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.