Radio observations of active galactic nuclei with mm-VLBI

TL;DR

This paper reviews recent advances in millimeter-wavelength VLBI observations of active galactic nuclei, highlighting how high-frequency imaging helps probe the jet formation regions near supermassive black holes.

Contribution

It provides an overview of open questions in AGN jet physics and discusses recent high-frequency VLBI results, including those from the Event Horizon Telescope.

Findings

High-frequency VLBI reveals jet structures close to black holes.

Recent 230 GHz observations provide new insights into black hole environments.

Advances in VLBI technology enable detailed studies of AGN jets.

Abstract

Over the past few decades, our knowledge of jets produced by active galactic nuclei (AGN) has greatly progressed thanks to the development of very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms involved in the formation of the plasma flow, as well as those driving its exceptional radiative output up to TeV energies, remain to be clarified. Most likely, these physical processes take place at short separations from the supermassive black hole, on scales which are inaccessible to VLBI observations at centimeter wavelengths. Due to their high synchrotron opacity, the dense and highly magnetized regions in the vicinity of the central engine can only be penetrated when observing at shorter wavelengths, in the millimeter and sub-millimeter regimes. While this was recognized already in the early days of VLBI, it was not until the very recent years that sensitive VLBI imaging at high frequencies has become possible. Ongoing technical development and wide band observing now provide adequate imaging fidelity to carry out more detailed analyses. In this article we overview some open questions concerning the physics of AGN jets, and we discuss the impact of mm-VLBI studies. Among the rich set of results produced so far in this frequency regime, we particularly focus on studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also presented.