June 30, 1999 COLUMBUS, Ohio -- Astronomers at Ohio State University have discovered more evidence of the existence of a black hole in the center of a galaxy.
The astronomers have determined that dense clouds of gas are orbiting the center of the galaxy NGC 5548 in such a way as to suggest the presence of a compact, supermassive object at the center.
Bradley M. Peterson, professor of astronomy at Ohio State, conducted this work with Amri Wandel, visiting professor at the University of California, Los Angeles. They presented their results June 1 in Chicago at the annual meeting of the American Astronomical Society.
NGC 5548, in the constellation Bootes, is one of a bright class of galaxies whose massive centers contain active galactic nuclei (AGN). Such nuclei are so bright, they outshine the stars in the galaxy.
While AGNs are among the brightest objects in the universe, they are also very small. Astronomers think their energy comes from gas heating up as it spirals into a massive black hole at the center.
One way to tell whether or not the mass in the center of an AGN is a black hole is to examine the motion of material orbiting the center. This can be done by studying changes in the light from the AGN over a period of time.
Peterson and Wandel examined data on NGC 5548 that they collected with the International Ultraviolet Explorer satellite and ground-based telescopes in 1989 and 1993. In 1993, this galaxy was also observed with the Hubble Space Telescope, and these data were also used in this study.
They looked at changes in the spectrum of ultraviolet and visible light from the galaxy, and used that information to calculate the motions of the clouds and their distance from the center.
Peterson and Wandel’s calculations implied the existence of a small, supermassive object at the center of NGC 5548 -- a mass equivalent to 70 million suns compressed into a space only about 250 times larger than the Earth’s orbit around the Sun.
“It’s very hard to believe that an object so massive and so small is anything other than a black hole,” said Peterson.
Peterson likened the orbit of the gas clouds around NGC 5548 to the motion of the planets around our solar system. Clouds far from the center of the galaxy move more slowly than clouds closer to the center. Likewise, the outer planets in our solar system, such as Jupiter or Saturn, move more slowly in their orbits around the Sun than inner planets such as Earth or Mars.
Because the detailed shapes of the clouds orbits cannot be determined from present data, the black hole mass is rather uncertain, but is almost certainly in the 10 to 100 million solar mass range, Peterson said.
Actual proof that the central masses in galaxies must be black holes would require detection of subtle effects predicted by Einstein’s theory of relativity. Such effects would be observable only about 100 times closer to the central mass than the gas clouds whose motions were analyzed by Peterson and Wandel.
“What we’re doing is we’re heaping on the circumstantial evidence,” said Peterson. “And this is among the best evidence yet.”
With UCLA professor of astronomy Matthew A. Malkan, Peterson and Wandel have recently extended this work to other AGNs. In the 19 active galaxies studied so far, the range of masses detected is about 1 million to about 100 million solar masses.
This work was funded by National Science Foundation and NASA.
The International Ultraviolet Explorer satellite, which ceased operations in 1996, resulted from a collaboration among NASA, the European Space Agency (ESA), and the United Kingdom’s Particle Physics and Astronomy Research Council (PPARC). The Hubble Space Telescope is a current NASA-ESA project.
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