At
left is the galaxy J0702+5002, which the researchers concluded is not
an X-shaped galaxy whose form is caused by a merger. At right is the
galaxy J1043+3131, which is a "true" candidate for a merged system. Credit: Roberts, et al., NRAO/AUI/NSF
There may be fewer pairs of supermassive black holes orbiting each other at the cores of giant galaxies than previously thought, according to a new study by astronomers who analyzed data from the National Science Foundation's Karl G. Jansky Very Large Array (VLA) radio telescope.
Massive
galaxies harbor black holes with millions of times more mass than our
Sun at their centers. When two such galaxies collide, their supermassive
black holes join in a close orbital dance that ultimately results in
the pair combining. That process, scientists expect, is the strongest
source of the long-sought, elusive gravitational waves, still yet to be
directly detected.
"Gravitational waves represent the next great
frontier in astrophysics, and their detection will lead to new insights
on the Universe," said David Roberts of Brandeis University, lead author
of the research. "It's important to have as much information as
possible about the sources of these waves," he added.
Astronomers
worldwide have begun programs to monitor fast-rotating pulsars
throughout our Milky Way Galaxy in an attempt to detect gravitational
waves. These programs seek to measure shifts in the signals from the
pulsars caused by gravitational waves distorting the fabric of
space-time. Pulsars are spinning, superdense neutron stars that emit
lighthouse-like beams of light and radio waves that allow precise
measurement of their rotation rates.
Roberts and his colleagues
studied a sample of galaxies called "X-shaped radio galaxies," whose
peculiar structure indicated the possibility that the radio-emitting
jets of superfast particles ejected by disks of material swirling around
the central black holes of these galaxies have changed directions. The
change, astronomers had suggested, was caused by an earlier merger with
another galaxy, causing the spin axis of the black hole as well as the
jet axis to shift direction.
Working from an earlier list of 100
such objects, they collected archival data from the VLA to make new,
more detailed images of 52 of them. Their analysis of the new images led
them to conclude that only 11 are "genuine" candidates for galaxies
that have merged, causing their radio jets to change direction. The jet
changes in the other galaxies, they concluded, came from other causes.
Extrapolating
from this result, the astronomers estimated that fewer than 1.3 percent
of galaxies with extended radio emission have experienced mergers. This
rate is five times lower than previous estimates.
"This could
significantly lower the level of very-long-wave gravitational waves
coming from X-shaped radio galaxies, compared to earlier estimates,"
Roberts said. "It will be very important to relate gravitational waves
to objects we see through electromagnetic radiation, such as radio
waves, in order to advance our understanding of fundamental physics," he
added.
Gravitational waves, ripples in space-time, were
predicted in 1916 by Albert Einstein as part of his theory of general
relativity. The first evidence for such waves came from observations of a
pulsar orbiting another star, a system discovered in 1974 by Joseph
Taylor and Russell Hulse. Observations of this pair over several years
showed that their orbits are decaying at exactly the rate predicted by
Einstein's equations that indicate gravitational waves carrying energy
away from the system.
Taylor and Hulse received the 1993 Nobel
Prize in physics for this work, which confirmed a predicted effect of
gravitational waves. However, no direct detection of such waves has yet
been made.
Roberts worked with Jake Cohen and Jing Lu, Brandeis
undergraduates who retrieved the data from the VLA archive and produced
the images of the galaxies; and Lakshmi Saripalli and Ravi Subrahmanyan
of the Raman research Institute in Bangalore, India. The researchers
reported their results and analysis in a pair of papers in the Astrophysical Journal Letters and the Astrophysical Journal Supplements.
The
National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
Contact:
Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu
