High resolution X-ray imaging offers a unique opportunity to probe the nature of dust in the z \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } } 2 universe .
Dust grains 0.1 - 1 \mu { m } in size will scatter soft X-rays , producing a diffuse “ halo ” image around an X-ray point source , with a brightness \sim few % confined to an arcminute-sized region .
We derive the formulae for scattering in a cosmological context and calculate the surface brightness of the scattering halo due to ( i ) an IGM uniformly enriched ( \Omega _ { d } \sim 10 ^ { -5 } ) by a power-law distribution of grain sizes , and ( ii ) a DLA-type ( N _ { H } \sim 10 ^ { 21 } cm ^ { -2 } ) dust screen at cosmological distances .
The morphology of the surface brightness profile can distinguish between the two scenarios above , place size constraints on dusty clumps , and constrain the homogeneity of the IGM .
Thus X-ray scattering can gauge the relative contribution of the first stars , dwarf galaxies , and galactic outflows to the cosmic metallicity budget and cosmic history of dust .
We show that , because the amount of intergalactic scattering is overestimated for photon energies < 1 keV , the non-detection of an X-ray scattering halo by Petric et al .
( 2006 ) is consistent with ‘ grey ’ intergalactic dust grains ( \Omega _ { d } \sim 10 ^ { -5 } ) when the data is restricted to the 1-8 keV band .
We also calculate the systematic offset in magnitude , \delta m \sim 0.01 , for such a population of graphite grains , which would affect the type of supernova survey ideal for measuring dark energy parameters within \sim 1 \% precision .