From: Henk Hoekstra (hoekstra@uvic.ca)
Date: Thu Jan 26 2006 - 17:10:02 PST
Hi Joe,
I agree that the strong lensing is compelling enough, but one has to
be careful not to overstate what can be done. It's one of those things
that sound good and make pretty pictures, but once looked at more closely
turns out to be much harder than people thought.
For instance, if I were on the TAC I would wonder about spectroscopic
follow up, etc. Also, I wouldn't be surprised to see arcs: provided one
observes to sufficient depth, you're pretty much guaranteed to see them.
With this in mind, should we get excited about a z=1.39 that shows and
arc?
I think we can focus the essence (I know it might not be the right word
to use in this collaboration) of the strong lensing much more.
Obviously I am interested in our results on the frequency on strong
lensing in clusters based on the RCS results, but I am not sure this
is the right sample... this is not a homogeneous sample. Not sure though
how much that will affect things.
> a) There is suggestive evidence for disagreement with LCDM at high
> redshift
What is the disagreement, and do we really think there's a problem with
LCDM? Or a limitation of the simulations, i.e. theory? The TAC will be
very sceptical of such claims...
> b) It is probing the nature of dark matter which is the kind of
> 'fundamental physics' the TAC will like
How do we probe the nature of dark matter? Do you think that with these
observations you can measure the interaction cross section? If not,
we're not doing much fundamental physics.
As you see, I am playing devil's advocate here, but the TAC will contain
specialists in this field, which may or may not be as critical (or be
much more critical).
How about the following (not sure it's shorter)? Note I also tried
to weave in the multi-color aspect:
Although the weak gravitational lensing measurements provide us with
an estimate of the overall cluster mass, accurate estimates of the
density profile in the centres of clusters can only be obtained using
strong lensing. The image quality delivered by the ACS dramatically
increases the number of the number of faint, low surface brightness
arcs and image families that can be detected. Such measurements nicely
complement the weak lensing measurements, as a single high redshift
giant arc determines the mass interior to the Einstein radius (~ 100
kpc/h). A combined analysis opens up the possibility to measure accurately
the concentration parameters of the clusters. In addition, comparison
with results at lower redshifts enables us the follow the evolution, and
test the CDM predictions. Based on the cycle 14 observations we already
discovered ???? giant arcs, and we expect a number of multiple image
families to be discovered using multicolor data (as proposed for cycle 15).
A related topic, that these observations can address, is the
outstanding question why high redshift clusters appear to be more
efficient lenses, compared to their lower redshift
counterparts. Gladders \etal (2003) find that most of the strong
lensing clusters in the RCS are at $z\gtrsim 0.7, whereas numerical
simulations predict the distribution of such clusters to peak at $z
\sim 0.4$ (Hennawi \etal 2006). A detailed strong lensing study
of large sample of high redshift clusters is a crucial part in
solving this intriguing problem.
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