Orbiting Water Molecules Dance to Tune Of Galaxy’s “Central Engine,” Astronomers Say

A disk of water molecules orbiting a supermassive black hole at the core
of a galaxy 60 million light-years away is “reverberating” in response to
variations in the energy output from the galaxy’s powerful “central
engine” close to the black hole, astronomers say. The team of astronomers
used the National Science Foundation’s (NSF) Very Large Array (VLA) radio
telescope in New Mexico and the 100-meter-diameter radio telescope of the
Max Planck Institute for Radio Astronomy at Effelsberg, Germany, to observe
the galaxy NGC 1068 in the constellation Cetus. They announced their
findings today at the American Astronomical Society’s meeting in Atlanta.

The water molecules, in a disk some 5 light-years in diameter, are acting
as a set of giant cosmic radio-wave amplifiers, called masers. Using
energy radiated by the galaxy’s “central engine,” the molecules
strengthen, or brighten, radio emission at a particular frequency as seen
from Earth. “We have seen variations in the radio ‘brightness’ of these
cosmic amplifiers that we believe were caused by variations in the energy
output of the central engine,” said Jack Gallimore, an astronomer at the
National Radio Astronomy Observatory (NRAO) in Charlottesville, VA. “This
could provide us with a valuable new tool for learning about the central
engine itself,” he added.

Gallimore worked with Stefi Baum of the Space Telescope Science Institute
in Baltimore, MD; Christian Henkel of the Max Planck Institute for Radio
Astronomy in Bonn, Germany; Ian Glass of the South African Astronomical
Observatory; Mark Claussen of the NRAO in Socorro, NM; and Almudena
Prieto of the European Southern Observatory in Munich, Germany.

“Our observations show that NGC 1068 is the second-known case of a giant
disk of water molecules orbiting a supermassive black hole at a galaxy’s
core,” Gallimore said. The first case was the galaxy NGC 4258 (Messier
106), whose disk of radio-amplifying water molecules was measured by the
NSF’s Very Long Baseline Array (VLBA) radio telescope in 1995. Further
VLBA observations of NGC 4258 allowed astronomers to calculate an extremely
accurate distance to that galaxy last year.

“We’re excited to find this phenomenon in a second galaxy, but we’re also
tantalized by the evidence that these masers respond to variations of
the central engine,” Gallimore said. In order to amplify radio signals,
masers, like their visible-light counterparts, lasers, require a source
of energy, called the pumping energy. The scientists believe the masers
in NGC 1068 get that pumping energy from a highly-energetic, superhot
disk of material that is being pulled into the black hole. That disk,
called an accretion disk, emits X-rays that the astronomers think start a
chain of events that powers the masers.

Such accretion disks can be unstable, dramatically changing their energy
output from time to time. “When the accretion disk puts out more energy,
the masers should brighten, and when it puts out less energy, they should
get fainter. If the accretion disk gets too bright, however, water
molecules are destroyed and the masers turn off. We think that’s what
we’re seeing in this galaxy,” Gallimore said. “We want to watch this in
the future to learn more, not only about the masers, but also about the
accretion disk itself,” he said.

The strongest evidence that the masers are responding to variations in
the output of the central engine came from watching variations in the
brightness of masers on opposite sides of the water molecule disk. The
masers on both sides of the molecular disk, some 5 light-years across,
brightened within about two weeks of each other. “If this were caused by
something within that molecular disk itself, it would take about 10,000
years to affect both sides of the disk, because of the orbital times
involved. However, both sides of the disk are the same distance from the
central engine, so they can both respond to the central engine
simultaneously,” Gallimore explained.

The black hole at NGC 1068’s center, the scientists say, is about 10
million times more massive than the Sun.

NGC 1068 also is known as Messier 77 (M77), one of the objects listed in
French astronomer Charles Messier’s catalog of non-stellar objects.
First observed in 1780, it appeared in the version of Messier’s catalog
published in 1781. In 1914, Lowell Observatory astronomer Vesto Slipher
measured the Doppler shift in the galaxy’s light, showing that the galaxy
is receding from Earth at a speed of about 1,100 kilometers per second.
The galaxy’s water masers, which amplify radio signals at a frequency of
22 GHz, were discovered in 1984. The galaxy is visible in moderate-sized
amateur telescopes.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.