THE MASS-DISTRIBUTION OF THE DWARF SPIRAL NGC-1560

H I synthesis observations with the WSRT and optical surface photometry of the dwarf spiral galaxy NGC 1560 are presented. This galaxy has an absolute luminosity of M(B) = -15.87. The observations show that the galaxy is gas rich, with an M(HI)/L(B) of 2.4. We obtained a very detailed rotation curve with more than four independent points per kiloparsec, extending to about twice the de Vaucouleurs radius. The curve shows remarkable structure. A mass model using a constant M/L stellar disk, and a thin gas disk as observable components has been constructed. These components alone cannot explain the observed rotation curve. The discrepancy between dynamical and observable mass is noticeable at a radius of about 2 kpc, well within the optical part of NGC 1560. At the last measured point of the rotation curve the global mass-to-light ratio is about 35, corresponding to a mass discrepancy of a factor 5, taking the gas mass into account. Models have been constructed, consisting of the two observable components and a dark, spherically symmetric halo. The parameter range allowed for the halo is in agreement with halo parameters determined for more massive galaxies. The neutral hydrogen distribution seems to be a quite precise tracer of the dark matter distribution. By adding a dominant mass component with a smooth density distribution (i.e. a dark halo) to the observable disk components, the fine structure in the rotation curve due to fluctuations in the gas surface density distribution is diluted. One way to match the observed shape of the rotation curve in detail using the disk-halo models is to increase the distance to NGC 1560 by at least a factor two. The models obtained by using modified dynamics (MOND) fit the observed rotation curve with all its detailed structure very well. This study shows that very detailed rotation curves can place severe limits on the distribution of luminous and non-luminous matter. Dark material in a dominant halo with a smooth density distribution could be ruled out if more rotation curves with prominent features become available.