Blue Color of Quasar Accretion Disk Verified

The United Kingdom’s Infra-Red Telescope (UKIRT), the world’s largest telescope dedicated solely to infrared astronomy, which had a polarizing filter attached to it to study accretion disk emissions (Credit: United Kingdom Science and Technology Facilities Council, the Joint Astronomy Centre)

Quasars are extremely powerful and distant active galactic nuclei. It is believed they are powered by accretion of interstellar gas onto the black holes at the centers of galaxies. The gas swirls in an accretion disk surrounding the black hole, thus heating up and emitting radiation. Ultimately, the gas falls into the black hole and contributes to its mass.

Theories predict that the spectrum of the radiation emitted from the accretion disk is mostly blue. However, this property as well as other suspected properties of the accretion disk, is difficult to verify with solely observational tools. The problem stems from the fact that the disk is surrounded by dust clouds that also emit radiation. Thus when studying the emission from a quasar it is difficult to isolate the radiation that traces back to the accretion disk from the rest of the radiation caused by the dust clouds.

The new research, conducted by an international team of astronomers including Makoto Kishimoto from the Max Planck Institute for Radio Astronomy in Bonn who led the team, is based on a new method of analysis of the emission spectra. Some of the light emitted from the accretion disk, which is in fact the part of the light that the astronomers wanted to study, is scattered, presumably off electrons. This scattering seems to take place before the light reaches the dust cloud and causes it to be polarized (meaning the electrical field of the light has a preferred direction).

Taking advantage of the fact that light emitted from the dust clouds is not polarized, the team attached polarizing filters to one of the VLTs (very large telescopes) in Chile and also to the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii. This resulted in an emission spectrum of the accretion disk, untainted by dust cloud emissions. This spectrum showed the accretion disk is blue, thus confirming theories on the matter.

Artist’s impression of an Active Galactic Nucleus (AGN). A supermassive black hole in the very center is surrounded by an accretion disk and messy dust clouds. Strong jets radiate perpendicular to the accretion disk. (Credit: NASA E/PO – Sonoma State University, Aurore Simonnet)

Many questions regarding the behavior of quasars remain, but the latest research shows astronomers are on the right track. Dr. Robert Antonucci, one of the team members from the University of California at Santa Barbara, said: “Our understanding of the physical processes in the disk is still rather poor, but now at least we are confident of the overall picture.”

Future research may address the issues of the accretion disk edge – it is still not clear exactly where the disk ends, as well as the mechanism that supplies matter to the nucleus. “In the near future, our new method may pioneer the way to address these questions,” Makoto Kishimoto said.

TFOT recently reported on research confirming the leading theory regarding the behavior of galactic black holes, according to which the particles are accelerated by tightly-twisted magnetic fields close to the black hole. Another story covered the observation of the light echo of an enormous X-ray flare, which may offer researchers a new method for mapping galactic nuclei. The importance of analyzing quasar spectra was demonstrated in an article about new evidence supporting the consistency of the proton-electron mass ratio, which resulted from quasar observations.

Further information on the new research can be found in the paper published in Nature magazine (PDF).