The dark matter distribution in disc galaxies

We use high-quality optical rotation curves of nine low-luminosity disc gal axies to obtain the velocity profiles of the surrounding dark matter haloes . We find that they increase linearly with radius at least out to the edge of the stellar disc, implying that, over the entire stellar region, the den sity of the dark halo is about constant. The properties of the mass structure of these haloes are similar to those f ound for a number of dwarf and low surface brightness galaxies, but provide a more substantial evidence of the discrepancy between the halo mass distr ibution predicted in the cold dark matter scenario and those actually detec ted around galaxies. We find that the density law proposed by Burkert repro duces the halo rotation curves, with halo central densities (rho (0) simila r to 1-4 x 10(-24) g cm(-3)) and core radii (r(0) similar to 5-15 kpc) scal ing as rho (0) proportional to r(0)(-2/3).