Easy method for estimating mass of distant black holes
Posted: January 17, 2002

An Ohio State University astronomer has developed a method for reliably estimating the mass of black holes in distant quasars.

Based on this method, Marianne Vestergaard, a postdoctoral astronomy researcher at Ohio State, has begun to address the longstanding issue of why only a small fraction of quasars are capable of producing very powerful radio emission.

Whereas normal galaxies emit energy proportionate to the number of stars contained within, a tiny subset known as "active" galaxies emit much more energy from their nuclei than can be readily explained by starlight alone. This radiation is detected at wavelengths that span from radio waves to X-rays, Vestergaard explained.

Quasars are thought to be the most energetic of the active galaxies, from which all the energy spills out of a very small region at the center, equal to about one-millionth of the diameter of the total galaxy. A small fraction of quasars emit more than 100 times as much radio energy as others, and they are deemed "radio-loud" quasars.

Current theory holds that black holes reside in these galaxies, and are ultimately responsible for most of the nuclear radiation by consuming and reprocessing a great deal of galactic material -- and spitting out the excess energy that results.

Vestergaard said the new results are important because they mean astronomers will need to look beyond the central "engine" that powers these active galaxies to understand why they would have different radio properties. She cited other recent studies that suggest the type of host galaxy and perhaps galaxy cluster environment aren't important either, as earlier thought.

"This means the puzzle is far from solved," she said.

To understand how distant black holes in quasars and other active galaxies form and evolve over time, astronomers must measure their mass, as well as their accretion properties -- meaning how much galactic material is being pulled into the center of the galaxy.

The problem -- the standard methods used for measuring the mass of nearby black holes either do not work with more distant luminous objects, or would take several decades to employ.

Vestergaard devised an easy method for estimating the masses of these distant black holes. Using measurements that Bradley Peterson, professor of astronomy at Ohio State, and his colleagues obtained for black holes in nearby active galaxies, she was able to calibrate measurements for black holes in far-away quasars.

Her method shows promise, she said, because for most quasars, she can estimate the black hole mass to within a factor of three -- quite a good estimate given the uncertainties in the source data, she said.

In previous studies, other astronomers have suggested that the black holes in radio-loud quasars are several times larger than those in radio-quiet quasars, implying that there is some threshold mass above which a black hole will cause a quasar to become radio-loud.

But based on a sample of objects much larger than in any previous study, Vestergaard's calibration method lends no support of these recent claims.

She applied her calibration technique to data from the Large Bright Quasar Survey, which documented more than 1,000 quasars. The survey was completed in 1995 by astronomers at the Multiple Mirror Telescope Observatory, a joint facility of the Smithsonian Institution and the University of Arizona located near Tucson.

Apart from their different levels of radio emission and sometimes X-ray emission, the radio-loud and radio-quiet quasars had relatively similar characteristics. She found no correlation between mass and radio signal.

When she looked closer at those earlier studies, she found intrinsic luminosity, or brightness, differences between the radio-loud and radio-quiet quasars that had been examined. Vestergaard determined that these brightness differences biased the earlier results in such a way as to make the mass of a black hole seem more important to the radio signal than it really is.

The same calibration method that allowed Vestergaard to reliably estimate the central black hole mass of distant quasars will be important for another Ohio State project: Kronos, a multi-wavelength observatory that Peterson and his colleagues have recently proposed to NASA. If approved, Kronos would map the environments of black holes with a resolution 10,000 times finer than that provided by the Hubble Space Telescope.

"The bottom line is that we need Kronos and its monitoring programs in order to improve the current calibration, and in turn improve our studies of the evolution of super-massive black holes in the universe," Vestergaard said.