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Revealing the hidden spectrum

Photonics Spectra
Dec 2008
Krista Zanolli, krista.zanolli@laurin.com

SANTA BARBARA, Calif. – Much like fishermen wear polarized sunglasses to see past the reflective glare of the water’s surface to spot their sport, astronomers use polarized filters in telescopes to isolate the reflected light of accretion disks, or gas formations that surround black holes.

The friction from the spiraling within an accretion disk causes matter to heat and produce light in all wavelengths of the electromagnetic spectrum, creating what is sometimes referred to as an optical “mess.” While most of the light from the accretion disk is direct light (unpolarized), a small amount of that light is reflected (polarized).

PNpolar_AccretionDisc.jpg

A polarizing filter attached to a telescope suppresses the light emitted by dust particles and ionized gas clouds around the quasar so that its true electromagnetic spectrum can be revealed. Photo courtesy of M. Kishimoto, with cloud image by M. Schartmann.


Now, for the first time, a team of international researchers has plotted the reflected light using polarized filters, enabling them to substantiate the theory that black holes and their accretion disks form quasars, a theory that previously had not been proved because the quasar spectrum could not be physically observed.

The team, led by Makoto Kishimoto, an astronomer with the Max Planck Institute for Radio Astronomy in Bonn, Germany, and including Robert Antonucci and Omer Blaes, professors of physics at the University of California, Santa Barbara, have surmised that when the polarized light is isolated using polarized filters, they are able to see light from the disk that had not been seen before.

“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.

Kishimoto explains further by adding, “The crucial observational difficulty here has been that the disk is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disk. Because the light coming from the disk is scattered in the disk vicinity and thus polarized, one can, by observing only polarized light from the quasars, uncover the buried light from the disk.”

PNpolar_BlackHole.jpg
Shown is a spiral galaxy powered by a black hole. Photo courtesy of the Hubble Heritage Team (AURA/STScI/NASA).


Antonucci goes on to explain, “There is also a little scattered gas just a few light-days from the black hole, and that is the secret to our new work. Since this bit of scattering gas very close to the black hole is placed well inside the contaminating source of light, it doesn’t scatter dust particles efficiently into the line of sight, but the actual black hole engine is scattered into the line of sight, and plotting the polarized (scattered) light spectrum allows us to see the true central engine spectrum for the first time.”

“This work has greatly strengthened the evidence for the accepted explanation of quasars,” Antonucci said. “With this knowledge, we have a better understanding of how black holes consume matter and expand.”


GLOSSARY
electromagnetic spectrum
The total range of wavelengths, extending from the shortest to the longest wavelength or conversely, that can be generated physically. This range of electromagnetic wavelengths extends practically from zero to infinity and includes the visible portion of the spectrum known as light.
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