2000ApJ...539...73G

Standard big bang nucleosynthesis predicts the average baryon density of the universe to be a few percent of the critical density. Only about one-tenth of the predicted baryons have been seen. A plausible repository for the missing baryons is in a diffuse ionized intergalactic medium (IGM). In an attempt to measure the IGM, we searched for Thomson-scattered halos around strong high-redshift radio sources. Observations of the radio source 1935-692 were made with the Australia Telescope Compact Array. We assumed a uniform IGM, and isotropic steady emission of 1935-692 for a duration between 10⁷ and 10⁸ yr. A model of the expected halo visibility function was used in χ² fits to place upper limits on Ω_IGM. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2 σ limits of Ω_IGM<0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements in the technique will probably allow this in future studies.