Fine-tuning in the rotation curves of spirals and the uniqueness of their mass modelling

Abstract

We construct a generic mass model of a disk galaxy by combining the usual two mass components, an exponential thin stellar disk and a dark halo for which we alternatively assume one of the two most common mass distributions. We construct its generic rotation curve, with two free parameters linked to its shape and one to its amplitude, namely the halo characteristic scale lenght, the fractional amount of Dark Matter and the total mass at one disk scale lenght. We find that there is no need of a fine-tuning in the above parameters to produce smooth, featureless rotation curves like the ones actually observed. The rotation curves obtained from different mass distributions are significanlty affected by the variation of the parameters, and show substantial difference in their slope; however, they do not feature irregularities marking the transition from a disk-dominated to a halo-dominated regime. On the other hand we show, by direct analysis of a large number of plausible and different mass models, that an observed rotation curve fulfilling well defined quality requirements can be uniquely and properly decomposed in its dark and luminous components; through a suitable mass modelling together with a maximum-likelihood fitting method, in fact, the disentangling of the mass components is accomplished with a very high resolution.