From: Mike Gladders (gladders@ociw.edu)
Date: Fri Jan 27 2006 - 00:04:01 PST
Piero, good points, all. I particularly agree with the last one - adding
such stuff just opens us to attack without adding signifcantly to our
chances...
M
On Fri, 27 Jan 2006, Piero Rosati wrote:
> 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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