Cosmic 'blob' divulges supermassive black hole's violent past


Echoes of a quasar survive in the glow of a nearby gassy cloud. Belinda Smith reports.


A composite image of galaxy IC 2497 (top object) and the Green Blob (bottom), which shows a remarkable change in the past behaviour of the galaxy's supermassive black hole.
X-ray: NASA / CXC / ETH Zurich / L. Sartori et al, Optical: NASA / STScI

In a piece of cosmological detective work, astronomers have connected clues from a galaxy and giant gas cloud around 650 million light-years away to trace the ferocity of the galaxy's supermassive black hole.

An international team calculated that the distance between the Green Blob – an gassy structure discovered in 2007 by a Dutch biology teacher as part of the Galaxy Zoo citizen science project – and galaxy IC 2497 is around 200,000 light-years. But new observations by the Chandra telescope found the supermassive black hole lying in the centre of IC 2497 isn't active enough to explain the presence of the Green Blob, which glows emerald thanks to excited oxygen atoms.

So, suggest Lia Sartori from ETH Zurich, Switzerland, and colleagues, the supermassive black hole once underwent a growth spurt, where it gobbled up surrounding gas and blasted humungous amounts of radiation – the echoes of which manifest as the Green Blob's glow.

The work was published in the Monthly Notices of the Royal Astronomical Society.

Astronomers suspect most large galaxies have a supermassive black hole residing in their centre. Our galaxy, the Milky Way, has one – it's around 27,000 light-years from Earth and a whopping 4.6 million times the mass of the sun.

Still, it's nowhere near the biggest supermassive black hole ever detected – they can range up to billions of times the sun's mass.

Supermassive black holes are mysterious objects, too. They've been found growing faster than they should, blasting massive jets out of their galaxy and shaping the space between galaxies.

So given the proximity of the brilliant Green Blob to IC 2497, astronomers thought the galaxy's supermassive black hole might be behind the Green Blob.

But when Sartori and her colleagues checked out Chandra telescope X-ray data, taken in January 2012, they were surprised to find that the black hole, while still active, was too quiet to provide enough energy to give the blob its vivid glow.

So what was going on?

The most likely culprit, they write, was recent a quasar – an incredibly compact stellar object that voraciously consumes material and blasts huge amounts of radiation that heats their surrounds.

But quasars live fast and die young – once their fuel is exhausted, they fade away, leaving their host galaxy behind.

This, Sartori and her crew think, is the likely scenario for IC 2497. They calculated the quasar's broad beam of radiation "switched off" only 200,000 years ago, very recently in cosmological terms.

The supermassive black hole continued to tick along blasting radiation – although not at quasar levels. The astronomers think a pair of jets from the black hole blew away surrounding hot gas, which explains why Chandra saw evidence of a cool bubble of gas in the centre of the galaxy.

But echoes of the quasar remain in the Green Blob's verdant blush.

And because objects similar to the Green Blob have been uncovered, they could also provide clues to nearby galaxies' histories.

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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