Supermassive black holes lie at the center of most galaxies (not to be confused with the Muse song). The cores of some of these galaxies, known as active galactic nuclei, put out a lot of electromagnetic radiation (in particular radio waves). Astronomers think these emissions are powered by the black holes: accretion disks of material spiraling inward, counterintuitively, eject material along the axes that are perpendicular to the disks. This high-energy, high-speed material, called radio or relativistic jets, can account for the radiation we observe. We just haven't fully accounted for the processes that form these jets.

The galaxy M87 is one of the strongest sources of radio waves in the sky. Its black hole, accretion disk, and radio jet have been studied in the past, but it has been difficult to pin down the relative positions of the black hole and the base of the jets. Until now, at least: in today's Nature, scientists report observations that map out the shape of the jet and, from that, extrapolate the position of the black hole—the two appear to be much closer than in other galaxies.

The radio jets emit what is known as synchrotron radiation, which is generated by relativistic (moving near the speed of light) charged particles traveling on a curved path. Although the phenomenon is named after the man-made cyclical particle accelerators (synchrotrons) it also occurs naturally. In this case, it is thought that the spiraling material in the accretion disk generates strong magnetic fields that drive this acceleration. The mechanism that drives the formation of the jets themselves isn’t actually understood—it’s been known since the 1940s that material must be ejected to ensure the conservation of angular momentum, but no one knows exactly how this actually happens.

In order to get a better understanding of the mechanism behind jet formation, astronomers have observed their structures. But, so far at least, the resolution has been limited, and the location of the black hole driving the jet hasn't been determined with any accuracy.

The authors of the new paper used the radio jet itself to find the location of the black hole with respect to the base of the jet. They precisely measured the position (angular position, in milliseconds of arc) of six frequencies in the radio jet, and, assuming the jet is cone-shaped, fit the points to a power-law function. This can be done because of what’s known as the core-shift effect, which indicates that the frequency of the radio jet increases as you move towards the black hole generating the jet.

Once they had the equation describing the shape of the jet, where distance is roughly dependent on the inverse of frequency, they just extrapolated to find the base of the jet, where the frequency wouldn't increase further (the asymptotic limit)—this is at 43GHz.

To find the distance between the jet base and the actual black hole, the authors had to consider the viewing angle—the angle between the jet axis and line of sight. With this taken into account, they found that the separation distance was 0.007-0.01 parsec, or around 1011 km; this is equivalent to 14-23 multiples of the Schwarzschild radius of the black hole (which can be considered equivalent to the event horizon, for non-rotating black holes).

For context, that's also about 10 times the distance from the Sun to Voyager 1, which is now on the edge of the solar system.

This is a significantly smaller separation than any previous measurements, which ranged from 0.1-10 parsec, but were derived using different methods. The authors believe this discrepancy may be explained by different viewing angles, as well as the recent theory that radio jets may have more complex structure, with a faster internal flow surrounded by a slow layer. If this is the case, these conflicting reports may actually be measuring different components of the jets.

This study presents the first accurate measurement for the location of the black hole driving the radio jet at the center of M87. This is certainly an advance, and the results suggest additional complexity in the structure of the jet. Higher-resolution images will still be needed to probe the flow of matter from the accretion disk to the jet if we want to better understand the underlying mechanism.

Nature, 2011. DOI: 10.1038/nature10387 (About DOIs)