All galaxies have monsters lurking in their cores: supermassive black holes that are millions or even billions of times the mass of our Sun. And when galaxies merge, so, too, do these black holes, which slowly creep closer over millions of years.
During that time, the gravitational effects of the merging galaxies and their black holes funnel stars, planets, gas, and dust to the center. That material begins to swirl into the black holes, forming a hot, bright accretion disk around each one that is visible across the universe.
Such a brightly shining disk around a supermassive black hole is called a quasar.
Normally, quasars hang out by themselves, pulling in material from the young galaxies in which they form. And once the galaxy settles down, its quasar shuts down as the black hole consumes all the nearby material and runs out of food. But chaotic events like galaxy mergers can reignite quasars, resulting in a unique double quasar pair that ultimately merges into a single, brighter, and even more massive black hole. Astronomers estimate that one out of every 1,000 quasars is actually a double quasar, although some of them sit too close to one another to be visibly separated at such great distances.