HOLLOW GALACTIC HALOS OF FERMIONIC DARK MATTER

Fermi statistics and velocity anistropy give a surprisingly rich structure to massive neutrino dark matter halos. If a spherically symmetric halo has an anisotropic phase space, then hollow halos with a minimum of the mass density at the center are possible. Hollowness of a halo is controlled by dimensionless constant Ko = (g-upsilon/4-pi)(m2/M(p)2(L0(2)/h2)(c/[mu]1/2) involving the fermion mass, m, the Planck mass M(p), and parameters L0 and mu-1/2 fixing the halo anistropy. All dependence of halo on fermion mass and the maximum phase-space density is contained in K0. For K0 << 1, i.3., for small enough fermion mass and effective core size, most halos must be hollow to satisfy Fermi statistics. Thus, existing neutrino lower mass bounds not only are overly restrictive but become inapplicable. It follows that neutrinos or other fermions in the mass range m less-than-or-similar-to 100 eV are viable candidates for galactic dark matter. A flat rotation curve is also shown to be a generic result of anisotropy in the same limit of interest. We discuss observable consequences of the possibility that isolated dwarf galaxies are associated with hollow dark matter halos.