Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
The image of the X-ray jet at http://chandra.harvard.edu/photo/2000/pictor/index.html and it's presumed hot spot appears to be a straight line (at least from our perspective). The source and the hot spot are estimated to be at least 800,000 light years apart. This would imply that the location and direction and jet must have remained constant for almost a million years (I'm assuming the X-rays travel at the speed of light). How can such a narrow beam maintain such a precise direction for such a long time?

A:
You have identified one of the major puzzles connected with radio galaxies and quasars. The basic idea is that the jet is produced by the amplification of magnetic fields in matter that is swirling into a black hole. The shearing motion in the disk creates a magnetic coil that expels some of the infalling matter, and pinches it into a narrow jet. Supercomputer simulations indicate that jets moving close to the speed of light can be produced in this way. The rotation of the black hole defines an axis that would be stable over many millions of years. Many details, such as how the high-energy particles needed to explain the X-rays, and the relation between the rate of infall, spin of black hole, mass of black hole, and power of the jet. For a nice summary of the present state of research, see D. Meier et al, Science, 291, 84 (2001).