Long ago astronomers anticipated finding in the Galactic center a counterpart to the power plants of the distant quasars and other galaxies with active galactic nuclei (AGN). The universal belief in the astrophysical community is that the AGNs are powered by black holes with masses ranging from 1 million solar masses to 1 billion solar masses. The discovery 3 decades ago of Sgr A*, a bright point radio source that resembles radio-loud AGNs, intensified the expectation that our own Galaxy contains a massive black hole that generates energy in the same way as the black holes in AGNs. We now know that Sgr A* is a 4 million solar mass black hole candidate.
While the central Galactic black hole is bright by solar standards, with the power radiated as radio waves over 100 times the total power generated by the Sun, it is very tame by extragalactic standards. AGNs can generate a billion times the power of Sgr A*. Despite this difference in power, researchers had hoped that similar physics powered both Sgr A* and the distant active galactic nuclei, which would enable them to learn about the physics of the very distant AGNs by studying the relatively nearby central Galactic black hole.
This hope is vain. To be a low-luminosity proxy of the AGNs, Sgr A* must release about 10% of its power as x-rays, but instead it is a dim x-ray source, with the power generated as x-ray less than 0.1% of the radio power. The physics of Sgr A* is likely to be too different from the physics of AGNs to be instructive.
Personally I prefer seeing a quiet black hole at the Galactic center. Sgr A* gives us the best chance of testing general relativity where gravitational fields are strong. It covers a larger area on the sky than any other black hole candidate. If the region close to the Sgr A* event horizon is relatively free of gas, which is not expected in an AGN-like environment, the chances that Sgr A* can provide a test of general relativity improve.
The properties of the radio, infrared, and x-ray radiation emitted by Sgr A*, and their incongruity with the radiation from AGNs are discussed in more detail in a new page added to the “Milky Way Galaxy” path in this issue.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for September 19.
The Appearance of Sgr A*. The largest object in our Galaxy at 3.6 million solar masses is the massive black hole candidate Sagittarius A*. The power generated by this object, however, does not match its size. The central black hole is only bright at radio frequencies, producing only several hundred times the power generated by the Sun. In the infrared and x-ray energy bands, the black hole candidate is unexpectedly dim. Numerous sources within the Milky Way Galaxy outshine this giant at the Galactic center. The low power output at at x-ray frequencies dashed hopes that Sgr A* could serve as a nearby proxy for the distant active galactic nuclei. (continue)