Imagine winning the Powerball jackpot-more than once. You may have a sense of how a team of astronomers feels after their discovery of a set of four quasars at the visible universe's edge. These brilliant beacons of light are typically spread far apart, but this quartet exists shoved together in only 650,000 light-years of space-equivalent to around a quarter of the distance between our closest big neighbor galaxy Andromeda and the Milky Way.
"On average, quasars are about 100 million light-years apart," says lead study author Joseph Hennawi, an astrophysicist at Germany's Max Planck Institute for Astronomy. "The odds against finding four so close together are ten million to one."
The first quasars were discovered in the early 1960s. At that time no one knew for certain what these glowing lights were, in particular because it seemed impossible that anything could glow so brightly and be visible to us billions of light-years away. Today we know that quasars are gigantic black holes that suck up gas at rates so monumental that the engulfed gas radiates energy out into space as it heats up to millions of degrees.
"The discovery is significant both because there are four of them, and because they are so close together," Hennawi says.
The core of a galaxy's central black hole has gravity so strong that even light photons are trapped within it. The period of time that the black hole sucks in gases and shines as a quasar is brief but intense; this is why quasars are so much brighter than their host galaxies, and are in fact the universe's brightest objects.
Only around 100 quasars in pairs have been found. Only two groups of triplets have ever been observed. This discovery is the first and only quartet.
The newly discovered quasar quartet is nestled inside a nebula-a tremendous cloud of cold hydrogen gas which has a mass equal to 100 billion stars. All of this has thrown the researchers for a loop.
"If you find something theory says is very unlikely, "you either have to conclude you got incredibly lucky, or that the theory is flawed," says Hennawi.
One reason the quasar quartet is so unusual is that quasars in general are rare. Black holes which cause quasars are common, but they only produce the brilliant light of the quasar when they are actively sucking in gas. There are about 100 billion galaxies in the portion of the universe we can see, but not many of these are quasars. Approximately 500,000 of these are in the active quasar phase, because the quasar represents a tiny fraction of the life of a black hole. This is rare in the lifetimes of most galaxies even though almost every galaxy has a black hole at its center.
Interestingly, National Geographic reports that the black hole at the center of our Milky Way galaxy weighs only about as much as four million suns-in other words, it's a lightweight and could never have been a quasar. The black hole at the core of our neighbor, Andromeda, is a much heavier player at 100 million suns; therefore it probably was once a quasar.
The nature of the nebula the four quasars call home is also probably part of the mystery. Scientists think that galaxies were first born as dark matter clumps sucked in gas expelled by the Big Bang. This dark matter is still poorly described although it is five times more prevalent than visible galaxies and stars.
In addition, the area of the universe that is home to the nebula and the quartet holds an unusually large amount of matter.
"There are several hundred times more galaxies in this region than you would expect to see at these distances," principal investigator of the Keck observations, a professor of astronomy and astrophysics at UC Santa Cruz, J. Xavier Prochaska says.
This nebula is relatively cool-only around 10,000° Celsius even though gas typically heats as it collapses. The cool gas of the nebula may be the right type of fuel to allow these quasars to glow actively for longer. Ohio State University astrophysicist David Weinberg, unconnected to the study, discussed this issue with National Geographic.
"By cosmological standards," says Weinberg, "that's actually pretty cold. You'd expect the temperature to be more like ten million degrees."
This region is a proto-cluster, the ancestor of a modern galaxy cluster. The system resembles the galaxy clusters that scientists observe in the universe today, but because the light has been traveling for 10 billion years, we are seeing the area as it was less than 4 billion years after the Big Bang. However, the fact that this massive nebula was found in the pro-cluster is a surprise.
"Our current models of cosmic structure formation based on supercomputer simulations predict that massive objects in the early universe should be filled with rarefied gas that is about ten million degrees, whereas this giant nebula requires gas thousands of times denser and colder," says Sebastiano Cantalupo of ETH Zurich, who led Keck Observatory imaging observations during his previous UCSC research appointment. "It is really amazing that this discovery was made the same night of the Slug Nebula while we were hunting for giant Lyman alpha nebulae illuminated by quasars-my first night at Keck Observatory and definitely the most exciting observing night I have ever had!"
This discovery by Hennawi and his team suggests that quasars are more likely to occur in certain environments. This cool nebula is unusual in that its characteristics were previously thought to be mutually exclusive:
"It may be that quasar episodes are more likely to be triggered in such an unusual environment, which is rich in both gas and galaxies," Hennawi says. "Current models of how structure forms in the universe would never predict that there would be so much cool, dense gas around. Instead, those models predict that the gas in such a massive object should be 1,000 times hotter and 1,000 times less dense."
Durham University's astronomer Michele Fumagalli, who with her team discovered quasar triplets in 2013, sees this discovery as an unique opportunity for scientists to explore how black holes grow.
"With this study, we are taking a ride back in time-roughly 10 billion years ago-and get to see how massive clusters are made," says Fumagalli.
The findings were published this week in Science.
