1995A&A...296...45S

Measurement of pattern speeds has been an observational imperative in the last years since a clear determination of the corotation resonance radius will give some insight in the nature of density waves and in the mechanism of their maintenance. In this paper we analyse in detail the practical application to the grand design spiral NGC 4321 of two different methods, intended to derive the pattern speed ({OMEGA}_p) of its wave-based structure. The first method, based on the change of sign of the radial streaming motions beyond the corotation circle, is an observational diagnostic discussed by Canzian (1993ApJ...414..487C). The final morphology of the residual velocity field after substraction of the rotation curve from the observed HI velocity field is analyzed in order to find the signature of two different morphological regions delimited by the corotation, as predicted by the theory. The first successful application of this test to a real galaxy is presented. We have estimated a value for {OMEGA}_p∼20km/s/kpc that locates corotation in the middle of the disc (R_CR=8-11kpc). The only restriction for a more accurate determination comes from the intrinsic nature of the galaxy. The central bar potential distorts the spiral arm structure and the corotation is not a narrow, but a relatively extended region. The second method, involving numerical simulations of the molecular cloud hydrodynamics, and supplying the behaviour of the neutral gas in the central parts of the galaxy, complete the observational information. This technique consists of a model fitting of the spiral gas response, the free parameter being {OMEGA}_p. The best fit solution is also obtained for {OMEGA}_p∼20km/s/kpc. From this estimate we definitely validate the picture where the stellar bar ends within the corotation and the outer spiral structure, stretching out from the bar, lies outside the corotation circle. The reassuring agreement in the solutions obtained using the two methods allows us to be optimistic about the applicability of the Canzian test as a method to detect the presence of density waves in the discs of "real" galaxies and to derive their pattern speeds with reasonable accuracy.