A Radio Census of Binary Supermassive Black Holes

TL;DR

This study conducted a radio survey of over 3000 active galactic nuclei to identify binary supermassive black holes, finding only one confirmed case and providing insights into their merger timescales and radio activity correlations.

Contribution

It introduces a radio spectral index mapping technique for detecting binary supermassive black holes and constrains their evolutionary timescales and prevalence in the universe.

Findings

Only one binary supermassive black hole was detected.

Binary evolution from merger to stalling is less than 500 Myr.

No evidence found for an excess of stalled binaries at small separations.

Abstract

Using archival VLBI data for 3114 radio-luminous active galactic nuclei, we searched for binary supermassive black holes using a radio spectral index mapping technique which targets spatially resolved, double radio-emitting nuclei. Only one source was detected as a double nucleus. This result is compared with a cosmological merger rate model and interpreted in terms of (1) implications for post-merger timescales for centralisation of the two black holes, (2) implications for the possibility of "stalled" systems, and (3) the relationship of radio activity in nuclei to mergers. Our analysis suggests that the binary evolution of paired supermassive black holes (both of masses >= 1e8 Msun) spends less than 500 Myr in progression from the merging of galactic stellar cores to within the purported stalling radius for supermassive black hole pairs. The data show no evidence for an excess of stalled binary systems at small separations. We see circumstantial evidence that the relative state of radio emission between paired supermassive black holes is correlated within orbital separations of 2.5 kpc.