From: Joseph Hennawi (jhennawi@astro.princeton.edu)
Date: Thu Jan 26 2006 - 15:08:07 PST
I agree that the weak lensing should not be compressed, but I think
the strong lensing science is compelling because
a) There is suggestive evidence for disagreement with LCDM at high
redshift
b) It is probing the nature of dark matter which is the kind of
'fundamental physics' the TAC will like
Chris Mullis has found an arc in 2 z-band orbits and 1 i-band orbit
behind an XMM cluster at z = 1.39, which makes this the highest
redshift lensing cluster known to my knowledge, and the SN cluster
sample has roughly doubled the number of arcs known behind z ~ 1
clusters. These discoveries should be mentioned.
I agree that the comparison to Abell 1689 is a stretch, but I was
trying to relate the lensing science to the HST GTO work which has
had a very high profile. That said, RCS2319+0038
(z=0.91), which is on the cycle 14 target list has three giant arcs
in a shallow ground based image. So a detailed reconstruction could
be possible there. This does not make a good argument for cycle 15
though, unless there is an analogous system on the cycle 15 target list.
How about if we put the strong lensing science at the end of the WL
section, since the two are actually complementary. Weak lensing + 1
giant arc can measure the
concentration of the high redshift cluster. We have done simulations
that show this is feasible but the paper is not out yet (although we
didn't simulate z ~ 1).
So we could say something like the following
(first part should be shortened).
Strong gravitational lensing by clusters is a powerful test of the
$\Lambda$CDM model, probing the rare, highest mass concentrations in
the Universe, where the dark matter density is highest. However, the
frequency of giant arcs observed in the RCS survey (Gladders \etal
2003) suggests that strong lensing by high redshift clusters is at
odds with the predictions of $\Lambda$CDM. Although numerical
simulations predict that the distribution of lensing clusters should
be peaked at $z \sim 0.4$ (Hennawi \etal 2006), {\em all} the lensing
clusters in the RCS sample are at $z\gtrsim 0.7$. In addition, the
presence of several high redshift clusters with multiple arcs at large
Einstein radii, have led many to speculate that these systems might
constitute a distinct population of `superlenses', with extremely
large lensing cross sections (Gladders et al. 2003; Dalal et al. 2004)
-- whereas $\Lambda$CDM predicts no such structures. Our cycle 14
sample
already contains 3 new giant arc systems lensed by $z \gtrsim 1$
clusters, doubling the number
known, including a giant arc lensed by a cluster at $z = 1.39$
which is the highest redshift lensing cluster known. These are in
addition to the
known high redshift lensing cluster on our cycle 15 target list
(mention how many).
The image quality delivered by the ACS dramatically increases the
number of the number
of faint, low surface brightness arcs and image families
detectable. A single high redshift giant arc in a cluster measures
the mass interior to the Einstein radius (~ 100 kpc/h), and is thus
highly complementary
to the larger scale weak lensing measurements. By combining this small
scale strong lensing constraint with the larger scale weak lensing
analysis,
which constrains the mass interior to the virial radius, we will be able
to measure the concentration parameters of the clusters. Our
unprecendented
deep imaging survey of high redshift clusters will thus allow us to
determine if there really are
'superlenses' at high redshift and make the first measurements of the
profiles of dark matter
halos at z \gtrsim 1.
On Jan 26, 2006, at 2:34 PM, Henk Hoekstra wrote:
> Hi Saul et al.,
>
> I have been looking at the proposal pdf file on the website (not sure
> this is the right place), but I am wondering whether we should start
> the proposal differently, highlighting the NEW things we can do with
> the cycle 15 observations.
>
> The way it reads now it's more of the same (everybody wants more
> data...)
> and only later on it becomes clear we want to do all kinds of new
> things.
>
> I noticed the weak lensing has been cut considerably, but I think
> we should be careful not to compress it too much. For instance, my
> feeling is that the strong lensing part is too long. So far the
> strong lensing hasn't been overwhelming, and I am not sure it will
> ever be... these are high-z clusters after all. I don't think the
> sample of clusters studied here will really advance the science
> questions
> posed in this section by a significant amount. Also I don't believe
> we'll do as well as what has been done for A1689. Nothing obvious
> is seen in the current data. A somewhat longer integration time
> won't help
> there.
>
> Virial masses: also too long, as it describes other data that we want
> to obtain. We could simply say that
>
> "We are acquiring X-ray and plan to obtain SZ measurements as part
> of a
> multiwavelength strategy of determining cluster masses through
> lensing, X-ray and SZ in order to understand cluster physics and
> formation."
>
> and copy some stuff from my lensing write up about how this helps
> studies of cluster abundance and cosmology.
>
> In short I think we can condense the strong lensing section and virial
> mass section in a short paragraph and tie it in with the weak lensing
> as a "Determination of cluster masses" section.
>
> Cheers,
> Henk
>
>
> On Thu, Jan 26, 2006 at 01:10:09PM -0800, Saul Perlmutter wrote:
>> Our Cycle 14 program has now proven a new extremely efficient
>> approach
>> to obtain z>1 dust-free Type Ia supernovae, and we here propose to
>> capitalize on this new technique . We will collect a total sample of
>> ~20 z>1 SNe Ia in cluster ellipticals, yielding dark energy
>> measurements
>> that do not suffer from the major systematic uncertainty at these
>> redshifts, that of the extinction correction with a prior. By
>> targeting
>> massive galaxy clusters at z>1, we obtain a five-times [CHECK]
>> higher
>> efficiency in detection of Type Ia supernovae in elliptical galaxies,
>> and provide a well-understood host galaxy environment. These same
>> deep
>> cluster images then also yield fundamental calibrations required for
>> future weak lensing and Sunyaev-Zel'dovich measurements of dark
>> energy,
>> as well as an entire program of cluster studies. The data will make
>> possible a factor of two improvement on supernova constraints on dark
>> energy time variation, and much larger improvement in systematic
>> uncertainty, taking advantage of the uniquely well-controlled host
>> environment that clusters provide. They will provide both a cluster
>> dataset and a SN Ia data set that will be a longstanding scientific
>> resource.
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