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. The author 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. Models have been constructed, consisting of the two observable components and a dark, spherically symmetric halo. The models obtained by using modified dynamics 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.