Outside the boundary of the diffusive cosmic-ray halo, the cosmic-ray flux must drop significantly because (as with photons leaving the surface of the sun) the cosmic-rays are allowed to stream freely away from the surface. A reasonable estimate of the intergalactic cosmic-ray ionization rate can be obtained by comparing the relative volumes of space and the relative times spent in each volume, that is, z IG ~ z CR ( Vhalo / VIG ) ( tIG / thalo ) ~ z CR ( 3 x 10-6 ) ( 102 ) ~ 3 x 10-4 z CR ~ 3 x 10-21 s-1 If a GMC were introduced to this intergalactic environment, its fractional ionization would drop significantly; the relevant ambipolar diffusion time would drop below the free-fall time; the cloud may actually be able to cool a little; and it ought to be able to make stars like crazy! Return to: Model Figure Continue