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.
Ray Villard, STScI
Dr. Holland Ford, STScI/JHU