VLBI Studies of DAGN and SMBHB Hosting Galaxies

Abstract

Dual active galactic nuclei and supermassive black hole binaries at kiloparsec and parsec-scale separations, respectively, are expected during stages of galaxy merger and evolution. Their observational identification can address a range of areas of current astrophysics frontiers including the final parsec problem and their contribution toward the emission of low-frequency gravitational waves. This has however been difficult to achieve with current spectroscopy and time domain strategies. Very long baseline interferometry as a method of directly imaging radio structures with milli-arcsecond and submilli-arcsecond resolutions is introduced as a possible means of detecting dual active galactic nuclei and supermassive black hole binaries. We motivate its usage with expected observational signatures and cite some studies from literature to illustrate its current status and present an updated list of candidates imaged with high-resolution radio observations. We then recall some shortcomings of the method with possible solutions and discuss future directions, relevant to large surveys with the upcoming Square Kilometer Array and future space very long baseline interferometry missions.

The quest for dual and binary supermassive black holes: A multi-messenger view

“…The technique is thus ideally suited for observing kpc-scale dual AGN (e.g. An et al, 2018). However, a severe limitation of its applicability is that only a minority of AGN, less than 10 per cent of the population, are strong (i.e.…”

VLBI Non-detection of a Candidate Dual AGN in a Galaxy Merger

“…Confirmation of each case would ideally require AGN indicators at multiple wavebands. Although radio emission is associated with a minority of AGNs, the very long baseline interferometry (VLBI) technique plays a unique role in confirming emission from dual AGNs, owing to its high angular resolution (e.g., An et al 2018).…”

A self-lensing supermassive binary black hole at radio frequencies: the story of Spikey continues

“…Detecting binary AGNs separated by a small fraction of a pc is practically impossible with direct imaging observations, even with the high resolution offered by VLBI and in the most nearby universe (e.g. An et al 2018). But radio interferometric observations could help in another way, by detecting a discernible effect of a binary companion on the appearance of the relativistic jet produced by the other AGN in the system, because the orbital motion of the jetted AGN may result in a helical shape of the jet (for a recent review, see De Rosa et al 2019, and references therein).…”