Mass and Light distributions in Clusters of Galaxies - Henry A ...

Weak Lensing Dilution in A1689
limits the radius to which a reliable subtraction of the background can be
made.
We have also established that the bluest galaxies in the field of A1689
lie predominantly in the background, as their radial distortion profile follows
closely the red galaxies, but with an offset indicating the blue population lies
at a greater mean distance than the red background galaxies, and consistent
with the estimated mean redshifts of these two populations. With a larger
sample of clusters, this purely geometric effect can potentially be put to use to
provide a simple model-independent measure of the cosmological curvature.
The mass profile of A1689 was reexamined using our combined background
sample of red and blue galaxies. The distortion profile derived from
this sample is consistent with our earlier work, but somewhat more statistically
significant, so we have examined the mass profile more carefully out
to a larger radius. We have found that the distortion profile is steeper than
predicted for CDM halos appropriate for cluster sized masses C vir ∼ 5. This
discrepancy is particularly clear at large radius r > 2 ′ , where an acceptable
fit is found to an NFW profile but with a concentration C vir = 27 +3.5
−5.7. This
finding is consistent with our earlier work which showed that although an
overall best fit profile of C vir ≃ 14 to the joint strong and weak lensing based
data presented in Broadhurst et al. (2005b), the curvature of the data is
more pronounced than an NFW profile being shallower in the inner region
and steeper at larger radius, so that the derived value of the concentration
increases with radius depending on the radial limits being examined.
This result is surprising and may require a significant departure from the
standard CDM model, either in terms of the mass content, or the epoch at
which the bulk of the cluster was assembled. For example, one possibility to
achieve earlier formation of clusters is to allow deviation from Gaussianity
of the primordial density fluctuation field, as has been considered recently
by, e.g., Sadeh et al. (2006).
A1689 is amongst the most massive known
clusters, and projection effects may play a role in boosting somewhat the
lensing signal along the line of sight. We therefore aim to test the generality
of this result with a careful study of a statistical sample of clusters.
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