Cosmological decrease in brightness and angular broadening in the ionized inter-galactic mediumdetected in the MASIV quasar survey

The MASIV survey of 482 compact sources at 6 cm found that half of them exhibit low level interstellar scintillation on timescales of two days. The ISS amplitude is quantified by the structure function of the flux density at a time lag of 2 d, which is found to decrease with redshifts in the range 2–4. A structure function model is given depending on the fraction of flux density and angular diameter in the most compact core in each source. If the core emission is limited to a maximum brightness temperature, the minimum diameter should increase as (1 + z)0.5. However the observed decrease in ISS is significantly faster than this prediction. We consider a possible explanation in terms of angular broadening in the ionized IGM, based on optical observations of the Lyα forest.