Astronomers Baffled by Discovery of Rare Quasar Quartet

W. M. Keck Observatory press release

Using the W. M. Keck Observatory in Hawaii, a group of astronomers led by Joseph Hennawi of the Max Planck Institute for Astronomy have discovered the first quadruple quasar: four rare active black holes situated in close proximity to one another. The quartet resides in one of the most massive structures ever discovered in the distant universe, and is surrounded by a giant nebula of cool dense gas. Because the discovery comes with one-in-ten-million odds, perhaps cosmologists need to rethink their models of quasar evolution and the formation of the most massive cosmic structures. The results are being published in the May 15, 2015 edition of the journal Science.

Quasar Quartet
Image of the region of the space occupied by the rare quasar quartet. The four quasars are indicated by arrows. Credit: Hennawi & Arrigoni-Battaia, MPIA
Hitting the jackpot is one thing, but if you hit the jackpot four times in a row you might wonder if the odds were somehow stacked in your favor.

Quasars constitute a brief phase of galaxy evolution, powered by the in-fall of matter onto a supermassive black hole at the center of a galaxy. During this phase, they are the most luminous objects in the Universe, shining hundreds of times brighter than their host galaxies, which themselves contain hundreds of billions of stars. But these hyper-luminous episodes last only a tiny fraction of a galaxy’s lifetime, which is why astronomers need to be very lucky to catch any given galaxy in the act. As a result, quasars are exceedingly rare on the sky, and are typically separated by hundreds of millions of light years from one another. The researchers estimate that the odds of discovering a quadruple quasar by chance is one in ten million. How on Earth did they get so lucky?

Clues come from peculiar properties of the quartet’s environment. The four quasars are surrounded by a giant nebula of cool dense hydrogen gas, which emits light because it is irradiated by the intense glare of the quasars. In addition, both the quartet and the surrounding nebula reside in a rare corner of the universe with a surprisingly large amount of matter. “There are several hundred times more galaxies in this region than you would expect to see at these distances,” said J. Xavier Prochaska, professor at the University of California Santa Cruz and the principal investigator of the Keck Observatory observations.

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