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Another quasar mystery solved

An astronomer has explained why quasars appear to have different colours. Cathal O’Connell reports.

A Hubble telescope image of Bright Quasar 3C 273. Now scientists know why the light from quasars is emitted at different wavelengths. – ESA/Hubble & NASA

When astronomers aimed radio telescopes at the sky in the 1960s they got a surprise. Beaming back at them were pinpoint sources of light a trillion times brighter than the Sun. These quasi-stellar objects – or quasars – are the most luminous objects in the known universe. Astronomers now realise quasars are caused by voracious black holes in the centre of newborn galaxies. But an enduring mystery has been why their light is emitted at very different wavelengths. Now Yue Shen and his team at the Carnegie Observatories in California have come up with an explanation as reported in Nature this September.

Astronomers were stumped when they first detected quasars shining 100 times brighter than our Milky Way. Some dismissed them as impossible; others suggested that perhaps an alien civilization had learnt to harness the energy of an entire galaxy. But by the 1980s astronomers agreed there was something in the known universe that could explain them – the supermassive black holes at the centres of distant galaxies. A black hole sucks in surrounding galactic material to create a swirling, flat donut-shaped cloud around it called an accretion disc. As matter is sucked into the black hole at extraordinary densities and speeds, it radiates a white-hot glow far hotter than the centre of any star. That intense glow is the quasar we see from Earth.

Quasars are seen in the most distant galaxies at the edge of the visible universe. Because their light takes so long to reach us, we see them as they were in their youthful first billion years with black holes gorging on the plentiful material at their galactic centres. Nearby galaxies like our Milky Way reveal a picture of a more mature black hole that has already eaten the heart out of the galaxy and no longer generates quasars.

An artist's concept of the accretion disk around a binary star system. – P. Marenfeld and NOAO/AURA/NSF

But quasars still presented a puzzle. Though they were all intensely bright they shone with different colours. Some could be seen with infrared telescopes, but others could only be seen using radio wave or X-ray telescopes. It became “a real mess,” says Paul Francis, an astronomy researcher at the Australian National University. “There’s a whole zoo of these things.” So was it a zoo of different animals or all the same species?

The new research reveals they are all the same species. It’s just a matter of how voracious they are and what angle we are viewing them from. Yue Shen and his team came to this conclusion by mining the data from the Sloan Digital Sky Survey, a repository of 20,000 known quasars. They measured how voraciously the black hole swallows galactic matter from the Eddington Ratio. It’s calculated by comparing the luminosity of the quasar to the mass of the black hole (estimated from the event horizon, the radius beyond which light cannot escape. They also examined how the angle from which we view the accretion disc could affect the wavelength. If we could see it, the flat donut-shaped cloud might look either like a thin band or like a ring. Likewise depending on the angle of view, the disc would emit x-rays while from another angle it would emit infrared.

While the new analysis helps to explain the diversity of quasars, Francis suspects this is not the end of the story. “That would be like saying you can define a person by age and gender,” he says. “Quasars are like people, there’s more to them than two numbers.”

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Cathal O'Connell is a science writer based in Melbourne.
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