Cosmological X-ray Scattering from Intergalactic Dust

TL;DR

This paper explores how X-ray scattering by intergalactic dust can reveal dust properties and distribution in the universe, using theoretical models and observational constraints to distinguish dust scenarios and their cosmological implications.

Contribution

It derives cosmological scattering formulae, models surface brightness profiles for different dust distributions, and interprets non-detections to constrain intergalactic dust properties and their cosmological impact.

Findings

X-ray scattering halos depend on dust distribution and grain size.

Non-detection of halos at 1-8 keV constrains grey dust models.

Intergalactic dust can cause measurable magnitude offsets in supernova surveys.

Abstract

High resolution X-ray imaging offers a unique opportunity to probe the nature of dust in the z ~< 2 universe. Dust grains 0.1- 1 um in size will scatter soft X-rays, producing a diffuse "halo" image around an X-ray point source, with a brightness ~ 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 ~ 10^-5) by a power-law distribution of grain sizes, and (ii) a DLA-type (N_H ~ 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 ~ 10^-5$) when the data is restricted to the 1-8 keV band. We also calculate the systematic offset in magnitude, delta m ~ 0.01, for such a population of graphite grains, which would affect the type of supernova survey ideal for measuring dark energy parameters within ~ 1% precision.