CONTACT: Ray Villard, STScI Wednesday, May 25, 1994
(410) 338-4514
PRESS RELEASE NO.: STScI-PR94-23
Dr. Holland Ford, STScI/JHU
(410) 338-4803
(410) 516-8653
HUBBLE CONFIRMS EXISTENCE OF MASSIVE BLACK HOLE AT HEART OF
ACTIVE GALAXY
Astronomers using NASA's Hubble Space Telescope have found
seemingly conclusive evidence for a massive black hole in the
center of the giant elliptical galaxy M87, located 50 million
light years away in the constellation Virgo. Earlier
observations suggested the black hole was present, but were not
decisive.
This observation provides very strong support for the existence
of gravitationally collapsed objects, which were predicted 80
years ago by Albert Einstein's general theory of relativity.
"If it isn't a black hole, then I don't know what it is," says
Dr. Holland Ford of the Space Telescope Science Institute and The
Johns Hopkins University in Baltimore, Maryland.
"A massive black hole is actually the conservative explanation
for what we see in M87. If it's not a black hole, it must be
something even harder to understand with our present theories of
astrophysics," adds fellow investigator Dr. Richard Harms of the
Applied Research Corp. in Landover, Maryland.
The discovery is based on velocity measurements of a whirlpool of
hot gas that is orbiting around the black hole in the form of a
disk. The presence of the disk, discovered in recent Hubble
images, allows for an unprecedented, precise measurement of the
mass of the object at the hub of the disk.
A black hole is an object that is so massive yet compact nothing
can escape its gravitational pull, not even light. The object at
the center of M87 fits that description. It weights as much as
three billion suns, but is concentrated into a space no larger
than our solar system.
Now that astronomers have seen the signature of the tremendous
gravitational field at the center of M87, it is clear that the
region contains only a fraction of the number of stars that would
be necessary to create such a powerful attraction. There must be
something else there that cannot be seen.
Ford and Harms were astounded by the M87 images taken with the
telescope's Wide Field Planetary Camera-2 (in PC mode) on Feb.
27. They hadn't anticipated seeing such clear evidence of a
gaseous disk in the center of M87.
"It's just totally unexpected to see the spiral-like structure in
the center of an elliptical galaxy," Ford says.
Ford and Harms used HST's Faint Object Spectrograph to measure
the speeds of orbiting gas on either side of the disk from
regions located about 60 light-years from the black hole at the
center.
They calculated that the disk of hot (about 10,000 Kelvin),
ionized gas is rotating at tremendous speeds around a central
object that is extremely massive but extraordinarily compact -- a
black hole.
"Once you get that measurement, all you need is straightforward
Newtonian physics to calculate the mass of the central object
that's making the disk spin," says Harms.
The measurement was made by studying how the light from the disk
is blueshifted and redshifted -- as one side of the disk spins
toward us and the other side spins away from us. The gas on one
side of the disk is speeding away from Earth, at a speed of about
1.2 million miles per hour (550 kilometers per second). The gas
on the other side of the disk is whipping around at the same
speed, but in the opposite direction, as it approaches viewers on
Earth.
"Now, it all knits together," Ford said. "We see a disk-like
structure that appears to have spiral structure, and it's
rotating. One side is approaching, and the other is receding."
The cloud of gas is composed mostly of hydrogen. The hydrogen
atoms have been ionized, or stripped of their single electron,
possibly by radiation originating near the black hole.
Over the next few months, they will attempt to peer even closer
to the center, where the disk should be spinning at even higher
speeds, improving the measurement of the black hole's mass.
M87: A NEARBY ACTIVE GALAXY
Since observations as early as 1917, astronomers have suspected
that unusual activity was taking place in the center of M87.
They discovered a long finger of energy emanating from the
nucleus. Investigations using radio telescopes in the 1950s
detected large emissions of energy from the galaxy. This made it
clear that the bright optical jet and radio source were the
result of energy released by something in the center of the
galaxy.
In high resolution images, the jet appears as a string of knots
(some as small as ten light-years across) within a widening cone
extending out from M87's core. A massive black hole had been the
suspected "engine" for generating the enormous energies that
power the jet. The gravitational energy is released by gas
falling into the black hole, producing a beam or jet of electrons
spiraling outward at nearly the speed of light.
HUNTING FOR BLACK HOLES
Hubble's observation confirms more than two centuries of theory
and conjecture about the reality of black holes. The term black
hole was coined in 1967 by American physicist John Wheeler.
However, French scientist Simone Pierre LaPlace first speculated
that "dark stars" might exist, which would have such intense
gravitation that light itself could not escape. This conjecture
was put into a theoretical framework with Einstein's general
theory of relativity, published in 1915, which postulated that
very massive objects actually warp space and time. The theory
was supported in 1916 when German physicist Karl Schwarzschild
described the mathematical basis behind black holes.
For decades, however, black holes were regarded not as real
astronomical objects, but merely as mathematical curiosities.
With the discovery of active galaxies and quasars, black holes
have become the favored "engine" for explaining a wide array of
powerful and energetic events seen in the universe.
Earlier Hubble Space Telescope observations found strong
circumstantial evidence for the presence of a massive black hole
in the core of M87, as well and other galaxies -- both active and
quiescent. These observations show a rapid increase in starlight
toward the center of a galaxy. This suggests that stars are
concentrated around the center due to the gravitational pull of a
massive black hole. However, the black hole's mass could not be
determined until Hubble's spectroscopic capabilities were used to
measure the actual motion of gas around the black hole. Such
high spatial resolution spectroscopic observations were not
possible prior to the installation of the COSTAR by the
astronauts during the December 1993 First Servicing Mission.
The research team included Holland Ford at the Johns Hopkins
University and STScI; Richard Harms at Applied Research Corp. in
Landover, Md.; and astronomers Zlatan Tsvetanov, Arthur Davidsen,
and Gerard Kriss at Johns Hopkins; Ralph Bohlin and George Hartig
at Space Telescope Science Institute; Linda Dressel and Ajay K.
Kochhar at Applied Research Corp. in Landover, Md.; and Bruce
Margon from the University of Washington in Seattle.
*********************************
The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.
(AURA) for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, MD. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space
Agency (ESA).
