Weak lensing by triaxial galaxy clusters
  Weak gravitational lensing studies of galaxy clusters often assume a
spherical cluster model to simplify the analysis, but some recent studies have
suggested this simplifying assumption may result in large biases in estimated
cluster masses and concentration values, since clusters are expected to exhibit
triaxiality. Several such analyses have, however, quoted expressions for the
spatial derivatives of the lensing potential in triaxial models, which are open
to misinterpretation. In this paper, we give a clear description of weak
lensing by triaxial NFW galaxy clusters and also present an efficient and
robust method to model these clusters and obtain parameter estimates. By
considering four highly triaxial NFW galaxy clusters, we re-examine the impact
of simplifying spherical assumptions and found that while the concentration
estimates are largely unbiased except in one of our traixial NFW simulated
clusters, for which the concentration is only slightly biased, the masses are
significantly biased, by up to 40%, for all the clusters we analysed. Moreover,
we find that such assumptions can lead to the erroneous conclusion that some
substructure is present in the galaxy clusters or, even worse, that multiple
galaxy clusters are present in the field. Our cluster fitting method also
allows one to answer the question of whether a given cluster exhibits
triaxiality or a simple spherical model is good enough.
