Velocity dispersion

The velocity dispersion is a statistical measure of the distribution of the residual velocities of particles towards a bonded system. In astronomy , velocity dispersion is used to characterize galaxies : spiral galaxies have a small dispersion of 10 to 30 km / s, elliptical galaxies have a dispersion up to 400 km / s. ${\displaystyle \sigma }$

calculation

The calculation is based on the assumption that the spectrum of the galaxy can be determined from the spectrum of a single star, provided that the luminosity- weighted mean of the entire galaxy is considered. In addition, the distribution of the velocities along the line of sight should follow a Gaussian distribution . Then the spectrum of the single star is shifted until the absorption lines match those of the galaxy. The radial velocity of the single star can then be used to deduce that of the galaxy. The spectrum of the single star is folded with a Gaussian function of the width . The value of with the best match gives the velocity dispersion. ${\displaystyle \sigma }$ ${\displaystyle \sigma }$

Importance in astronomy

The existence and mass of the central black hole can be deduced from the velocity dispersion of the stars in the center of the Milky Way . The relatively high speed of rotation in the outer spiral arms with small dispersion provides an indication of dark matter (see rotation curve ).

If a group of stars of the same type has the same velocity dispersion as other stars in a star cluster or a galaxy, conclusions can be drawn about the physical affiliation of this star group to the star cluster or galaxy - e.g. B. used for the assignment of brown dwarfs .

swell

• Instructions for calculating the velocity dispersion
• H.-Röser, W. Tscharnuter, Abriss der Astronomie, Weinheim 2012, p. 495
• KL Luhman, in: Annual Review of Astronomy and Astrophysics, 2012, Palo Alto 2012, pp. 65f

See also

• Faber-Jackson relation : relationship between velocity dispersion and luminosity of a galaxy