Measuring the brightness temperature distribution of extragalactic radio sources with space VLBI

We have used VLBI Space Observatory Programme (VSOP) observations to measure the brightness temperature distribution of a well-defined subset of the Pearson-Readhead sample of extragalactic radio sources. VLBI, which is restricted to Earth-diameter baselines, is not generally sensitive to emitting regions with brightness temperatures greater than approximately 10(12) K, coincidentally close to theoretical estimates of brightness temperature limits, 10(11)-10(12) K. We find that a significant proportion of our sample have brightness temperatures greater than 10(12) K; many have unresolved components on the longest baselines, and some remain completely unresolved. These observations begin to bridge the gap between the extended jets seen with ground-based VLBI and the microarcsecond structures inferred from intraday variability, evidenced here by the discovery of a relationship between intraday variability and VSOP-measured brightness temperature, likely due to the effects of relativistic beaming. Also, lower limits on jet Lorentz factors, estimated from space VLBI observations, are starting to challenge numerical simulations that predict low Lorentz factor jets.