Description: | Active Galactic Nuclei are among the brightest sources in the Universe, thus visible up to the earliest epochs. Among them, those that are also radio bright come at prime, as we can observe them at the highest possible angular resolution with Very Long Baseline Interferometry. Hence, they provide a viable way to study the smallest scales out to the largest distances, and can be used as cosmological probes. In this talk, we will exploit the high angular resolution of VLBI (sometimes enhanced by the strong gravitational lensing effect) to directly study the sub-galactic scales at high redshift.
Our broad-band sensitive radio campaign allows us to spatially resolve the jet in the most distant blazar known (PSO J0309+27 at z=6.1) and measure its properties from low to high frequencies. The plethora of multi-wavelength observations and the presence of an extended jet from pc- to kpc-scales makes PSO J0309+27 a unique source for studying jetted AGN at the end of the reionization epoch. To go beyond the tip of iceberg and study the faint (hence more common) sources at high-z, we take advantage of the lensing magnification. We will show results for several systems, sometimes revealing dual and offset AGN candidates at z>1. We will discuss the implication of our findings and future prospects for this kind of studies with the next generation of telescopes. |