The results are reported in the July 10th issue of The Astrophysical Journal by John Kormendy (Institute for Astronomy, University of Hawaii) and Douglas Richstone (University of Michigan).
The observations consist of high-resolution spectra of the galaxy's center taken with the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii. These were used to measure velocities of stars via the Doppler shift - the redshift of the color of light produced by the motion of stars away from us. Kormendy and Richstone found that the nucleus of NGC 3115 spins very rapidly. Random velocities of stars also increase dramatically toward the center. Both observations weigh the galaxy. They show that a large mass is present in the nucleus; otherwise it would fly apart. A computer analysis of stellar orbits was carried out in Michigan; it rules out alternatives that might have explained the data without a black hole. A comparison of the measured mass with the known mass in stars then shows that most of the material is dark. This proves that the galaxy hides a central object that is not made of ordinary stars. It is probably a black hole.
The technique is unique because it directly detects the gravitational influence of the black hole on its surroundings. Only two other nuclei have similarly been weighed - the Andromeda Galaxy and M32. Both contain black holes with masses of a million to ten million Suns. The black hole in NGC 3115 is a hundred times more massive. To grow to one billion solar masses, it swallowed so much material that it could have been brighter than the brightest galaxies. Kormendy comments, "This is the best black hole candidate that is massive enough to have powered a quasar."
These findings strengthen the popular view that quasars - the brightest objects in the Universe - are powered by accretion onto massive black holes. Quasars can be seen farther away than any other object. In many cases, their light has been traveling toward us for most of the age of the Universe. Therefore we see quasars as they were long ago. As a result, astronomers can infer how the quasar population evolved with time. They find that quasars were numerous when the Universe was 1/4 of its present age. Now they have mostly died out. So dead quasars should be hiding in many nearby galaxies. Quasar energies imply that the dead remnants should have masses of a billion Suns. The discovery of a supermassive black hole is a crucial confirmation of the black hole accretion theory of quasars.
Ironically, NGC 3115 is otherwise undistinguished. It's name comes from its listing as object number 3115 in J. Dreyer's "New General Catalog" of nebulae and star clusters, published in 1888. The galaxy is visible in moderate-sized amateur telesopes as a faint fuzzy patch in the constellation Sextans, The Sextant. But at a distance of 30 million light years, NGC 3115 is more than ten times farther from us than Andromeda or M32. In reality, it is several times bigger than our own Milky Way. But its stars are mostly old, it contains virtually no gas, and little is going on now apart from the stately orbits of its stars. In particular, its nucleus is extremely inactive. The growth of the black hole and the nuclear activity that it feeds are over, unless additional stars wander too close to the center. Whenever that happens, the nucleus is expected to experience a brief but energetic rebirth.
Although these findings support our general picture of quasars, they also highlight a number of unresolved issues. "We have only a very speculative idea of how supermassive black holes form," Richstone said. "The processes that control their feeding, make them shine, and later turn them off are also poorly understood." Finding nearby black holes is crucial to further progress. NGC 3115 provides a billion-solar-mass example.
This work was supported by the National Science Foundation.
Released by NSF, the University of Hawaii Institute for Astronomy, and the University of Michigan.