A Global 86 GHz VLBI Survey of Compact Radio Sources

We present results from a large global VLBI survey of compact radio sources at 86 GHz. The main goal of the survey is to increase the total number of objects accessible for future 3-mm VLBI imaging by factors of 3-5. The observations have yielded images for 109 sources, extending the database of the sources imaged at 86 GHz with VLBI observation by a factor of 5, and only 6 sources have not been detected. 12 objects have been detected but could not be imaged due to insufficient closure phase information. Radio galaxies are less compact than quasars and BL Lacs in sub-milliarcsecond scale. Flux densities and sizes of core and jet components of all imaged sources have been estimated using Gaussian model fitting. The cores of 70\% of the imaged sources are resolved. The core brightness temperatures of the sources peak at $\sim 10^{11}$K and only 1\% have brightness temperatures higher than $10^{12}$K. Cores of Intraday Variable (IDV) sources are smaller in angular size than non-IDV sources, and so yield higher brightness temperatures. Using the relation of the intrinsic and observed properties of relativistic jets, the intrinsic brightness temperature of $T_{\rm 0} = 6.5_{-0.8}^{+1.1}\times 109$\,K is deduced for 85 sources selected from our 3mm-survey data. This value is less than the one found from the database at 15 GHz in the case of the median-low state by a factor of ~5. Despite the difference in both samples, the decrease of the intrinsic brightness temperature may imply that the ultra compact cores in AGNs at 86 GHz are magnetic field dominated. Under the equipartition condition between the magnetic field energy and particle energy density, the absolute distance of the VLBI core can be predicted. From the database of VLBI survey at lower frequencies (2, 8, 15GHz) and our measurement, the brightness temperatures in source frame are investigated in the sub-parsec scale of the compact radio sources. From the very vicinity of the central engine, the brightness temperatures increase slowly and then rise with steeper slope. This implies that the jets are collimated and accelerated by the magnetically driven force, as predicted by the relativistic jet models for the magnetic acceleration