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Velocity dispersion

From Wikipedia, the free encyclopedia
Statistical dispersion

Inastronomy, thevelocity dispersion (σ) is thestatistical dispersion ofvelocities about themean velocity for a group ofastronomical objects, such as anopen cluster,globular cluster,galaxy,galaxy cluster, orsupercluster. By measuring theradial velocities of the group's members throughastronomical spectroscopy, the velocity dispersion of that group can be estimated and used to derive the group's mass from thevirial theorem.[1] Radial velocity is found by measuring theDoppler width ofspectral lines of a collection of objects; the more radial velocities one measures, the more accurately one knows their dispersion. Acentral velocity dispersion refers to the σ of the interior regions of an extended object, such as a galaxy or cluster.

The relationship between velocity dispersion andmatter (or the observedelectromagnetic radiation emitted by this matter) takes several forms – specific correlations – in astronomy based on the object(s) being observed. Notably, theM–σ relation applies for material orbiting manyblack holes, theFaber–Jackson relation forelliptical galaxies, and theTully–Fisher relation forspiral galaxies. For example, theσ found for objects about theMilky Way'ssupermassive black hole (SMBH) is about 100 km/s, which provides an approximation of the mass of this SMBH.[2] TheAndromeda Galaxy (Messier 31) hosts a SMBH about 10 times larger than our own, and has aσ ≈ 160 km/s.[3]

Groups and clusters of galaxies have more disparate (contrasting in degree) velocity dispersions than smaller objects. For example, while our ownpoor group, theLocal Group, has aσ = 61±8 km/s,[4]rich clusters of galaxies, such as theComa Cluster, have aσ ≈ 1,000 km/s.[5] Thedwarf elliptical galaxies within Coma, as with all galaxies, have their own internal velocity dispersion for their stars, which is aσ ≲ 80 km/s, typically.[6] Normalelliptical galaxies, by comparison, have an averageσ ≈ 200 km/s.[7]

Forspiral galaxies, the increase in velocity dispersion inpopulation I stars is a gradual process which likely results from the near-random incidence ofmomentum exchanges, specificallydynamical friction, between individual stars and largeinterstellar media (gas and dust clouds) with masses greater than 105 M☉.[8] Face-on spiral galaxies have a centralσ ≲ 90 km/s; slightly more if viewed edge-on.[9]

See also

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References

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  1. ^Collins Dictionary of Astronomy, 2nd Ed.; Harper Collins Publishers; 2000; pp. 444, 449
  2. ^Tremaine, Scott; Gebhardt, Karl; Bender, Ralf; Bower, Gary; Dressler, Alan; Faber, S. M.; Filippenko, Alexei V.; Green, Richard; Grillmair, Carl; Ho, Luis C.; Kormendy, John; Lauer, Tod R.; Magorrian, John; Pinkney, Jason; Richstone, Douglas (March 2002). "The slope of the black-hole mass versus velocity dispersion correlation".The Astrophysical Journal.574 (1). Chicago, Illinois, USA:740–753.arXiv:astro-ph/0203468.Bibcode:2002ApJ...574..740T.doi:10.1086/341002.S2CID 15482979.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^Gebhardt, Karl; Bender, Ralf; Bower, Gary; Dressler, Alan; Faber, S. M.; Filippenko, Alexei V.; Green, Richard; Grillmair, Carl; Ho, Luis C.; Kormendy, John; Lauer, Tod R.; Magorrian, John; Pinkney, Jason; Richstone, Douglas; Tremaine, Scott (June 2000). "A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion".The Astrophysical Journal.539 (1). Chicago, Illinois, USA: The University of Chicago Press:L13–L16.arXiv:astro-ph/0006289.Bibcode:2000ApJ...539L..13G.doi:10.1086/312840.S2CID 11737403.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^van den Bergh, Sidney (1999). "The local group of galaxies".The Astronomy and Astrophysics Review.9 (3–4). Springer: 273–318 (1999).Bibcode:1999A&ARv...9..273V.doi:10.1007/s001590050019.S2CID 119392899.
  5. ^Struble, Mitchell F.; Rood, Herbert J. (November 1999)."A Compilation of Redshifts and Velocity Dispersions for ACO Clusters".The Astrophysical Journal Supplement Series.125 (1). Chicago, Illinois, USA: The University of Chicago Press:35–71.Bibcode:1999ApJS..125...35S.doi:10.1086/313274.
  6. ^Kourkchi, E.; Khosroshahi, H. G.; Carter, D.; Karick, A. M.; Mármol-Queraltó, E.; Chiboucas, K.; Tully, R. B.; Mobasher, B.; Guzmán, R.; Matković, A.; Gruel, N. (March 2012)."Dwarf galaxies in the Coma cluster – I. Velocity dispersion measurements".Monthly Notices of the Royal Astronomical Society.420 (4). Wiley Online Library:2819–2834.arXiv:1110.2649.Bibcode:2012MNRAS.420.2819K.doi:10.1111/j.1365-2966.2011.19899.x.S2CID 62800506.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^Forbes, Duncan A.; Ponman, Trevor J. (November 1999)."On the relationship between age and dynamics in elliptical galaxies".Monthly Notices of the Royal Astronomical Society.309 (3):623–628.arXiv:astro-ph/9906368.Bibcode:1999MNRAS.309..623F.doi:10.1046/j.1365-8711.1999.02868.x.S2CID 13468195.
  8. ^Spitzer, Lyman Jr.;Schwarzschild, Martin (July 1953)."The Possible Influence of Interstellar Clouds on Stellar Velocities. II".Astrophysical Journal.118: 106.Bibcode:1953ApJ...118..106S.doi:10.1086/145730.
  9. ^Bershady, Matthew A.; Martinsson, Thomas P. K.; Verheijen, Marc A. W.; Westfall, Kyle B.; Andersen, David R.; Swaters, Rob A. (October 2011). "Galaxy Disks are Submaximal".The Astrophysical Journal Letters.739 (2): L47.arXiv:1108.4314.Bibcode:2011ApJ...739L..47B.doi:10.1088/2041-8205/739/2/L47.S2CID 119264732.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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