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

We have used VLBI Space Observatory Programme (VSOP) observations to measur e the brightness temperature distribution of a well-defined subset of the P earson-Readhead sample of extragalactic radio sources. VLBI, which is restr icted to Earth-diameter baselines, is not generally sensitive to emitting r egions with brightness temperatures greater than approximately 10(12) K, co incidentally close to theoretical estimates of brightness temperature limit s, 10(11)-10(12) K. We find that a significant proportion of our sample hav e brightness temperatures greater than 10(12) K; many have unresolved compo nents on the longest baselines, and some remain completely unresolved. Thes e 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 intr aday variability and VSOP-measured brightness temperature, likely due to th e effects of relativistic beaming. Also, lower limits on jet Lorentz factor s, estimated from space VLBI observations, are starting to challenge numeri cal simulations that predict low Lorentz factor jets.