Direct measurements of bar pattern speeds
  The dynamics of a barred galaxy depends on the angular velocity or pattern
speed of its bar. Indeed, it is related to the location of corotation where
gravitational and centrifugal forces cancel out in the rest frame of the bar.
The only direct method for measuring the bar pattern speed is the
Tremaine-Weinberg technique. This method is best suited to the analysis of the
distribution and kinematics of the stellar component in absence of significant
star formation and patchy dust obscuration. Therefore, it has been mostly used
for early-type barred galaxies. The main sources of uncertainties on the
directly-measured bar pattern speeds are discussed. There are attempts to
overcome the selection bias of the current sample of direct measurements by
extending the application of the Tremaine-Weinberg method to the gaseous
component. Furthermore, there is a variety of indirect methods which are based
on the analysis of the gas distribution and kinematics. They have been largely
used to measure the bar pattern speed in late-type barred galaxies. Nearly all
the bars measured with direct and indirect methods end close to their
corotation radius, i.e., they are as rapidly rotating as they can be.
