Press Release

An Easy Method For Estimating Mass Of Distant Black Holes

By SpaceRef Editor
January 9, 2002
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COLUMBUS, Ohio – 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.

She will present her results Tuesday, January 8, at the American
Astronomical Society
meeting in Washington, DC.

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.

#

KRONOS: An Ohio State Proposal

In the not-too-distant future, an Ohio State-inspired
satellite observatory could be orbiting high above the earth, watching
black holes feast on far-away galaxies.

Bradley Peterson, professor of astronomy, has submitted
a proposal to NASA for the Kronos observatory, a satellite that will be
able to image material spiraling into black holes with a resolution 10,000
times finer than now possible with the Hubble Space Telescope.

Ohio State’s partners in this endeavor include
NASA Goddard Space Flight Center, the Space Telescope Science Institute,
Battelle Memorial Institute, Penn State University and Rutgers University,
as well as a host of national and international universities and observatories.

Should NASA choose to fund Kronos, the observatory
will turn its eye to some of the brightest objects in the universe.

Among galaxies, a special subset known as "active
galaxies" have very bright centers, or nuclei. These nuclei are so
bright as to outshine all the other stars in those galaxies. When astronomers
look to the sky to learn how galaxies form, it’s the active galactic nuclei
(AGNs) that are practically screaming for attention.

Current theory holds that very massive black holes
may literally be consuming the center of these galaxies. So much gas and
dust is being pulled into these black holes, astronomers think, that the
material becomes superheated and emits light. Large black holes in the
center of galaxies could play a major role in how galaxies are created,
and how they change over time.

Sometime in May 2002, NASA will announce which
four of its 40 received proposals have passed the first cut in its Medium
Explorer program. These four will undergo proof-of-concept studies, and
six months later a final two projects will be selected for spaceflight.

If Kronos is chosen, Ohio State will be the prime
contractor for the observatory, creating data analysis software, calibrating
the satellite’s orbit, and managing the mission in general. Until then,
details about the Kronos proposal can be found on the Web at http://www.astronomy.ohio-state.edu/~kronos/.

#

Contact: Marianne Vestergaard, (614) 292-5807; Vestergaard.1@osu.edu

During the AAS conference, Vestergaard can be reached at the Hilton Washington
and Towers at (202) 797-5820.

Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu

SpaceRef staff editor.