COSMOLOGY
(7a) Light Propagation in Inhomogeneous Universes
Premadi, Martel, and Matzner (1998) presented and applied a new
ray-tracing, multiple lens-plane algorithm for studying the effect of
large-scale structure formation and galaxies on the gravitational lensing of
light from distant sources.
They use a Particle-Particle/Particle-Mesh (P3M) N-body numerical code
to simulate the formation of large scale structure in the universe.
We extend the length resolution of the simulations to sub-Megaparsec
scales by using a Monte-Carlo method for locating galaxies inside
the computational volume according to the underlying distribution
of background matter. The observed galaxy 2-point correlation function
is reproduced. This algorithm constitutes a major improvement over
previous methods, which either neglected the presence of large-scale
structure, neglected the presence of galaxies, neglected the
contribution of distant matter (matter located far from the beam),
or used the Zel'dovich approximation for simulating the formation of
large-scale structure.
In addition, we take into account the observed morphology-density
relation when assigning morphological types to galaxies, something
that was ignored in all previous studies.
To test this algorithm, they perform 1981 simulations, for three different
cosmological models: an Einstein-de~Sitter model with
density parameter Omega_0=1, an open model with Omega_0=0.2, and a flat,
low density model with Omega_0=0.2 and a cosmological
constant lambda_0=0.8. In all models, the
initial density fluctuations correspond to a Cold Dark Matter power spectrum
normalized to COBE.
In each simulation, we compute the shear and magnification resulting from
the presence of inhomogeneities. The results are the
following:
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The magnification is totally dominated by the convergence,
with the shear contributing less than one part in 10^4.
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Most of the cumulative
shear and magnification is contributed by matter located at intermediate
redshifts z=1-2.
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The actual value of the redshift where the
largest contribution to shear and magnification occurs
depends on the
cosmological model. In particular, the lens planes contributing the most
are located at larger redshift for models with smaller Omega_0.
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The number of galaxies directly hit by the beam increases with redshift,
while the contribution of lens planes to the
shear and magnification decrease with
increasing lens-plane redshift for z>2, indicating
that the bulk of the shear and magnification does not originate from direct
hits, but rather from the tidal influence of nearby and
more distant galaxies, and background matter.
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The average contributions of background matter and nearby
galaxies to the shear is comparable for models with small Omega_0.
For the Einstein-de~Sitter model, the contribution of the background
matter exceeds the one of nearby galaxies by nearly one order of magnitude.