![]() | Annu. Rev. Astron. Astrophys. 1999. 37:
127-189 Copyright © 1999 by Annual Reviews. All rights reserved |
7.2. Statistics of Arc(let)s
Up to now, several tens of multiply imaged distant galaxies have already been detected in clusters of galaxies, and this number will probably increase by a large factor within the next few years. Since the fraction of rich clusters (and therefore lensing-clusters) strongly depends on the cosmological scenario, we expect the number of arc(let)s to depend on cosmological parameters (Wilson et al 1996b).
It is well known that present-day statistical studies of clusters of
galaxies are limited by the few samples of cluster catalogs with well
understood selection function.
Wu & Mao (1996)
analyzed the statistics of arcs in the EMSS cluster sample
(Gioia et al 1990)
and show that the fraction is twice
the one expected for an EdS Universe, but compatible with a flat
= 0.7 model (see also
Cooray 1999).
Unfortunately, the geometry of the mass distribution and the
substructures present in the
lensing cluster increase the shear contribution to the
magnification and change dramatically the expected number of arcs
(Bartelmann et al 1995,
Bartelmann 1995b,
Hattori et al 1997,
Bartelmann et al 1998).
The importance of accurate simulations of clusters is clearly obvious
from the recent studies.
Bartelmann et al (1998)
find a totally
opposite result to that of Wu & Mao, and conclude that an open model
(
= 0.3 and
= 0) is preferred to
any flat models to reproduce the number of arcs observed. The other
models, including
the
= 0.7 model, fail
by about of factor of ten. This is a clear demonstration that the use of
statistics of arc(let)s to constrain the
cosmological scenario is very sensitive to the assumptions. The constraint
on
from the fraction
of arc(let)s is therefore rather weak and hopeless for the moment.