# Galactic disc

A galactic disc is a component of disc galaxies, such as spiral galaxies and lenticular galaxies. Galactic discs consist of a stellar component ( composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). 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 (very little vertical motion). The Milky Way's disc, for example is approximately 1 kpc thick but thickness can vary for discs in other galaxies.

The Sculptor Galaxy (NGC 253) is an example of a disc galaxy

## Stellar Component

### Exponential Surface Brightness Profiles

Galactic discs have surface brightness profiles that very closely follow exponential functions in both the radial and vertical directions.

The surface brightness radial profile of the galactic disc of a typical disc galaxy (viewed face-on) roughly follows an exponential function:

${\displaystyle I(R)=I_{0}\exp \left[{-{\frac {R}{h_{R}}}}\right]}$

Where ${\displaystyle I_{0}}$ is the galaxy's central brightness and ${\displaystyle h_{R}}$ is the scale length.[1] The scale length is the radius at which the galaxy is a factor of e (~2.7) less bright than it is at its center. Due to the diversity in the shapes and sizes of galaxies, not all galactic discs follow this simple exponential form in their brightness profiles.[2][3] Some galaxies have been found to have discs with profiles that become truncated in the outermost regions.[4]

#### Vertical profile

When viewed edge-on, the vertical surface brightness profiles of galactic discs follow a very similar exponential profile that is proportional to the disc's radial profile:

${\displaystyle I(R,z)=I(R)\exp \left[-{\frac {\vert z\vert }{h_{z}}}\right]=I_{0}\exp \left[-\left({\frac {R}{h_{R}}}+{\frac {\vert z\vert }{h_{z}}}\right)\right]}$

Where the scale height ${\displaystyle h_{z}\approx 0.1h_{R}}$.[5] Although exponential profiles serve as a useful first approximations, vertical surface brightness profiles can also be more complicated. For example, the scale height ${\displaystyle h_{z}}$, although assumed to be a constant above, can in some cases increase with the radius.[6]

## Gaseous Component

Most of a disc galaxy's gas lies within the disc. Both cool atomic hydrogen (HI) and warm molecular hydrogen (HII) make up most of the disc's gaseous component. This gas serves as the fuel for the formation of new stars in the disc. Although the distribution of gas in the disc is not as well-defined as the stellar component's distribution it is understood (from 21cm emission) that atomic hydrogen is distributed fairly uniformly throughout the disc.[7] 21 cm emission by HI also reveals that the gaseous component can flare out at the outer regions of the galaxy.[8] The abundance of molecular hydrogen makes it a great candidate to help trace the dynamics within the disc. Like the stars within the disc, clumps or clouds of gas follow approximately circular orbits about the galactic center. The circular velocity of the gas in the disc is strongly correlated with the luminosity of the galaxy (see Tully-Fisher Relation).[9] This relationship becomes stronger when the stellar mass is also taken into consideration.[10]

## Structure of the Milky Way Disc

Three stellar components with varying scale heights can be distinguished within the disc of the Milky Way (MW): the young thin disc, the old thin disc, and the thick disc.[11] The young thin disc is a region in which star formation is taking place and contains the MW's youngest stars and most of its gas and dust. The scale height of this component is roughly 100 pc. The old thin disc has a scale height of approximately 325 pc while the thick disc has a scale height of 1.5 kpc. Although stars move primarily within the disc, they exhibit a random enough motion in the direction perpendicular to the disc to result in various scale heights for the different disc components. Stars in the MW's thin disc tend to have higher metallicities compared to the stars in the thick disc.[12] The metal-rich stars in the thin disc have metallicities close to that of the sun (${\displaystyle Z\approx 0.02}$) and are referred to as population I (pop I) stars while the stars that populate the thick disc are more metal-poor (${\displaystyle Z\approx 0.001}$) and are referred to as population II (pop II) stars (see stellar population). These distinct ages and metallicities in the different stellar components of the disc points to a strong relationship between the metallicities and ages of stars.[13]

