Intergalactic Dust and Observations of Type Ia Supernovae

Estimates of the cosmic star formation rate and of cluster metallicities independently imply that at z ∼< 0.5 the gas in the universe has substantial average metallicity: 1/10 ∼< Z/Z⊙ ∼< 1/3 for Ωgas = 0.05. This metal density probably cannot be contained in known solar-metallicity galaxies of density parameter Ω∗ ≈ 0.004, implying significant enrichment of the intergalactic medium (IGM) by ejection of metals and dust from galaxies via winds, in mergers or in dust efflux driven by radiation pressure. Galaxies have a dust/metal ratio of ∼ 0.5 in their interstellar media, but some fraction (1− f) > 0 of this must be destroyed in the IGM or during the ejection process. Assuming the Draine & Lee dust model and preferential destruction of small grains (as destruction by sputtering would provide), I calculate the reddening and extinction of a uniform cosmological dust component in terms of f and the minimum grain size amin. Very small grains provide most of the reddening but less than half of the opacity for optical extinction. For f ∼> 0.3 and amin ∼> 0.1μm, the intergalactic dust would be too grey to have been detected by its reddening, yet dense enough to be cosmologically important: it could account for the recently observed type Ia supernova dimming at z ∼ 0.5 without cosmic acceleration. It would also have implications for galaxy counts and evolutionary studies, and would contribute significantly to the cosmic infrared background (CIB). The importance of grey intergalactic dust of the described type can be tested by observations of z = 0.5 supernovae in (rest) R−band or longer wavelengths and by the fluxes of a large sample of supernovae at z > 1. Subject headings: cosmology: observations, distance scale – intergalactic medium – dust, extinction