DARK BARYONS AND ROTATION CURVES

The best measured rotation curve for any galaxy is that of the dwarf s piral DDO 154, which extends out to about 20 disk scale lengths. It pr ovides an ideal laboratory for testing the universal density profile p rediction from high-resolution numerical simulations of hierarchical c lustering in cold dark matter-dominated cosmological models. We find t hat the observed rotation curve cannot be fitted either at small radii , as previously noted, or at large radii. We advocate a resolution of this dilemma by postulating the existence of a dark spheroid of baryon s amounting to several times the mass of the observed disk component a nd comparable to that of the cold dark matter halo component. Such an additional mass component provides an excellent fit to the rotation cu rve provided that the outer halo is still cold dark matter-dominated w ith a density profile and mass-radius scaling relation as predicted by standard CDM-dominated models. The universal existence of such dark b aryonic spheroidal components provides a natural explanation of the un iversal rotation curves observed in spiral galaxies, may have a simila r origin and composition to the local counterpart that has been detect ed as MACHOs in our own Galactic halo via gravitational microlensing, and is consistent with, and even motivated by, primordial nucleosynthe sis estimates of the baryon fraction.