Re: Reworked strong lens section

From: Mike Gladders (gladders@ociw.edu)
Date: Fri Jan 27 2006 - 00:04:01 PST

  • Next message: Lori Lubin: "Small comments"

    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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