Probing the Milky Way's Black Heart

Astronomers have taken their closest look yet at the black hole in the heart of our galaxy and uncovered a mystery. Just outside the hole, electrons torn from matter falling into the black hole gyrate around magnetic field lines, broadcasting radio waves. By mapping the radio emission with the Very Long Baseline Array, a system of linked telescopes that spans North America, astronomers have found that the source region is drastically elongated, suggesting that the black hole is somehow shooting jets of material out of the plane of the galaxy. Their report will appear in the November Astrophysical Journal Letters.

Earlier attempts to gauge the size and shape of the radio source near the black hole failed because of scattering by interstellar electrons, which made the radio source look larger than it really is, just as a streetlight looks larger when viewed in the mist. By combining near-simultaneous measurements at five different wavelengths, Kwok-Yung Lo of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei and his colleagues from Taiwan and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, were able to extract the true size and structure of the source from the scattering.

In the plane of the galaxy, they found, the radio source measures 150 million kilometers across--about the distance from Earth to the sun, and about 10 times the calculated diameter of the black hole itself. In the perpendicular direction, the source stretches nearly four times that distance. Those proportions imply that the black hole is somehow spurting out material, probably in two opposite directions. This geometry conflicts with a popular model of black holes, which holds that the radio waves come from extremely hot electrons in the inner parts of an accretion disk. Since the hot electrons would occupy a near-spherical region, the model is hard to reconcile with the observed elongated shape, says Lo.

There is a slight chance that the elongated shape is not genuine, cautions Cambridge University astronomer Martin Rees. It might instead be due to scattering in a preferred direction, as a result of asymmetric turbulence in the gas surrounding the black hole. But with the galactic center black hole only 26,000 light-years away, Lo and other radio astronomers have a good chance of ultimately sorting out its puzzles.