Why Do Galaxies Have Extended Flat Rotation Curves?

Abstract

Recent observations by Mistele et al . show that the circular velocity curves of isolated galaxies remain flat out to the largest radii probed so far, i . e ., ≈1 Mpc. The velocity decline beyond the expected virial radius is not observed. These results imply that the galaxy halo is in thermal equilibrium even at large radii where particles did not have time to relax. The galaxies must have already formed in the isothermal state. How is this possible? In the present note we try to understand the formation of galaxies with warm dark matter in the expanding universe.