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H. L. Helfer (Dept of Physics & Ast; Lab for Laser Energetics, U Rochester Rochester NY 14627)
We focus on the problem of representing only the dark matter presently associated with the halos of isolated galaxies. This manifests itself in gravitational lenses and as the unexplained source of the extended flat portion of the rotation curves Vrot(r) \approx const. of the Milky Way and other galaxies. For example,any halo fluid with a simple equation of state must have finite density at the origin and be isothermal in those regions for which Vrot is flat. Such fluids are represented by Bonnor-Ebert models; these are gravitationally unstable for halos extending beyond ~20 kpc. We also note a halo of exotic non-interacting particles satisfying the collisionless Boltzman equation cannot be restored to even approximate spherical symmetry in regions where galaxy-galaxy collisions have occured. Provisionally, we suggest that the halos arose as very small amplitude unstable fluctuation in a classical scalar field of the generic form c-2\partial2tt\phi -\nabla2\phi= m2\phi (1-\phi2) where 1/m ~3 kpc; the growth of spacial fluctuations is limited to finite amplitude by the non-linear term. From an initial value \delta\phi ~ 10-4 it takes ~105 yrs to develop to finite amplitude; they could not have been important in the early days of the universe. These disturbances today play the role of `dark' halos around galaxies.The gravitational fields the produce can well represent the observed rotation curves of the Milky Way, NGC4605, F583-1, & DDO 154, with 1/m ~ 2-5 kpc determining the onset of the flat fortion of Vrot. When galaxies are clustered, the boundaries between halos is described by discontinuities in the second derivatives of the \phi-field and a \Lambda-term appears naturally in representing the field's energy density. The universe's present energy density associated with these halo fields is \Omega ~1/2 -2/3. If the universe were modeled as an enormous cluster of equally spaced galactic halos, the resulting cosmological fluid's energy density would scale with the cosmic scale factor as {\cal R}(t)-2. Using this, a possible scenario in which {\cal R}(t) would have experienced a recent acceleration, representing a `quintessence' effect.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.