Separating intrinsic alignment and galaxy-galaxy lensing
  The coherent physical alignment of galaxies is an important systematic for
gravitational lensing studies as well as a probe of the physical mechanisms
involved in galaxy formation and evolution. We develop a formalism for treating
this intrinsic alignment (IA) in the context of galaxy-galaxy lensing and
present an improved method for measuring IA contamination, which can arise when
sources physically associated with the lens are placed behind the lens due to
photometric redshift scatter. We apply the technique to recent Sloan Digital
Sky Survey (SDSS) measurements of Luminous Red Galaxy lenses and typical (L*)
source galaxies with photometric redshifts selected from the SDSS imaging data.
Compared to previous measurements, this method has the advantage of being fully
self-consistent in its treatment of the IA and lensing signals, solving for the
two simultaneously. We find an IA signal consistent with zero, placing tight
constraints on both the magnitude of the IA effect and its potential
contamination to the lensing signal. While these constraints depend on source
selection and redshift quality, the method can be applied to any measurement
that uses photometric redshifts. We obtain a model-independent upper-limit of
roughly 10% IA contamination for projected separations of approximately 0.1-100
Mpc/h. With more stringent photo-z cuts and reasonable assumptions about the
physics of intrinsic alignments, this upper limit is reduced to 1-2%. These
limits are well below the statistical error of the current lensing
measurements. Our results suggest that IA will not present intractable
challenges to the next generation of galaxy-galaxy lensing experiments, and the
methods presented here should continue to aid in our understanding of alignment
processes and in the removal of IA from the lensing signal.
