Intergalactic dust and observations of type Ia supernovae

Estimates of the cosmic star formation rate and of cluster metallicities in dependently imply that at z less than or similar to 0.5 the gas in the univ erse has substantial average metallicity: 1/10 less than or similar to Z/Z. less than or similar to 1/3 for Omega(gas) = 0.05. This metal density prob ably cannot be contained in known solar-metallicity galaxies of density par ameter Omega(*) approximate to 0.004, implying significant enrichment of th e intergalactic medium (IGM) by ejection of metals and dust from galaxies v ia winds, in mergers or in dust efflux driven by radiation pressure. Galaxi es have a dust-to-metal ratio of similar to 0.5 in their interstellar media , but some fraction (1 - f) > 0 of this must be destroyed in the IGM or dur ing the ejection process. Assuming the Draine & Lee dust model and preferen tial destruction of small grains (as destruction by sputtering would provid e), I calculate the reddening and extinction of a uniform cosmological dust component in terms off and the minimum grain size a(min). Very small grain s provide most of the reddening but less than half of the opacity for optic al extinction. For f greater than or similar to 0.3 and a(min) greater than or similar to 0.1 mu m, the intergalactic dust would be too gray to have b een detected by its reddening, yet dense enough to be cosmologically import ant: it could account for the recently observed Type Ia supernova dimming a t z similar to 0.5 without cosmic acceleration. It would also have implicat ions for galaxy counts and evolutionary studies and would contribute signif icantly to the cosmic infrared background (CIB). The importance of gray int ergalactic 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.