content_image/354539.jpg
therefore be a relic of a now dissolved bar. In a scenario of ring formation by bar destruction, the outer exponential represents the ‘true’ disc, and the Type II hump is mainly due to the formation of bright stars that do not trace the mass. The results in Foyle et al. (2008) suggest that the Type II profile could be reproduced by the emergence of a bar in a disc of scale length 1.3 kpc. Therefore, it is incumbent to investigate the SB profile by fitting only the points exterior to 2 kpc. 

A third possibility is that Type II profiles are caused by a genuine mass deficit in the inner region, and therefore we also fit the full SB profile directly by assuming an inner truncated disc (Kormendy 1977). This naturally produces a better fit to the hump than to the outer points; along with the first scenario, this scenario provides a direct comparison with the modelling of BG97, who did not account for the outer exponential. 

## 2.2. Rotation curves 

Several rotation curves for NGC 6503 have been measured. H I observations by Begeman (1987) reveal a remarkably flat rotation curve between 3 kpc and 20 kpc deviating by no more than ∼ 4 − 5%, and a maximum circular velocity of ∼ 120 km s − $^{1}$. Greisen et al. (2009) find a similar H I curve in addition to evidence for a thick and thin H I disc in NGC 6503. The thick disc appears to rotate more slowly than the thin disc and the former does not extend beyond the optical radius. However, this distinction does not affect Begeman’s measurements of the total H I rotation curve. B89 maps the inner rotation curve with H β and O III emission as well as the stellar features, while de Vaucouleurs & Caulet (1982) supply H α data. Where they overlap, all data sets are in agreement; the H I observations appear to be slightly but systematically larger than the H β . The stellar rotation curve largely coincides with the H β and O III curves, with an average difference of only ∼ 2 km s − $^{1}$. At about 1.5 kpc, the stellar rotation exceeds the gas rotation, possibly owing to the effect of a spiral arm. For our models, we fit the H I , H β and stellar data sets. These are found in Fig. 3, along with fits based on the fitting formula (Courteau 1997): 

<img src='content_image/354539.jpg'>

where v$_{0}$ is an asymptotic velocity, R$_{c}$ is a projected length scale and ζ is a shape parameter governing the sharpness of the ‘turnover’ to the asymptotic velocity. The fit suggests a projected asymptotic circular velocity of 117 ± 1 km s − 1 and shows how close the two rotation curves are. 

The coincidence of the stellar and gas rotation curves must be accounted for. If the gas rotation traces the gravitational potential, the difference between the two rotation curves 

