A close-pair binary in a distant triple supermassive black-hole system

TL;DR

This paper reports the discovery of a close-pair supermassive black hole system within a distant triple system, revealing that such tight pairs are more common than previously thought, impacting gravitational wave research.

Contribution

It presents the first observation of a supermassive black hole pair separated by about 140 parsecs in a triple system, using radio jet properties as an indirect detection method.

Findings

Tight supermassive black hole pairs are more frequent than previously believed.

Radio jet modulation can indicate the presence of close black hole pairs.

The discovery constrains models of black hole evolution and gravitational wave sources.

Abstract

Galaxies are believed to evolve through merging, which should lead to multiple supermassive black holes in some. There are four known triple black hole systems, with the closest pair being 2.4 kiloparsecs apart (the third component is more distant at 3 kiloparsecs), which is far from the gravitational sphere of influence of a black hole with mass $\sim$10$^9$ M$_\odot$ (about 100 parsecs). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs. Here we report observations of a triple black hole system at redshift z=0.39, with the closest pair separated by $\sim$140 parsecs. The presence of the tight pair is imprinted onto the properties of the large-scale radio jets, as a rotationally-symmetric helical modulation, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments.