With Polarization Studies, Astronomers Creep Closer to Black Holes

Hank Hogan

By measuring the polarization of radiation from distant objects to an accuracy of better than 1/20 of a percent, astronomers have pushed their observations perhaps 100 times closer to the edge of a black hole than previously had been accomplished. The international group, led by Makoto Kishimoto of the University of Edinburgh in the UK, studied the polarized visible and near-UV radiation emitted by quasars.

Quasars are extremely distant point sources in the sky that are as bright as entire galaxies. Some astronomers theorize that they are the result of matter falling into supermassive black holes at the center of galaxies in the early formation of the universe. Verifying the theory is difficult because even these monster black holes are small, far-off and shrouded.

The team, working with instruments in Hawaii and Chile, looked at the radiation from a handful of quasars. The polarized output, Kishimoto noted, represented only 1 to 2 percent of the total energy spilling out from the quasars. The rest of the photons were unpolarized. What the researchers found in the polarized signal was a Balmer edge, a sudden and revealing change in the spectral output. The quasars' maximum polarized flux was near 400 nm and its minimum, around 360 nm.

The unpolarized broad emission lines mask telltale spectral features from within 100 light days of a black hole. The researchers believe that the polarized light originates inside this distance and so carries information about the region. The polarization, Kishimoto said, is probably the result of some form of scattering, but it's unclear just how near the black hole this scattering is taking place. "If the polarization is from the quasar 'engine' itself, then the polarized flux is coming from the region of on the order of one light-day," he said.

Kishimoto added that the orientation of the polarization with respect to a quasar's radio jet indicates that the polarization phenomenon probably arises at a distance from the black hole of between one and 100 light days. Resolving this and other questions requires more data, and the researchers plan additional observations.

He and others are looking at polarization in the near-IR. He hopes eventually to have infrared polarization-measurement instruments with accuracies as good as those available for the visible spectrum.