Black holes get close in triplets

The discovery of a triple supermassive black hole system in a distant (redshift z = 0.39) galaxy provides a rare opportunity to observe what may be the result of galactic mergers. In the four known triple black hole systems, the smallest distance between a pair of black holes is 2.4 kiloparsecs, but the newly discovered triple system includes a ‘tight pair’ separated by around 140 parsecs. The authors show that the presence of the tight pair is imprinted onto the properties of the large-scale radio jets generated by the black holes, providing a useful way of searching for other tight pairs without the need for extremely high-resolution observations. Six candidate galaxies were surveyed in this study, a ‘hit rate’ that suggests that tight pairs are more common than was thought. Close-pair binaries are useful targets for gravitational wave studies, so the development of an efficient way of finding them, and the prospect of there being more of them, should stimulate interest in work on predicting the strength of gravitational waves and assist in their eventual detection.