Weekly Science Update

Wednesday, August 13, 2008
Probing the Galaxy's Supermassive Black Hole
There is now overwhelming evidence that the center of our Milky Way galaxy contains a giant black hole with a mass of about four million suns. The most convincing data come from the motions of stars near the object. Their orbits, traced over sixteen years of observations, show them looping around an unseen mass of this size. Moreover, the data imply that the huge mass is concentrated into less than only 100 AU (one AU - astronomical unit - is the average distance of the earth from the sun). Finally, the stars are observed moving faster than 5000 km/s while an extremely bright radio source associated with the unseen mass appears motionless at the 1 km/s level or less, implying that it must contain much more mass than the stars swinging around it.

Recent measurements of the size of the radio source make it smaller than one AU. Combining the mass and radius of the radio source yields an incredibly high density that can only be achieved by a black hole. The formation and development of our galaxy was enormously influenced by this supermassive black hole (SMBH), and astronomers are therefore trying to understand as many of its properties as they can. It is, furthermore, also by far the closest such dramatic object to us, making it much easier to study than than its cousins at the centers of distant galaxies.

Astronomers think that a disk of very hot material surrounds most SMBHs, and that the disk is likely to have a hot spot (or spots) that rotates around the black hole. A team of CfA astronomers, Mark Reid, Avery Broderick, and Avi Loeb, along with two colleagues, used ultra-precise radio astronomy techniques to try to trace the motions of any hot spot via the apparent location of its radio emission over a few hours, the time it might take for such a spot to move enough to be detected. The measurement is extremely difficult because the radiation is faint yet the observation must be relatively brief (less than an hour), and because the hot disk is expected to have other processes that can confuse the result. Despite the difficulties, the team succeeded in setting a limit of less than about 0.6 AU for any motion of a hot spot - a remarkable precision that comes close to testing some of the fundamental relativistic theories of SMBH behavior. Further research, probably at millimeter or submillimeter rather than radio wavelengths, can extend this result into the regime where models predict hot spots should be detected on these short timescales.


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