From: Joseph Hennawi (jhennawi@astro.princeton.edu)
Date: Wed Jan 25 2006 - 00:03:59 PST
Hi All,
Below is a revision of the strong lensing text. I think it is
stronger, but unfortunately it is not much shorter. How hard pressed
are we for space?
If anybody has found any new giant arcs in their HST images please
report them to me now so we can mention them in the text. The tally
so far is 1 (RCS 0220-03), plus another arc in
one other cluster (not sure which one), but I believe this was
already discovered in previous HST imaging that was not part of this
program. If there are other known lensing clusters on the target list
for cycle 14 or cycle 15, let me know. Currently the only one I know
of is RCS2319+0038 at z=0.91. These should be mentioned.
\SUBSECTION{Strong lenses at $z \gs 1$}
Does the currently popular $\Lambda$CDM cosmological model explain the
detailed distribution of dark matter in galaxy clusters? Strong
gravitational lensing by clusters is a powerful test of this model,
probing the rare, highest mass concentrations in the Universe, where
the dark matter density is highest. However, the
frequency of giant arcs observed in the Red-Sequence Cluster survey
(RCS) (Gladders \etal 2003) highlights potential conflicts with the
predictions of $\Lambda$CDM, particularly at high redshifts.
While $\Lambda$CDM predicts that the distribution of
lensing clusters should be peaked at $z \sim 0.4$, {\em all} the lensing
clusters in the RCS sample are at $z\gtrsim 0.7$. In addition, the
presence of several high redshift clusters with multiple arcs at large
Einstein radii, have led many to speculate that these systems might
constitute a distinct population of `superlenses', with extremely
large lensing cross sections (Gladders et al. 2003; Dalal et al. 2004;
but see also Ho \& White 2005) -- whereas $\Lambda$CDM makes no such
prediction.
Our cycle 14 sample already contains 2 (?) new giant arc systems lensed
by $z \gtrsim 1$ clusters, nearly doubling the number known. These are
in addition to the known high redshift lensing cluster RCS2319+0038
(z=0.91), which is on our cycle 14 target list but not yet
observed. The superb image quality delivered by the ACS dramatically
increases the number of the number of faint, low surface brightness
arcs and image families detectable. This will enable detailed modeling
of the distorted image positions, resulting in high fidelity mass
reconstructions which will measure the distribution of dark matter,
similar to the recent exciting analysis of Abell 1689 by the ACS GTO
team (Broadhurst et al. 2005), but for clusters at $z \gtrsim 1$.
Does $\Lambda$CDM predict the correct mass distributions at high
redshift? Or could there be new physics lurking in the dark matter
sector on small scales or high overdensities? Our unprecedented deep
imaging survey of high redshift clusters will allow us to address this
fundamental question.
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