since only a small region of the slit used for observation would have intersected the bar. Part of the σ − drop could be due to the bar, and thus the bulge may not be as large and have as low a density as found in § 6.2. Other bulge parameters are less likely to be affected by the bar; in particular, the conclusion that there is a star forming component in the galaxy centre is unchanged. 

## 8. Conclusion 

We have deployed a Bayesian/Markov chain Monte Carlo technique to model the disc galaxy NGC 6503. We find models for NGC 6503 that satisfy the observational constraints using this technique, and are able to constrain the input parameters. The data includes a Freeman Type II surface brightness profile, ionized gas and H I rotation curves, the stellar rotation curve and the stellar line of sight velocity dispersion. Four different scenarios were considered: an inner truncated disc, an exponential disc with dust, a model in which gas traces the gravitational potential, and a model in which the true underlying disc scale length is revealed in the outermost portion of the surface brightness profile. We find that the second scenario leads to models that strongly bar unstable, and the third scenario cannot reproduce the stellar rotation curve. The first scenario provides the best fit to the data and is most likely to offer the bar stability required to correctly model the galaxy, but the last scenario also provides a realistic model for the galaxy. 

Further properties of the galaxy are discerned; the bulge is a pseudobulge, and the bulge M/L is lower than the disc M/L , suggesting a star forming component that is probably responsible for the σ − drop. We also find that the halo must be cusped with γ ≳ 1, a result that is robust to all fitting methods. The Bayesian/MCMC technique used to discover these results is robust and flexible; we find it to be an effective tool for fitting galaxy models in complex parameter spaces. 

We thank K. Spekkens, P. Teuben, R. Bottema, J. Dubinski, M. Bershady and C. Arsenault for useful and informative discussions. S.C. and L.M.W. wish to acknowledge support through respective Discovery Grants from the Natural Sciences and Engineering Research Council of Canada. 

