Journal: Transactions of IAA RAS vol. 40
Keywords: VLBI, Active Galaxies, Jets, Radio continuum, Millimetre surveys, Brightness temperature
Very Long Baseline Interferometry (VLBI) Observations at 86 GHz reach a resolution of about 50 µas and sample the scales as small as 10³ − 10⁴ Schwarzschild radii of the central black hole in Active Galactic Nuclei (AGN), uncovering the jet regions where acceleration and collimation of the relativistic flow takes place. We present the results from a large global VLBI survey of 162 ultra-compact radio sources at 86 GHz conducted in 2010–2011. All the sources were detected and imaged; increasing by a factor of ∼ 2 the total number of AGN so far imaged with VLBI at 86 GHz. The survey data attained a baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy/beam. We have used Gaussian model fitting to represent the structure of the observed sources and to estimate the flux densities and sizes of the core and jet components. The model fitting yields estimates of the brightness temperature (Tb) of the compact VLBI (base) of the jet and inner jet components of AGN, taking into account the resolution limits of the data at 3 mm. We have applied a basic population model with a single value of intrinsic brightness temperature, T0, in order to reproduce the observed distribution of Tb. The modelling yields T₀ ∼ 1 − 7 × 10¹¹ K in the VLBI cores and T₀ ≤ 5 × 10¹⁰ K in the jets. This finding indicates that the VLBI cores reflect the inverse-Compton limit on brightness temperature, while the properties of the inner jets can be described by the equipartition between the particle and magnetic field energy. We also find a correlation between the brightness temperatures obtained from the model fits with estimates of the brightness temperature limits made directly from the visibility data. For objects with sufficient structural detail detected, we investigated the effect of adiabatic energy losses on the evolution of brightness temperature along the jet.