Agalactic disc(orgalactic disk) is a component ofdisc galaxies,such asspiral galaxieslike theMilky Wayandlenticular 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 kly thick,[1]but thickness can vary for discs in other galaxies.
Stellar component
editExponential surface brightness profiles
editGalactic discs have surface brightness profiles that very closely followexponential functionsin both the radial and vertical directions.
Radial profile
editThe surface brightnessradial profileof 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],}](https://wikimedia.org/api/rest_v1/media/math/render/svg/25641b2ace80b7760417c50d3320310377d14928)
whereis the galaxy's central brightness andis the scale length.[2]The scale length is the radius at which the galaxy is a factor ofe(≈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.[3][4]Some galaxies have been found to have discs with profiles that become truncated in the outermost regions.[5]
Vertical profile
editWhen 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],}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1dffa65ab4bb219de0bc9a92e349c044916afdc6)
whereis the scale height.[6]Although exponential profiles serve as a useful first approximations, vertical surface brightness profiles can also be more complicated. For example, the scale height,although assumed to be a constant above, can in some cases increase with the radius.[7]
Gaseous component
editMost 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 (from21cm emission) that atomic hydrogen is distributed fairly uniformly throughout the disc.[8]21 cm emission by HI also reveals that the gaseous component can flare out at the outer regions of the galaxy.[9]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 (seeTully–Fisher relation).[10]This relationship becomes stronger when the stellar mass is also taken into consideration.[11]
Structure of the Milky Way disc
editThree stellar components with varying scale heights can be distinguished within the disc of the Milky Way (MW): theyoung thin disc,theold thin disc,and thethick disc.[12]Theyoung thin discis 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. Theold thin dischas a scale height of approximately 325 pc while thethick dischas 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.[13]The metal-rich stars in the thin disc have metallicities close to that of the sun () and are referred to as population I (pop I) stars while the stars that populate the thick disc are more metal-poor () and are referred to as population II (pop II) stars (seestellar population). These distinct ages and metallicities in the different stellar components of the disc point to a strong relationship between the metallicities and ages of stars.[14]
See also
editReferences
edit- ^"Scale".Archivedfrom the original on 2023-03-06.Retrieved2021-11-30.
- ^Sparke, Linda Siobhan;Gallagher, John S. (2007).Galaxies in the universe: an introduction(2nd ed.). Cambridge: Cambridge University Press. p. 199.ISBN978-0521855938.OCLC74967110.
- ^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(1): 618–628.arXiv:0907.4884.Bibcode:2009ApJ...704..618T.doi:10.1088/0004-637X/704/1/618.S2CID16368604.
- ^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.ISSN0004-6361.S2CID5400689.Archivedfrom the original on 2020-08-31.Retrieved2018-06-14.
- ^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.ISSN0004-6256.S2CID6433626.
- ^Sparke & Gallagher (2007),pp. 201–202.
- ^de Grijs, R.; Peletier, R. F. (1997-02-25). "The shape of galaxy disks: how the scale height increases with galactocentric distance".Astronomy and Astrophysics.320.arXiv:astro-ph/9702215.Bibcode:1997A&A...320L..21D.
- ^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.ISSN0004-6256.S2CID13975982.
- ^Wouterloot, J. G. A.; Brand, J.; Burton, W. B.; Kwee, K. K. (1990). "IRAS sources beyond the solar circle. II – Distribution in the Galactic warp".Astronomy and Astrophysics.230:21.Bibcode:1990A&A...230...21W.ISSN0004-6361.
- ^Tully, R. B.; Fisher, J. R. (1977). "A new method of determining distances to galaxies".Astronomy and Astrophysics.54:105.Bibcode:1977A&A....54..661T.ISSN0004-6361.
- ^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.ISSN0004-6256.S2CID38472632.
- ^Schneider, P. (2006).Extragalactic astronomy and cosmology: an introduction.Berlin: Springer. p. 55.ISBN9783540331759.OCLC262687285.
- ^Schneider, P. (2006).Extragalactic astronomy and cosmology: an introduction.Berlin: Springer. p. 56.ISBN9783540331759.OCLC262687285.
- ^Schneider, P. (2006).Extragalactic astronomy and cosmology: an introduction.Berlin: Springer. p. 58.ISBN9783540331759.OCLC262687285.