## References

1. ^ Siobhan., Sparke, Linda (2007). Galaxies in the universe : an introduction. Gallagher, John S. (John Sill), 1947- (2nd ed.). Cambridge: Cambridge University Press. p. 199. ISBN 978-0521855938. OCLC 74967110.
2. ^ Trujillo, Ignacio; Martinez-Valpuesta, Inma; Martínez-Delgado, David; Peñarrubia, Jorge; Gabany, R. Jay; Pohlen, Michael (2009). "UNVEILING THE NATURE OF M94's (NGC4736) OUTER REGION: A PANCHROMATIC PERSPECTIVE". The Astrophysical Journal. 704: 618–628. doi:10.1088/0004-637X/704/1/618.
3. ^ Pohlen, M.; Trujillo, I. (2006-07-17). "The structure of galactic disks". Astronomy & Astrophysics. 454 (3): 759–772. arXiv:astro-ph/0603682. Bibcode:2006A&A...454..759P. doi:10.1051/0004-6361:20064883. ISSN 0004-6361.
4. ^ Erwin, Peter; Pohlen, Michael; Beckman, John E. (2008-01-01). "The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies". The Astronomical Journal. 135 (1): 20–54. arXiv:0709.3505. Bibcode:2008AJ....135...20E. doi:10.1088/0004-6256/135/1/20. ISSN 0004-6256.
5. ^ Siobhan., Sparke, Linda (2007). Galaxies in the universe : an introduction. Gallagher, John S. (John Sill), 1947- (2nd ed.). Cambridge: Cambridge University Press. pp. 201–202. ISBN 978-0521855938. OCLC 74967110.
6. ^ de Grijs, R.; Peletier, R. F. (1997-02-25). "The shape of galaxy disks: how the scale height increases with galactocentric distance". arXiv:astro-ph/9702215.
7. ^ Leroy, Adam K.; Walter, Fabian; Brinks, Elias; Bigiel, Frank; de Blok, W. J. G.; Madore, Barry; Thornley, M. D. (2008-11-19). "THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY". The Astronomical Journal. 136 (6): 2782–2845. arXiv:0810.2556. Bibcode:2008AJ....136.2782L. doi:10.1088/0004-6256/136/6/2782. ISSN 0004-6256.
8. ^ A., Wouterloot, J. G.; J., Brand; B., Burton, W.; K., Kwee, K. (1990). "IRAS sources beyond the solar circle. II - Distribution in the Galactic warp". Astronomy and Astrophysics. 230: 21. Bibcode:1990A&A...230...21W. ISSN 0004-6361.
9. ^ B., Tully, R.; R., Fisher, J. (1977). "A new method of determining distances to galaxies". Astronomy and Astrophysics. 54: 105. Bibcode:1977A&A....54..661T. ISSN 0004-6361.
10. ^ McGaugh, Stacy S. (2012-01-12). "THE BARYONIC TULLY-FISHER RELATION OF GAS-RICH GALAXIES AS A TEST OF ΛCDM AND MOND". The Astronomical Journal. 143 (2): 40. arXiv:1107.2934. Bibcode:2012AJ....143...40M. doi:10.1088/0004-6256/143/2/40. ISSN 0004-6256.
11. ^ 1958-, Schneider, P. (Peter) (2006). Extragalactic astronomy and cosmology : an introduction. Berlin: Springer. p. 55. ISBN 9783540331759. OCLC 262687285.
12. ^ 1958-, Schneider, P. (Peter) (2006). Extragalactic astronomy and cosmology : an introduction. Berlin: Springer. p. 56. ISBN 9783540331759. OCLC 262687285.
13. ^ 1958-, Schneider, P. (Peter) (2006). Extragalactic astronomy and cosmology : an introduction. Berlin: Springer. p. 58. ISBN 9783540331759. OCLC 262687285.
14 Boötis

14 Boötis is a binary star system in the northern constellation of Boötes. Its apparent magnitude is 5.53.

51 Arietis

51 Arietis (abbreviated 51 Ari) is a star in the northern constellation of Aries. 51 Arietis is the Flamsteed designation. It has an apparent visual magnitude of 6.6 and is an estimated 67.3 light-years (20.6 parsecs) distant from the Earth. This is a G-type main sequence star with a stellar classification of G8 V. Similar to the Sun, it has 1.04 times the mass and 0.99 times the radius. It radiates 92% of the Sun's luminosity from its outer atmosphere at an effective temperature of 5,666 K. This heat gives it the golden-hued glow of a G-type star.

Cassiopeia (constellation)

Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is easily recognizable due to its distinctive 'W' shape, formed by five bright stars. It is opposite Ursa Major.

In northern locations above latitude 34ºN it is visible year-round and in the (sub)tropics it can be seen at its clearest from September to early November. Even in low southern latitudes below 25ºS it can be seen low in the North.

At magnitude 2.2, Alpha Cassiopeiae, or Schedar, is generally the brightest star in Cassiopeia, though is often shaded by Gamma Cassiopeiae, which has brightened to magnitude 1.6 on occasion. The constellation hosts some of the most luminous stars known, including the yellow hypergiants Rho Cassiopeiae and V509 Cassiopeiae and white hypergiant 6 Cassiopeiae. The semiregular variable PZ Cassiopeiae is one of the largest known stars. In 1572, Tycho Brahe's supernova flared brightly in Cassiopeia. Cassiopeia A is a supernova remnant and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. Fourteen star systems have been found to have exoplanets, one of which—HR 8832—is thought to host seven planets. A rich section of the Milky Way runs through Cassiopeia, containing a number of open clusters, young luminous galactic disc stars, and nebulae. IC 10 is an irregular galaxy that is the closest known starburst galaxy and the only one in the Local Group of galaxies.

Dark matter halo

A dark matter halo is a theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy. The halo's mass dominates the total mass. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution. The dark matter halo is not fully explained by the presence of massive compact halo objects (MACHOs).

