yields a measure of the asymmetric drift, v$_{a}$ . The latter, defined as the difference between the circular velocity v$_{c}$ calculated from the potential and the streaming velocity v$_{s}$ of the stars, is given by for an exponential disc, where R$_{d}$ is the disc scale length and σ$_{R}$ is the radial velocity dispersion (Binney & Tremaine 2008). When applied to the observed H β rotation curve, Eq. 3 yields an asymmetric drift of order of 5 − 15 km s − $^{1}$, depending on the adopted central radial dispersion. 1 The asymmetric drift is much larger than the observed difference between the gas and stellar curves; a possible cause of the discrepancy is that gas does not trace the gravitational potential. Traditionally, gas is assumed to travel on circular orbits, so that the H β /H I rotation curves would trace the gravitational potential in this galaxy. The assumption of circularity in gas rotation curves underlies much of the work on mass modelling in the literature. However, it is not possible for this assumption to strictly hold because gas dispersion is non-zero. There are several sources of this dispersion including turbulence caused by supernova feedback and the magnetorotational instability (Tamburro et al. 2009). In addition, non-circular motions can impact the gas rotation curves of disc galaxies (e.g., Hayashi et al. 2004, Valenzuela et al. 2007), thereby reducing the observed velocity relative to the actual circular velocity. Being collisionless, our models cannot capture these effects; therefore, the hypothesis that the gas rotation traces the circular velocity can only be tested using the gravitational potential of our models. However, the stellar asymmetric drift can easily be modeled to obtain a separate value for the circular velocity. The stellar asymmetric drift is itself subject to several caveats; it is, for example, highly sensitive to the orientation of the velocity ellipsoid. These issues are addressed in § 7.3. The LOS velocity dispersion profile for NGC 6503 from B89 is found in Fig. 4. There is a σ -drop within 300 pc and an exponential decline beyond that range. We fit the data to an outer exponential with an inverse Gaussian in the centre: σ ( R ) = σ$_{0}$ exp( − R/R$_{σ}$ ) [ 1 − A exp( − R $^{2}$/ (2 B $^{2}$)) ] (4) v$_{a}$ = σ 2 R 2 v$_{c}$ [ 2 R R$_{d}$ + 1 2 ( R v$_{s}$ ∂v$_{s}$ ∂R − 1 )] (3) 2.3. The LOS velocity dispersion $^{1}$B89 claims that the calculated asymmetric drift implied by the rotation curves is ∼ 3 − 5 km s $^{$_{−}$1}$, which is much lower than our calculated values. We have been unable to reproduce this number; every permutation of scalelength and dispersion that we tried in Eq. 3 yields asymmetric drifts of at least ∼ 5 km s $^{$_{−}$1}$. 7