Density waves can in fact be a rather efficient tool to constrain the mass distribution in galaxies. By examining the gas response to the potential of a tumbling bar, Weiner et al. [18, 19] studied the pattern speed p and M / L of the bar in NGC 4123 (Fig. 4). The presence of strong shocks is clearly revealed in their Fabry-Perot H maps and this significantly narrows down the range of allowed values for p and M / L. The study of the warp in NGC 5055 Battaglia et al.  was not as successful, as it does not seem to bring any strong constraint on the presence of dark matter in the outer part of that galaxy. This contrasts with the use of outer HI rings in early-type galaxies, as emphasised by Franx, van Gorkom & de Zeeuw  who showed that the gravitational potential is nearly circular in the plane of the observed ring, and that there is a significant increase of M / L with radius. A new case, namely the early-type disc galaxy NGC 2974, is now under study by Weijmans et al. using both HI data for the outer ring and integral-field spectroscopic SAURON data for the inner ionised gaseous component.
Figure 4. Gas response for different models of NGC 4123: these models are used to constrain the pattern speed of the bar as well as the mass-to-light ratio. Extracted from Weiner et al. (see  and  for details).