Quasars are the extremely luminous state that some supermassive black hole can achieve. If these enormous cosmic objects are undergoing a feeding frenzy, they start throwing some of the material out with such energy that they can outshine their host galaxies. Galaxy collision can provide the fuel to start a quasar and, on rare occasions, quasars periods will begin in the supermassive black holes of both colliding galaxies. Now, astronomers have snapped two pairs of quasars – and they are the closest double quasars ever seen in the distant universe.

The findings, reported in Nature Astronomy, suggest that the black holes in each pair are just several thousand light-years apart and that the galaxies are well underway their merger, getting close to being a single object. The light from the quasars has traveled for at least 10 billion years, with the furthest one coming from 11,5 billion years ago. That pair has a projected separation of 11,400 light-years.

"We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack,"  lead researcher Yue Shen of the University of Illinois at Urbana-Champaign, said in a statement.

The evolution of supermassive black holes goes hand in hand with the evolution of their host galaxy. This relation is complex and driven by many factors, and understanding how these interactions unfold can be critical to our models of galaxy formation and evolution.

"This truly is the first sample of dual quasars at the peak epoch of galaxy formation with which we can use to probe ideas about how supermassive black holes come together to eventually form a binary," said research team member Nadia Zakamska of Johns Hopkins University in Baltimore, Maryland. "Quasars make a profound impact on galaxy formation in the universe. Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together."

The observations were possible thanks to the Hubble Space Telescope, the European Space Agency's Gaia satellite, as well as ground observatories such as the Sloan Digital Sky Survey and the Gemini North Telescope.

All these instruments were necessary to find distant quasars, identify possible dual quasars, and confirm if they were actually pairs. The team now hopes to look for many more objects such as these four quasars, expanding what we know of galaxy mergers in the distant universe.