Christopher
Hunter
Ph.D. Cambridge 1960
I am working on several problems that are related to the
internal dynamics of galaxies. Galaxies are composed primarily
of stars, and so the nature of the orbits of those stars is of
fundamental importance for the issue of which forms of
galaxies can be sustained. Some of my work is directed toward
constructing self-consistent models of elliptical galaxies.
Here one needs to find out which combinations of stars in
orbit reproduce the density that is needed to cause the
gravitational field that one assumed in the first place when
computing the orbits. The problems are complicated by the fact
that the three-dimensional shapes of most elliptical galaxies,
which are seen only in projection on the sky, may well be
triaxial. A further challenge is to build models which are
consistent with observations of line-of-sight velocities. Like
the distribution of light, the kinematics of a galaxy is also
observed only in projection on the plane of the sky.
I have recently developed some simple and direct algorithms
for deriving the Fourier series which describe the
quasi-periodic motion of regular orbits from numerical
integrations of those orbits. These algorithms are based
entirely on discrete Fourier transforms. They reproduce test
orbits accurately, satisfy constraints which are consequences
of Hamiltonian theory, and are faster than other methods
currently in use. They were developed because galactic models
require the use of large numbers of orbits. Hence efficient
methods of representing orbits are needed for modeling, and my
student Balsa Terzic is now using the new algorithms to
construct triaxial galactic models.
| Stability of
Stellar Systems |
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Another current interest is that of the stability of
stellar systems such as galaxies. Stability is a fundamental
requirement of any galactic model, but much more theoretical
understanding of this stability is still needed. Stellar
dynamic stability problems are harder than those of
hydrodynamic stability because they arise in a phase space
with twice as many dimensions as physical space. Some results
on the stability of particular models have come from numerical
N-body simulations, which can act as a guide to theoretical
understanding, but cannot replace it. For instance, it was
N-body simulations that first showed how prone flat stellar
disks are to bar-like instabilities and many galaxies do,
indeed, have bar-like features. I am working to obtain a
dynamical understanding of bar-like instabilities and what it
takes to overcome them.
Gravitational lensing provides another way of investigating
galaxies. Light and other rays are bent when they pass close
to a massive object like a galaxy, and the light is slowed
down. Consequently a galaxy which happens to lie between us
and a distant quasar can cause us to see multiple images of
that quasar. This phenomenon is known as gravitational lensing.
It provides a tool for investigating the combined effects of
the visible and dark matter content of a galaxy because both
contribute to the lensing. There are many instances of
galaxies which produce four images of the same distant quasar.
Wyn Evans (Oxford) and I have developed ways for deducing what
those image systems, their configurations and their relative
strengths, tell us about mass content of those galaxies. It is
that mass which produces the gravitational forces which
control the orbits of the stars of the galaxy, and hence which
is responsible for the dynamical structure. Hence dynamics and
lensing give us two complementary means of studying galaxies;
two means which we have been trying to interrelate
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