Intergalactic Ionization Rates Due to Cosmic-Rays

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!

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