Figure 12.6 is an HST picture of a rich cluster of
galaxies - you can see many massive elliptical galaxies in the
cluster, as you'd expect. Because of the superb image quality of HST,
you also see tens of radial arcs, centred on the cluster centre. These
are images of galaxies that lie far behind the cluster, that have been
lensed by the dark matter in the cluster. Clearly, their images have been
distorted a lot by the lensing process.
Several of these lensed galaxies have multiple images, i.e. you see the
same galaxy at various positions in the cluster. In addition, some
images of galaxies may have been magnified by a lot, sometimes as much
as a factor of 30. Because of this, the cluster really acts as a
telescope, allowing you to study faint and distant galaxies, using the
dark matter in the cluster to increase the brightness of the galaxy
light. Astronomers are now deliberately targeting foreground clusters
in order to study better the faint galaxies behind it, that would not
be bright enough to observe in the absence of lensing.
The positions and magnifications of these lensed galaxies can also be
used to infer the distribution of matter in the cluster, since it is
that which causes the lensing. This is a very important application of
lensing. Recall how we inferred the presence of dark matter in clusters
so far. Assuming virial equilibrium, we estimated the mass in the
cluster from the velocities of the galaxies. Another way was to assume
that the hot gas in the cluster was in hydrostatic equilibrium, and
obtain the mass from the X-ray observations of the hot gas. In either
case, the computed masses were far higher than what we could account
for in either stars or gas - hence our claim that galaxy clusters
contained a lot of dark matter.
But we had to assume something - virial equilibrium for the galaxies,
hydrostatic equilibrium for the gas. The great thing about
gravitational lensing is that all we care about is the amount of mass
present: we do not need to assume any type of equilibrium, since the
velocity of the mass is irrelevant. The lensing mass and X-ray masses
are in reasonably good agreement for the still relatively few clusters
studied so far, and so this is again very convincing evidence for the
presence of dark matter.