Polarizing filter allows astronomers to see disks surrounding black holes

July 23, 2008
A polarizing filter attached to a telescope suppresses the light emitted by dust particles and ionized gas clouds around the quasar so its true electromagnetic spectrum can be revealed. Image: Makoto Kishimoto, with cloud image by Schartmann

(PhysOrg.com) -- For the first time, a team of international researchers has found a way to view the accretion disks surrounding black holes and verify that their true electromagnetic spectra match what astronomers have long predicted they would be. Their work will be published in the July 24 issue of the science journal Nature.

A black hole and its bright accretion disk have been thought to form a quasar, the powerful light source at the center of some distant galaxies. Using a polarizing filter, the research team, which included Robert Antonucci and Omer Blaes, professors of physics at the University of California, Santa Barbara, isolated the light emitted by the accretion disk from that produced by other matter in the vicinity of the black hole.

"This work has greatly strengthened the evidence for the accepted explanation of quasars," said Antonucci.

According to Antonucci, the physical process that astronomers find most appealing to explain a quasar's energy source and light production involves matter falling toward a supermassive black hole and swirling around in a disk as it makes its way to the event horizon - the spherical surface that marks the boundary of the black hole. In the process, friction causes the matter to heat up such that it produces light in all wavelengths of the spectrum, including infrared, visible, and ultraviolet. Finally, the matter falls into the black hole and thereby increases the black hole's mass.

"If that's true, we can predict from the laws of physics what the electromagnetic spectrum of the quasar should be," said Antonucci. But testing the prediction has been impossible until now because astronomers have not been able to distinguish between the light emanating from the accretion disk and that of dust particle and ionized gas clouds in the area of the black hole.

By attaching a polarizing filter to the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii, the research team, led by Makoto Kishimoto, an astronomer with the Max-Plank Institute for Radio Astronomy in Bonn, and a former postdoctoral fellow at UCSB, eliminated the extraneous light and was able to measure the spectrum of the accretion disk. Doing so, they demonstrated that the spectrum matches what previously had been predicted. The researchers also used extensive data gathered from the polarization analyzer of the Very Large Telescope, an observatory in Chile that is operated by the European Space Observatory.

What makes the polarizing filter able to perform its magic is the fact that direct light is not polarized - that is, it has no preference in terms of the directional alignment of its electrical field. The accretion disk emanates direct light, as do the dust particles and ionized gas. However, a small amount of light from the accretion disk, which is the exact light the researchers want to study, reflects off gas located very close to the black hole. This light is polarized.

"So if we plot only polarized light, it's as if the additional light isn't there and we can see the true spectrum of the accretion disk," Antonucci said. "With this knowledge we have a better understanding of how black holes consume matter and expand."

Studying the spectrum of a glowing object such as a quasar provides astronomers with an incredible amount of valuable information about its properties and processes, Antonucci noted. "Our understanding of the physical processes in the disk is still rather poor, but now at least we are confident of the overall picture," he said.

Provided by University of California - Santa Barbara

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5 / 5 (1) Jul 24, 2008
I'm neither a physicist or astronomer but my common sense take is this:-

If the accretion disk is radiating at all frequences due to the heat developed by friction then that must include the frequencies radiated by the neighbouring dust/gas clouds. How, then, can they tell the difference between the polarised reflections from the disk and the polarised reflections from the surrounding gas/dust? Both are direct radiators and both will be reflected in a polarised way.

Go on, shoot me down in flames.
5 / 5 (1) Jul 24, 2008
Hi Smiffy!
I believe to understand that surrounding gas and dust absorb the light from the accretion disk and reemit it at other wavelength, which isn't a reflection, and doesn't polarize the emitted light.

Only the fraction "bumping" on nearby gas would be polarized.

Anyway, I wish the articles would give more details. And what are exactly the provided pictures? Are the ones on right-hand side legitimate pictures, or just paintings, with the left-hand ones being true images?

The upper right for instance is a composite - if people could distinguish the disk from dust in that way, they wouldn't need extra processing. Hope it's a partially processing image, or a superposition.
not rated yet Jul 25, 2008
As far as the pictures are concerned they must be only illustrations. The ones on the right are
surely far too detailed for a quasar which as I understand it are hundreds of millions of light years away. The ones on the left amount to different coloured blots - and surely these can't be the true colours due to the large red shift involved. I suppose it's almost mandatory for the authors to provide a picture for the article - as in TV news.

I've been trying to get my head around what the
local effect of the disk on surrounding gas/dust would be. If it's confined to the rim of the disk then I just can't see how that could throw
up a polarised profile, and even if it did then the polarisation would be due to the outer part of the accretion disk only. This part would not be representative of the disk as a whole and would only radiate in a narrow temperature range, presumably at the lower end. (things getting hotter the nearer the centre)

If it's due to the the entire disk striking the surounding gas/dust then this would have a polarising effect if line-of-sight and radiated light were at right angles, meaning the polarisation would be strong when the disk was edge on. Perhaps even at an angle there is sufficient to get a result. If this is the case then the light from the centre of the disk has further to travel through the empty shell before
it strikes the cloud, and this light would have to penetrate more of the cloud on its way to observers on Earth, skewing the results?

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