From: Piero Rosati (prosati@eso.org)
Date: Thu Jan 26 2006 - 23:59:41 PST
Hi..
I dont' want to add noise at this point, I just want to say that I
concur with Henk's comments. Specifically:
a) it's good to point out that the new (including those previously
known) arcs in these clusters will be very useful when combined with the
weak-lensing analysis.
b) one could say that in some cases (after spectroscopy id) the strong
lensing inversion will give us first insight on the mass profile in the
core at these high redshifts, which can be compared with CDM predictions
(evolution of NFW profile due to variation of background density with
redshifts).
c) we have to be careful about overemphasizing the Arc statistics stuff,
we are asking for troubles here. It's easy to kill such an argument with
our sample *which is anything but homogeneous*. True, one could say I
have detected *at least* XX arcs at z>1, but it's still pretty weak. So
we just we have to be careful with the wording here.
d) it's important to empahsize that our feasibility argument on weak
lensing are based on the analysis *real ACS data on z>1 clusters*, i.e.
how the shear signal-to-noise varies as a function of number of orbits
and passband (e.g. in RDCS1252).
e) probing the nature of the dark matter ?? I would just drop this
point., this is done with low-z "bullet clusters".
piero
Henk Hoekstra wrote:
>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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