Apparent maximum rotation velocity of the gas: vmaxg

Observed maximum rotation velocity uncorrected for inclination effect. This quantity is calculated from 21-cm line widths at different levels and/or rotation curves (generally in H_alpha). It is expressed in km/s. The data are regularly maintained and the methods of homogenization are regular ly revisited in order to account for the evolution of measurements. In 1982 we collected HI measurements (21-cm line width, HI flux or HI radial velocity) for 1210 galaxies (Bottinelli, Gouguenheim, Paturel, 1982) and for 6439 galaxies in 1990 (Bottinelli et al., 1990).
Today, we have measurements for 16666 galaxies.
Many new measurements of rotation velocity obtained from rotation curves are available now in the literature. This gives us a new way to convert directly the observed 21-cm line widths into the true rotation velocity.

The last compilation provides us with 50520 measurements of 21-cm line widths or maximum rotation velocity. These data are characterized by some secondary parameters: telescope, velocity resolution, level of the 21-cm line width and bibliographic reference.

For data homogenization we use the following EPIDEMIC METHOD (Paturel, G. et al.; 2003, A&A 415,57):
we start from a standard sample (a set of measurements giving a large and homogeneous sample). All other measurements are grouped into homogeneous classes (for instance, the class of measurements made at a given level and obtained with a given resolution). The most populated class is cross-identified with the standard sample in order to establish the equation of conversion to the standard system. Then, the whole class is incorporated into the standard sample. So, the standard sample is growing progressively. The conversion to the standard propagates like an epidemy.

Actually, the order of inclusion of a new class is dictated by the quantity t= sigma/ sqrt{n}, where sigma is the standard deviation of a preliminar comparison of each class with the standard sample and n the number of measurements in common. Using t, the classes are sorted following the best compromise between quality (sigma) and quantity (n). References having no intersection with the standard sample during the prelimina ry comparison will be included after all others, the order of inclusion being simply given by their total number of measurements, the richest, the first included.

This kind of analysis allows us to convert directly the widths for a given resolution r and given level l into a quantity which is homogeneous to twice the maximum rotation velocity, uncorrected for inclination. A final correction is applied reference by reference to improve the homogenization.
See also vmaxs the observed maximum rotation velocity of the stars, and vdis the stellar velocity dispersion. The observed rotation is used to derive the physical maximum velocity rotation vrot by correcting for inclination.

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