EQ Virginis

EQ Virginis is a flare star in the constellation of Virgo. It is an orange dwarf of spectral type K5Ve and is a member of a IC 2391 moving group. The moving group of stars is between 30 and 50 million years old.

Einstein ring

In observational astronomy an Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring, is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational lensing of the source's light by an object with an extremely large mass (such as another galaxy or a black hole). This occurs when the source, lens, and observer are all aligned—a syzygy. The first complete Einstein ring, designated B1938+666, was discovered by collaboration between astronomers at the University of Manchester and NASA's Hubble Space Telescope in 1998.

Epsilon Equulei

Epsilon Equulei, Latinized from ε Equulei, is a star system of apparent magnitude +5.23 in the constellation of Equuleus. It is located 180 light years away from the Solar System, based on its parallax.

Galactic corona

The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way. A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.

This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernova remnants expand vertically through galactic chimneys into the halo. As the gas cools, it is pulled back into the galactic disc of the galaxy by gravitational forces.

Galactic coronas have been and are currently being studied extensively, in the hope of gaining a further understanding of galaxy formation. Although, considering how galaxies differ in shaping and sizing, no particular theory has been able to adequately illustrate how the galaxies in the Universe originally formed.

Gerard F. Gilmore

Gerard Francis Gilmore FRS FRAS FInstP (born 7 November 1951) is Professor of Experimental Philosophy, in the Institute of Astronomy, at the University of Cambridge. His research has centred on studying stars in the Galaxy to understand its structure and evolutionary history.

HD 131040

HD 131040 is a double star in the northern constellation of Boötes. The brighter component is a subgiant star that varies slightly in luminosity by 0.04 in magnitude. The magnitude 9.64 companion lies at an angular separation of 15.2″ along a position angle of 93°.

HD 156331

HD 156331 is double star in the southern constellation of Ara. As of 2014, the pair have an angular separation of less than an arc second along a position angle of 49°.

HD 211575

HD 211575 is a star in the constellation Aquarius in between "Gamma Aquarii", "Pi Aquarii" and "Sadalmelik". It is a member of the corona of the Ursa Major moving group.

List of unsolved problems in astronomy

Some of the unsolved problems in astronomy are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Some unresolved questions in astronomy pertain to one-off events, unusual occurrences that have not repeated and whose causes therefore remain unclear.

Monoceros Ring

The Monoceros Ring is a long, complex, ringlike filament of stars that wraps around the Milky Way three times. This is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy by tidal forces as part of the process of merging with the Milky Way over a period of billions of years, although this view has long been disputed. The ring contains 100 million solar masses and is 200,000 light years long.

NGC 612

NGC 612 is a lenticular galaxy in the constellation of Sculptor located approximately 388 million light-years from Earth. It is a type II Seyfert galaxy and thus has an active galactic nucleus. NGC 612 has been identified as an extremely rare example of a non-elliptical radio galaxy, hosting one of the nearest powerful FR-II radio sources.

NGC 7013

NGC 7013 is a relatively nearby spiral or lenticular galaxy estimated to be around 37 to 41.4 million light-years away from Earth in the constellation of Cygnus. NGC 7013 was discovered by English astronomer William Herschel on July 17, 1784 and was also observed by his son, astronomer John Herschel on September 15, 1828.

NGC 8

NGC 8 is an asterism of two completely unrelated stars (spectral types K6I and G4) in the constellation Pegasus, discovered on 29 September 1865 by Otto Wilhelm von Struve. It is approximately 2.7 arc minutes away from NGC 9.The two stars are completely unrelated to each other, with the whiter, dimmer star being at a distance of 10400+4400−2400 light years, and the yellower, brighter star having a minimum distance of 215,000 light years. While both stars are technically outside of the milky way's galactic disc, the nearer is 6400±2100 light-years south of the 1,000-light-year-thick disc, and the further is not only at least 130,000 light-years south of the disk, but is located entirely outside of the Milky Way itself, being at least 220,000 light-years from the galactic core.

Spiral galaxy

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has likely changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope.Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.

V528 Carinae

V528 Carinae (V528 Car, HD 95950, HIP 54021) is a variable star in the constellation Carina.

V528 Carinae has an apparent visual magnitude of +6.75. It is a distant star but the exact distance is uncertain. The Hipparcos satellite gives a negative annual parallax and is not helpful. Its Carina OB2 membership allows the distance to be estimated at 3,850 light-years.V528 Carinae is a red supergiant of spectral type M2 Ib with an effective temperature of 3,700 K. It has a radius of 700 solar radii, making it one of the largest stars. In the visible spectrum luminosity is 11,900 times higher than the sun, but the bolometric luminosity considering all wavelengths reaches 81,000 L☉. It loses mass at 0.5×10−9 M☉ per year.It is classified as a slow irregular variable whose prototype is TZ Cassiopeiae.

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Energetic galaxies
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