Groups and clusters of galaxies have a wider range of velocity dispersions than smaller objects. For example, our own poor group, the Local Group, has a σ = 61±8 km/s.[4] But rich clusters of galaxies, such as the Coma Cluster, have a σ ≈ 1,000 km/s.[5] The dwarf elliptical galaxies within Coma have their own internal velocity dispersion for their stars, which is a σ ≲ 80 km/s, typically.[6] Normal elliptical galaxies, by comparison, have an average σ ≈ 200 km/s.[7]
For spiral galaxies, the increase in velocity dispersion in population I stars is a gradual process which likely results from the random momentum exchanges, known as dynamical friction, between individual stars and large interstellar media (gas and dust clouds) with masses greater than 105M☉.[8] Face-on spiral galaxies have a central σ ≲ 90 km/s; slightly more if viewed edge-on.[9]
^Collins Dictionary of Astronomy, 2nd Ed.; Harper Collins Publishers; 2000; pp. 444, 449
^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. S2CID15482979.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^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. S2CID11737403.{{cite journal}}: CS1 maint: multiple names: authors list (link)
