The differential rotation of spiral disks possesses a peculiar characteristic: the rotation velocity 
is a constant except in the innermost regions. This behavior persists at the largest radii sampled by the best rotation curves which means that the edge of the mass distribution in spiral galaxies has not yet been reached. The rotation curves of spiral galaxies are said to be flat. For intermediate redshift galaxies, the rise of the rotation curve in the innermost regions cannot be resolved. Therefore, the simplest velocity field reproducing a flat rotation curve in this case is one in which is constant everywhere, 
is zero, and the direction of rotation is specified by the sign of the projected velocity V
sin 
.