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We use Bayesian statistics and Markov chain Monte Carlo (MCMC) techniques toconstruct dynamical models for the spiral galaxy NGC 6503. The constraintsinclude surface brightness profiles which display a Freeman Type II structure;HI and ionized gas rotation curves; the stellar rotation, which is nearlycoincident with the ionized gas curve; and the line of sight stellardispersion, with a sigma-drop at the centre. The galaxy models consist of aSersic bulge, an exponential disc with an optional inner truncation and acosmologically motivated dark halo. The Bayesian/MCMC technique yields thejoint posterior probability distribution function for the input parameters. Weexamine several interpretations of the data: the Type II surface brightnessprofile may be due to dust extinction, to an inner truncated disc or to a ringof bright stars; and we test separate fits to the gas and stellar rotationcurves to determine if the gas traces the gravitational potential. We test eachof these scenarios for bar stability, ruling out dust extinction. We also findthat the gas likely does not trace the gravitational potential, since thepredicted stellar rotation curve is then inconsistent with the observed stellarrotation curve. The disc is well fit by an inner-truncated profile, but thepossibility of ring formation by a bar to reproduce the Type II profile is alsoa realistic model. We further find that the halo must have a cuspy profile withgamma > 1; the bulge has a lower M/L than the disc, suggesting a star formingcomponent in the centre of the galaxy; and the bulge, as expected for this latetype galaxy, has a low Sersic index with n_b~1-2, suggesting a formationhistory dominated by secular evolution.

Dynamical Models for NGC 6503 using a Markov Chain Monte Carlo Technique