V. doi:10.1111/j.1365-2966.2008.13979.x. ISSN   0035-8711. S2CID   14881474.
  10. ↑ Steinmetz, M. (2012). "The Galactic thin and thick disk". Astronomische Nachrichten . 333 (5–6): 523–529. arXiv: 1205.6098 . Bibcode:2012AN....333..523S. doi:10.1002/asna.201211698. S2CID   119112515.
  11. ↑ Schoenrich, R.; Binney, J. (2009). "Chemical Evolution with Radial Migration" (PDF). Monthly Notices of the Royal Astronomical Society . 396 (1): 203–222. arXiv: 0809.3006 . Bibcode:2009MNRAS.396..203S. doi:10.1111/j.1365-2966.2009.14750.x. S2CID   14749516.
  12. ↑ Loebman, S.; et al. (2011). "The Genesis of the Milky Way's Thick Disk via Stellar Migration". The Astrophysical Journal . 737 (1): 8. arXiv: 1009.5997 . Bibcode:2011ApJ...737....8L. doi:10.1088/0004-637X/737/1/8. S2CID   21097464.
  13. ↑ Brook, C. B; Kawata, D.; Gibson, B. K.; Freeman, K. C. (2004). "The Emergence of the Thick Disk in a CDM Universe". The Astrophysical Journal . 612 (2): 894–899. arXiv: astro-ph/0405306 . Bibcode:2004ApJ...612..894B. doi:10.1086/422709. S2CID   54633942.
  14. ↑ Bournaud, Frédéric; Elmegreen, Bruce G.; Martig, Marie (2009-12-01). "The Thick Disks of Spiral Galaxies as Relics from Gas-rich, Turbulent, Clumpy Disks at High Redshift". The Astrophysical Journal Letters. 707 (1): L1–L5. arXiv: 0910.3677 . Bibcode:2009ApJ...707L...1B. doi:10.1088/0004-637X/707/1/L1. ISSN   0004-637X. S2CID   118528894.
  15. ↑ Minchev, I.; Martig, M.; Streich, D.; Scannapieco, C.; de Jong, R. S.; Steinmetz, M. (2015-04-24). "On the Formation of Galactic Thick Disks". The Astrophysical Journal. 804 (1): L9. arXiv: 1502.06606 . Bibcode:2015ApJ...804L...9M. doi:10.1088/2041-8205/804/1/L9. ISSN   2041-8213. S2CID   59389858.
  16. ↑ Fohlmeister, J. (24 April 2015). "The riddle of galactic thin–thick disk solved". Phys.org . Retrieved 24 May 2015.
  17. ↑ Bournaud, Frédéric; Elmegreen, Bruce G.; Martig, Marie (2009-12-01). "The Thick Disks of Spiral Galaxies as Relics from Gas-rich, Turbulent, Clumpy Disks at High Redshift". The Astrophysical Journal Letters. 707 (1): L1–L5. arXiv: 0910.3677 . Bibcode:2009ApJ...707L...1B. doi:10.1088/0004-637X/707/1/L1. ISSN   0004-637X. S2CID   118528894.
  18. ↑ Clarke, A.; Debattista, V. P.; Nidever, D. (2019-04-01). "The imprint of clump formation at high redshift - I. A disc alpha-abundance dichotomy". Monthly Notices of the Royal Astronomical Society. 484 (3): 3476–3490. arXiv: 1901.00931 . Bibcode:2019MNRAS.484.3476C. doi:10.1093/mnras/stz104. ISSN   0004-637X.
  19. ↑ Beraldo e Silva, L.; Debattista, V. P.; Khachaturyants, T. (2020-03-01). "Geometric properties of galactic discs with clumpy episodes". Monthly Notices of the Royal Astronomical Society. 492 (4): 4716–4726. arXiv: 1911.03717 . Bibcode:2020MNRAS.492.4716B. doi:10.1093/mnras/staa065. ISSN   0004-637X.
  20. ↑ Yoachim, P.; Dalcanton, J. (2012). "Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies". The Astronomical Journal . 131 (1): 226–249. arXiv: astro-ph/0508460 . Bibcode:2006AJ....131..226Y. doi:10.1086/497970. S2CID   2219155.
  21. ↑ Bovy, Jo; Rix, Hans-Walter; Liu, Chao; Hogg, David W.; Beers, Timothy C.; Lee, Young Sun (2012-07-01). "The Spatial Structure of Mono-abundance Sub-populations of the Milky Way Disk". The Astrophysical Journal. 753 (2): 148. arXiv: 1111.1724 . Bibcode:2012ApJ...753..148B. doi:10.1088/0004-637X/753/2/148. ISSN   0004-637X. S2CID   118530729.
  22. ↑ Bovy, J.; Rix, H.- W.; Hogg, D. W. (2012). "The Milky Way Has No Distinct Thick Disk". The Astrophysical Journal . 751 (2): 131. arXiv: 1111.6585 . Bibcode:2012ApJ...751..131B. doi:10.1088/0004-637X/751/2/131. S2CID   119299930.
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