It’s estimated that roughly three in every thousand quasars have two actively feeding black holes in their core. Created during the final stages of large galaxy mergers, these systems are a temporary configuration. In time, these black holes should merge, releasing tremendous bursts of gravitational energy. In the interim, however, they are doing their normal black hole thing of taking infalling material and using it to build disks, throw jets, and quash star formation by blasting out tremendous amounts of light that clear the galactic core. 

According to Shenli Tang: In spite of their rarity, they represent an important stage in the evolution of galaxies, where the central giant is awakened, gaining mass, and potentially impacting the growth of its host galaxy.

Unfortunately, finding these systems isn’t only difficult because they are rare; it is also difficult because they are just plain hard to see. Quasars are distant, and distinguishing between one very luminous blob, and a luminous blob that is actually two very close together objects requires using some of the most advanced telescopes in the world. 

An international collaboration of astronomers has used the combined abilities of the Subaru Telescope, W. M. Keck Observatory, and Gemini Observatory to search out and find three different binary SMBH quasars. This work appears in The Astrophysical Journal. 

According to lead author John Silverman: To make our job easier, we started by looking at the 34,476 known quasars from the Sloan Digital Sky Survey with Hyper Suprime-Cam imaging to identify those having two (or more) distinct centers. Honestly, we didn’t start out looking for dual quasars. We were examining images of these luminous quasars to determine which type of galaxies they preferred to reside in when we started to see cases with two optical sources in their centers where we only expected one. 

In the images, 421 systems appeared to have dual cores, but an image alone doesn’t allow the eye or software to distinguish between a system with two SMBHs and glowing accretion disks, and a system with one SMBH that happens to be lined up on the sky with something else that makes it look like there is are two objects at the same distance. To distinguish between actual binary systems and chance alignments the team used spectroscopy to look for two spinning accretion disks at the same redshift. The material around SMBHs can reach orbital speeds of 1000s of kilometers per second – a phenomenon unique to SMBH systems.

From these 421 candidates, they were able to identify three actual binary SMBHs. Two of these systems were already known. This simultaneously proves that their technique works and that these systems are rare and super hard to find. 

While this may seem like one heck of a waste of data – looking at 34,000 galaxies to find 421 binary candidates of which only 3 are actually binary systems – the data acquired in this project will be used for other purposes. This is your regular reminder that one person’s noise is another person’s research. 

More Information

Keck Observatory press release 

Kavli Institute for the Physics and Mathematics of the Universe press release 

Subaru Telescope press release 

National Astronomical Observatory of Japan press release 

“Dual Supermassive Black Holes at Close Separation Revealed by the Hyper Suprime-Cam Subaru Strategic Program,” J. D. Silverman et al., 2020 Aug. 26, Astrophysical Journal (Preprint on arxiv.org)