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HyperLEDA documentation: parameter: vdis

Central stellar velocity dispersion for stellar systems

The central velocity dispersion represents the internal motions of the stars along a column crossing the center of a galaxy or star cluster. It may be derived from spectra integrated over the line of sight (typically for a stellar population which is not resolved into individual stars) or from velocity measurements of individual stars (in star clusters or nearby galaxies which are resolved into stars) or planetary nebulae. The measured velocity dispersion corresponds to a certain defined region of the object and, after systematic corrections applied to series of measurements, it is reduced to a common standard aperture (for galaxies only).
The catalogue of collected velocity dispersions may be accessed in HyperLeda.
vdis is the velocity dispersion given in km/s.

The velocity dispersion gives to some approximation the scale for the specific kinetic energy contained in in un-ordered motions and combined with the velocity of rotation it may give an estimate of the specific kinetic energy. For early-type galaxies (E-S0) it is one of the parameters entering in the Fundamental-Plane relation, which reflects the self-similarity of all early-type galaxies and their equilibrium status.

The velocity dispersions are standardized to an aperture rnorm of 0.595 h-1kpc, which is equivalent to an angular diameter of 3.4 arcsec at the distance of Coma. The method of normalization is the one introduced by Jorgensen et al:

vdisc = vdis . [rmeas/rnorm]0.04 ,

where rmeas is the aperture defined by the slit width at the distance of the object. All raw vdis values are rescaled to the homogeneous system and thus corrected for systematic effects on the individual datasets (see Golev & Prugniel, 1998). At the end of 2003, the total available catalog contains 8596 measurements of vdis for 3636 galaxies

This parameter is used to determine the distance modulus mod0. 

HyperLeda Questions: prugniel@obs.univ-lyon1.fr