Washington, August 6 : An astronomer has discovered a strange green blob in the nearby universe that may be a 'light echo' from a long-dead quasar - an extremely bright object powered by a colossal black hole, which could give astronomers insight into the quasar's dying days.
For months, astronomers have been working to pinpoint what illuminates 'Hanny's Voorwerp', a mysteriously lit cloud that sits close to a spiral galaxy 700 million light years from Earth.
'Voorwerp' is Dutch for 'object', and 'Hanny' refers to Hanny van Arkel, a Dutch school teacher.
According to a report in New Scientist, van Arkel found the blob as she was classifying galaxy shapes from astronomical images as part of the online Galaxy Zoo project, which aims to harness the power of volunteers to answer cosmological questions.
To find a light source for the cloud, astronomers turned to NASA's Swift satellite to look for X-ray evidence of nearby energy sources. But the search came up dry.
Now astronomers think the best explanation is that Voorwerp got its energy from light that was once emitted by a bright quasar.
Quasars are considered 'active' galaxies because the bright objects are powered by supermassive black holes that are devouring their surroundings. When black holes stop eating, quasars can turn off, becoming dormant galaxies.
A galaxy called IC 2497 is thought to have once hosted the quasar that lit up Hanny's Voorwerp.
"We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar," explained Yale astrophysicist Kevin Schawinski.
Because of the distance between the two objects, any light that left the quasar would take tens of thousands of years to reach and energise Voorwerp. As a result, Voorwerp is still shining even though the quasar is thought to have turned off.
Because of uncertainties in the distance between Voorwerp and the galaxy, astronomers say the quasar likely turned off in the previous 100,000 years.
"Because of the vast scale of the galaxy and the Voorwerp, light from that past still lights up the nearby Voorwerp even though the quasar shut down sometime in the past 100,000 years, and the galaxy's black hole itself has gone quiet," he added.
"From the point of view of the Voorwerp, the galaxy looks as bright as it would have before the black hole turned off - it's this light echo that has been frozen in time for us to observe," said Chris Lintott, a co-organizer of Galaxy Zoo at Oxford University, UK.
Since Voorwerp is 65,000 light years across on its longer axis, the cloud could hold a record of tens of thousands of years of history, potentially allowing astronomers to discern how the quasar died.