Hanny's Voorwerp
Back in 2007, Dutch Astronomer and school teacher Hanny van Arkel, while participating as an amateur volunteer in the Galaxy Zoo citizen science project, discovered an astronomical object of what was termed 'an unknown nature.' When Dan Smith, Peter Herbert and Chris Lintott saw it again through the 2.5 metre Isaac Newton Telescope in La Palma, Spain, they saw it as an unusually green cloud with no stars whatsoever and with a huge hole a whopping 16,000 light year across. It was nothing like they had ever seen before. The object was almost immediately named Hanny's Voorwerp (voorwerp meaning object in Dutch) after its discoverer.
Naturally, the scientific community was looking for an explanation for this. One plausible reason given was that it was the remnants of the nearby galaxy IC 2497, picturing the probable death of a quasar previously existing in the very heart of IC 2497, close to a 100,000 years ago.
Light Echo
Two years after its discovery, the reason for the 'one-of-a-kind' Hanny's Voorwerp finally came to light. The answer was in a paper submitted to the Astrophysical Journal of Letters on November the 6th 2010, titled "The Sudden Death of the Nearest Quasar" by Kevin Schawinski and team. The researchers believe the glow of the green blob is due to the powerful light emitted by a quasar - a radiation source powered by a black hole - that has now either dimmed or died.
This quasar was believed to be present in IC 2497 and, theoretically, if present, would have now been visible from the Earth merely through binoculars. Since it is not, the quasar has clearly died. But the light it emitted has reached the Voorwerp, a good tens of thousands of years travel away from it, and, having met the gaseous object, resulted in the apparent emission of the green light that we now see.
"The quasar itself is no longer visible to us," says Dr Chris Lintott of Oxford University, co-presenter of BBC's 'The Sky at Night,' "but its light continues to travel through space and the Voorwerp is a massive 'light echo' produced as the (quasar's) light strikes the gas."
The Voorwerp Unraveled
"This system is really like the Rosetta Stone of quasars," said the Yale astronomer, Schawinski, co-founder of Galaxy Zoo. "The amazing thing is that if it wasn't for the Voorwerp being illuminated nearby, the galaxy never would have piqued anyone's interest."
The team calculated that the light from the quasar took close to 70,000 years to travel to the Voorwerp which meant that the quasar must have shut down during the past 70,000 years. It was never really known how long a black hole took to die out this way. The numbers expected were in millions and the super-fast, 70,000 year long death came as a true surprise. "This," Schawinski said, "has huge implications for our understanding of how galaxies and black holes co-evolve."
The green colouring of the Voorwerp has been credited to ionised oxygen as the light particles strike the gas. Schawinski said his team had solved the mystery of the Voorwerp, "but," he confessed, "this discovery has raised a whole new bunch of questions." Indeed it is true. The exciting possibility of there being such quasar deaths previously missed out by astronomers has arisen and Shawinski and his team have decided to investigate next whether the radio waves released by the dead quasar had anything to do directly with shutting down the black hole.
Venkatram Harish Belvadi - Physics populariser, fiddler, blogger, photographer, writer and chief of the grammar police.
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