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A new study of a strange green glowing object in intergalactic space has supported the theory that it's the ghostly glow of a long-dead quasar.

The finding has given astronomers a new understanding of how quasars live and die.

While sorting through hundreds of galaxy images as part of the Galaxy Zoo citizen science project two years ago, Dutch school teacher and volunteer astronomer Hanny van Arkel stumbled upon the strange-looking green object.

Floating all alone in intergalactic space the mysterious cloud became known as Hanny's Voorwerp, which is Dutch for Hanny's object.

Located near the spiral galaxy IC 2497 some 700 million light years away in the constellation Leo Minor, it's baffled professional astronomers for years.

Now a report accepted for publication in Astrophysical Journal Letters by a team of scientists lead by astronomer Kevin Schawinski from Yale University claims the Voorwerp is a large cloud of glowing gas illuminated by the light from a quasar.

Quasars are powerful beams of energy generated by supermassive black holes consuming matter in the centre of active galaxies.
Quasar light echo

What makes Hanny's Voorwerp different is that the quasar lighting up the gas, has since burned out almost entirely, even though the light it emitted in the past continues to travel through space, illuminating the gas cloud and producing a sort of "light echo" of the dead quasar.

The idea of the Voorwerp being a quasar light echo isn't new, but was dismissed following evidence that the quasar was still active.

Confusing the issue further was the discovery of another cloud of dust and gas preventing scientists from directly seeing the black hole generating the quasar.

Schawinski says "the system really is like the Rosetta Stone of quasars".

"The amazing thing is that if it wasn't for the Voorwerp being illuminated nearby, the galaxy never would have piqued anyone's interest."

Schawinski calculated that the light from a quasar in a galaxy 70,000 light years from the Voorwerp caused it to light up.

The quasar has since died down and could only be detected through weak X-ray emissions and radio wavelengths from matter being crushed and torn apart at the sub atomic level before falling beyond the black hole's event horizon.

The fact that the Voorwerp is still illuminated means the quasar must have shut down sometime within the past 70,000 years.

Until now scientists assumed supermassive black holes took millions of years to die down after reaching their peak energy output. However, the Voorwerp suggests they can shut down much more quickly.

Schawinski says "this has huge implications for our understanding of how galaxies and their supermassive black holes co-evolve".

Dr Darren Croton a theoretical astrophysicist at Melbourne's Swinburne University who was a supporter of the original light echo theory says "the new work shows that science is all about competing ideas".

"Observation always triumphs, we should never stop investigating."

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