Ray-Tracing through the Millennium Simulation

In this thesis, gravitational lensing in the concordance LambdaCDM
cosmology is investigated by carrying out ray-tracing along past
light cones through the Millennium Simulation, a very large N-body
simulation of cosmic structure formation. The method used for
tracing light rays substantially extends previous ray-tracing
methods that are based on the Multiple-Lens-Plane approximation.
Strong lensing is investigated by shooting random light rays
through the Millennium Simulation. The probability is evaluated
that an image of a small distant light source will be highly
magnified, will be highly elongated or will be one of a set of
multiple images. It is found that these probabilities increase
strongly with increasing source redshift. It is shown that
strong-lensing events can almost always be traced to a single
dominant lensing object, and the mass and redshift distribution of
these primary lenses is studied. The observed lens-mass range
extends to lower masses than those found in earlier studies using
simulations with lower spatial and mass resolution. Furthermore,
effects of additional material along the line-of-sight are
investigated. Although strong-lensing lines-of-sight are indeed
biased towards higher than average mean densities, this additional
matter typically contributes only a few percent of the total
surface density. The influence of stellar mass in galaxies on
strong lensing is investigated by comparing the results obtained
for lensing by dark matter alone to those obtained by also
including the luminous matter. The dark-matter component of the
lensing matter is constructed directly from the dark-matter
particle distribution of the Millennium Simulation, while the
luminous component is inferred from semi-analytic galaxy-formation
models implemented within the evolving dark-matter distribution of
the simulation. It is found that the inclusion of the stellar mass
strongly enhances the probability for strong lensing compared to a
'dark-matter only' universe. The identification of the lenses
associated with strong-lensing events reveals that the stellar mass
of galaxies (i) significantly enhances the strong-lensing
cross-sections of group and cluster halos, and (ii) gives rise to
strong lensing in smaller halos, which would not produce noticeable
effects in the absence of the stars. Even if only image splittings
>10 arcsec are considered, the luminous matter can still enhance
the strong-lensing optical depths by up to a factor of two.
Finally, the potential capabilities of future radio telescopes for
imaging the cosmic matter distribution are discussed. The
Millennium Simulation is used to simulate large-area maps of the
lensing convergence with the noise, resolution and
redshift-weighting achievable with a variety of idealised surveys.
It is shown that by observing lensing of 21-cm emission during
reionization with a sufficiently large radio telescope, an image of
the matter distribution could be obtained whose signal-to-noise far
exceeds that of any map made using galaxy lensing. These mass
images would allow the dark-matter halos of individual galaxies to
be viewed directly, giving a wealth of statistical and
morphological information about the relative distributions of mass
and light. For telescopes like the planned Square Kilometre Array,
mass imaging may be possible near the resolution limit of the core
array of the telescope.