Contraction of Dark Matter Galactic Halos Due to Baryonic Infall

Varied evidence suggests that galaxies consist of roughly 10 percent baryonic matter by mass and that baryons sink dissipatively by about a factor of 10 in. radius during galaxy formation. It is shown that such infall strongly perturbs the underlying dark matter distribution, pulling it inward and creating cores that are considerably smaller and denser than would have evolved without dissipation. Any discontinuity between the baryonic and dark matter mass distributions is smoothed out by the coupled motions of the two components. If dark halos have large core radii in the absence of dissipation, the above infall scenario yields rotation curves that are flat over large distances, in agreement with observations of spiral galaxies. Such large dissipationless cores may plausibly result from large internal kinetic energy in protogalaxies at maximum expansion, perhaps as a result of subclustering, tidal effects, or anisotropic collapse.