From: Anthony Gonzalez (anthony@astro.ufl.edu)
Date: Thu Jan 26 2006 - 17:47:45 PST
To offer my 2 cents on the matter, I agree with Henk on how the TAC will
view the strong lensing part. There have been a number of papers in the
last few years with different explanations for discrepancy between LCDM
expectations and the number of arcs in the RCS and LCDCS, and I suspect
that any members of the TAC who have been vaguely keeping up with this
will be skeptical that these z>1 arcs provide strong new constraints.
Additionally, the existence of the arcs in these clusters won't
necessarily mean much to people unless you provide information about the
arc properties (do these classify as classic giant arcs based on
criteria such as l/w?), which we don't have space to do. I think
something along the lines of what Henk wrote is about the right level,
but would add a sentence at the end stating that this sample contains
multiple z>1 clusters with arcs, including one at z=1.39.
-Anthony
> 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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