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