Two neighbouring supermassive black holes in their quasar phase are visible as they were less than a billion years after the Big Bang. This is the earliest known close pair of quasars. Both had already reached masses for which there is still no definitive theoretical explanation.
Unexpectedly Massive for Their Era
The pair has been given the catalogue designation J1602+4228AB and the Hawaiian name Nāpōwawāʻenakaulua. The larger of the two black holes has a mass of 1.5 billion solar masses, while the smaller is around 400 million solar masses. For comparison, Sagittarius A* at the centre of our Galaxy has a mass of approximately 4 million Suns, making the larger black hole in the pair more than 300 times as massive. The two quasars are separated by 18,100 light-years, roughly one-fifth of the Milky Way’s diameter.
We see the pair as it was 900 million years after the Big Bang, when the Universe was less than 7% of its current age. According to NOIRLab, lead researcher Silvia Onorato considers the existence of even one giant quasar at such an early epoch a mystery. “There is no definitive theoretical explanation for how even one such quasar can exist, let alone two,” she says.
The problem is that matter cannot fall into a black hole at an arbitrarily high rate. Gas near it heats up and emits radiation, and the pressure from that radiation pushes the remaining material away. If the initial seed was a stellar remnant, growing to a billion solar masses would have required accretion at nearly the maximum possible rate for almost the entire time since the Big Bang. Astronomers have already found individual rapidly growing black holes at the dawn of the Universe, but two such giants side by side make the problem even more difficult.
A Discovery in the Aether Survey
Galaxies frequently collided in the early Universe, so pairs of supermassive black holes should be common. Finding them, however, is difficult. The quasar phase is brief, distant sources are faint, and two nearby objects are hard to distinguish in an image. Only three such pairs are known at “cosmic dawn”, and the newly discovered pair is the closest together.
The second quasar was spotted in data from the Aether survey, led by Emanuele Paolo Farina of NOIRLab. Using the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope, the survey examined 200 quasars in the early Universe, representing 43% of those known from this epoch.
The presence of two distinct objects was confirmed using the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North telescope on Maunakea. The fainter quasar lies so close to the brighter one that it is almost impossible to distinguish from Earth, but the sky was exceptionally clear during the observations. The findings have been published in the peer-reviewed journal The Astrophysical Journal Letters.
A Pair or an Optical Copy?
A gravitational lens—a massive galaxy between us and the quasar—could bend its light and create a second, fainter image of the same source. Such copies have the same spectral shape, but data from the Gemini Multi-Object Spectrograph (GMOS) show noticeable differences between the two quasars’ spectra. The team therefore considers it more likely that these are two separate black holes. The lensing explanation can only be definitively ruled out once a model of the lens has been constructed.
The study’s authors suggest that the pair marks a region of the early Universe with an exceptionally high concentration of matter. Galaxy interactions there could have fuelled both star formation and rapid black hole growth at the same time. If so, other galaxies are likely to surround the pair, making a search for this surrounding population the next test.
