2000ApJ...539..155G

The universal coupling relation between the baryonic matter of spiral galaxy disks and the inner part of the corresponding dark halos requires the existence of a secondary dark component in faint late-type spirals. In the present paper, this empirical relation is used to predict the amount of this secondary component within 1.5-2 optical radii of the nearby spiral M33. At such radii there is no clear need of any additional disk dark mass, but the low value of the derived stellar mass-to-light ratio suggests masses of the order 0.5-1M_HI+He_ for this component, if any. At four optical radii, the coupling relation between detected disk baryons and dark halo cannot describe the observed rotation curve. The luminous mass (stars and gas) in M33 appears to have a higher collapse factor with respect to the halo than in galaxies where the coupling relation has been successfully tested, in bright galaxies in particular. An alternative solution is to invoke a disk dark baryonic component. The density distribution of such a component is not proportional to that of the neutral hydrogen at large radii and must have a larger scale length. A distribution obtained by scaling radially the H I distribution is a possible solution. The nature of this component cannot be easily inferred from its distribution. If it is made of cold molecular H₂, the observational constraint is the detection of a column density of N_H2∼6x10²¹ cm^–2.