Halo Shapes From Weak Lensing: The Impact of Galaxy--Halo Misalignment
  We analyse the impact of galaxy--halo misalignment on the ability of weak
lensing studies to constrain the shape of dark matter haloes, using a
combination of the Millennium dark matter N-body simulation and different
semi-analytic galaxy formation models, as well as simpler Monte Carlo tests.
Since the distribution of galaxy--halo alignments is not known in detail, we
test various alignment models, together with different methods of determining
the halo shape. In addition to alignment, we examine the interplay of halo mass
and shape, and galaxy colour and morphology with the resulting stacked
projected halo shape. We find that only in the case where significant numbers
of galaxy and halo minor axes are parallel does the stacked, projected halo
axis ratio fall below 0.95. When using broader misalignment distributions, such
as those found in recent simulations of galaxy formation, the halo ellipticity
signal is washed out and would be extremely difficult to measure
observationally. It is important to note that the spread in stacked halo axis
ratio due to theoretical unknowns (differences between semi-analytic models,
and between alignment models) are much bigger than any statistical uncertainty:
It is naive to assume that, simply because LCDM predicts aspherical haloes, the
stacked projected shape will be elliptical. In fact, there is no robust LCDM
prediction yet for this procedure, and the interpretation of any such
elliptical halo signal from lensing in terms of physical halo properties will
be extremely difficult.
