Researchers illuminate quasars

The lights went out 70,000 years ago for today’s brightest and most energetic object in the universe.

In a paper published in the Nov. 20 issue of The Astrophysical Journal Letters, a team of astrophysicists from around the country led by Kevin Schawinski of Yale’s Department of Physics answered what caused the appearance of the bright, mysterious and baffling gas cloud known as Hanny’s Voorwerp: the dying glow from a starlike object called a quasar whose light and energy output shut down around 70,000 years ago. This discovery, Schawinski said, will allow astronomers to better understand quasars and black holes, as well as the evolution of the universe. In addition to its scientific significance, astronomy researchers interviewed said the study revealed the importance of amateur astronomers in the field.

The Voorwerp was discovered two years ago by Dutch amateur astronomer Hanny van Arkel when she stumbled upon a strange-looking picture while sifting through the Hubble Telescope’s archive as part of a citizen science project called Galaxy Zoo. Schawinski, who is a co-founder of Galaxy Zoo, and his team spent about two years compiling data and observations before they came up with an explanation of what they saw in the photograph.

After verifying that the Voorwerp was a real object in the sky, the team analyzed the extragalactic object with the use of a spectrum — measurements of different wavelengths — which revealed the composition of the Voorwerp.

“Only when you take a spectrum of something can you understand what it is made of and what the light is that you are seeing,” Schawinski said.

The Voorwerp consists almost entirely of hot gas that is illuminated by a quasar, the researchers said, but they were unable to find an active one in the nearby galaxy. (A quasar is the central region of a galaxy where the light is emitted as materials spiral inward toward the center of a supermassive black hole, which sucks in gas and other diffuse materials around it.)

To rule out the possibility that the quasar responsible was hidden by a gas cloud, the research team looked for high-energy X-rays from the quasar that should be visible no matter how dense a gas cloud is, professor Meg Urry, director of the Yale Center for Astronomy and Astrophysics and a co-author of the paper, said.

This idea was ruled out after an experiment using space satellites revealed that the only black hole within a reasonable distance for it to form a quasar and thus light up the Voorwerp was not emitting enough energy, Schawinski and Urry said.

The only logical conclusion remaining was that the quasar responsible for lighting up the Voorwerp is no longer producing light, Schawinski said. “Maybe the quasar has shut down and the black hole has stopped feeding in the amount of time it took for the light to come from the quasar in the center of the galaxy to reach the Voorwerp,” Schawinski said.

The discovery provides valuable data and insight for the study of the life cycle of quasars and the supermassive black holes that power them, Urry said.

“[The study] reveals something very interesting about the light cycle of a quasar,” astronomy professor Priyamvada Natarajan and another co-author of the paper said. “We think quasars may start and stop multiple times. Now, we have the first evidence that there is a full cycle.”

The study will also help to refine existing theoretical models of black hole accretion, which is the physical process that lights a quasar, Schawinski said.

“The only constraint we have is on how much matter is accreted and how much energy is released, even those constraints are not well-known,” Urry said. “[The results of the study] give us a timescale on which accretion stops and starts.”

Schawinski added that the 70,000-year quasar shut down estimate the team made is very short cosmologically speaking, noting that before this study, astronomers did not know exactly how long this process takes.

The discovery of the Voorwerp by van Arkel, a schoolteacher, reveals the growing role amateur astronomers will play in the future, Stanford University professor Roger Romani said.

“One of the big pushes in modern astronomy is to gather large sky surveys,” he said. “While you can program computers to look for things you know, it’s much harder to program computers to look for things you can’t imagine.”

More than 120,000 quasars have been identified to date by the Sloan Digital Sky Survey, according to its website.