A strange green blob in the nearby universe may be a 鈥榣ight echo鈥� from a long-dead quasar 鈥� an extremely bright object powered by a colossal black hole. Future studies of the blob could give astronomers insight into the quasar鈥檚 dying days.
For months, astronomers have been working to pinpoint what illuminates 鈥楬anny鈥檚 Voorwerp鈥�, a mysteriously lit cloud that sits close to a spiral galaxy 700 million light years from Earth.
鈥榁oorwerp鈥� is Dutch for 鈥榦bject鈥�, and 鈥楬anny鈥� refers to Hanny van Arkel, a Dutch school teacher. She found the blob as she was classifying galaxy shapes from astronomical images as part of the online project, which aims to harness the power of volunteers to answer cosmological questions.
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To find a light source for the cloud, astronomers turned to NASA鈥檚 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 鈥榓ctive鈥� 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 quiescent galaxies.
A galaxy called IC 2497 is thought to have once hosted the quasar that lit up Hanny鈥檚 Voorwerp.
Travel time
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.
鈥淎ssuming that are our explanation is correct, this is the first time we鈥檝e seen anything like this ever in the sky,鈥� says astronomer Chris Lintott of the University of Oxford. 鈥淲e鈥檝e never seen a galaxy we could point at and say, 鈥楾hat galaxy used to be active and is no longer.'鈥�
In the past, astronomers have found light from supernovae reflected in surrounding clouds of dust and gas. But Voorwerp shines not by reflection but by the re-emission of light absorbed from its mysterious source.
Hidden in plain sight
If the explanation holds, Voorwerp could give astronomers insight into the quasar鈥檚 last days.
Follow-up observations with the Hubble Space Telescope could give astronomers time-lapse information on the last days of the quasar, by comparing light from different areas of Voorwerp. Areas close to IC 2497 will emit light that came from the galaxy more recently.
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.
More observations are needed to determine if Voorwerp was truly energised by an expired quasar, says astronomer Meg Urry of Yale University. It may be that Voorwerp gets its energy from a jet or from a still-active quasar at the heart of IC 2497, one that is largely obscured from view by gas and dust.
Still, quasars in the local universe are rare. If astronomers find Voorwerp is emitting light it once absorbed from a quasar, 鈥渢his would be some kind of pointer to the possibility that there is more local black hole growth than we might have estimated originally鈥�, Urry told New 杏吧原创.
Voorwerp looks green because its light is emitted mostly by ionised oxygen. Lintott says the object was hidden for years in plain sight. 鈥淭his object鈥檚 been in the archive for decades. It鈥檚 on the plates from old surveys, but no one ever looked at it,鈥� he told New 杏吧原创.
Lintott says the Galaxy Zoo project might turn up more strange finds. In the coming weeks, a new version of the program will be released that will allow users to report unusual objects, including gravitational lenses, more easily.
The research has been submitted to the .
