Cosmological Halos: A Search for the Ionized Intergalactic Medium

Abstract

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 ΩIGM. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2 σ limits of ΩIGM < 0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements in the technique will probably allow this in future studies.