257 research outputs found
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.Comment: 31 pages, 8 Figures. Minor changes to reflect published versio
Time evolution of intrinsic alignments of galaxies
Intrinsic alignments (IA), correlations between the intrinsic shapes and
orientations of galaxies on the sky, are both a significant systematic in weak
lensing and a probe of the effect of large-scale structure on galactic
structure and angular momentum. In the era of precision cosmology, it is thus
especially important to model IA with high accuracy. Efforts to use
cosmological perturbation theory to model the dependence of IA on the
large-scale structure have thus far been relatively successful; however, extant
models do not consistently account for time evolution. In particular, advection
of galaxies due to peculiar velocities alters the impact of IA, because galaxy
positions when observed are generally different from their positions at the
epoch when IA is believed to be set. In this work, we evolve the galaxy IA from
the time of galaxy formation to the time at which they are observed, including
the effects of this advection, and show how this process naturally leads to a
dependence of IA on the velocity shear. We calculate the galaxy-galaxy-IA
bispectrum to tree level (in the linear matter density) in terms of the evolved
IA coefficients. We then discuss the implications for weak lensing systematics
as well as for studies of galaxy formation and evolution. We find that
considering advection introduces nonlocality into the bispectrum, and that the
degree of nonlocality represents the memory of a galaxy's path from the time of
its formation to the time of observation. We discuss how this result can be
used to constrain the redshift at which IA is determined and provide Fisher
estimation for the relevant measurements using the example of SDSS-BOSS.Comment: 30 pages, 5 figures, 2 table
Cosmological constraints from the convergence 1-point probability distribution
We examine the cosmological information available from the 1-point
probability distribution (PDF) of the weak-lensing convergence field, utilizing
fast L-PICOLA simulations and a Fisher analysis. We find competitive
constraints in the - plane from the convergence PDF with
pixels compared to the cosmic shear power spectrum with an
equivalent number of modes (). The convergence PDF also partially
breaks the degeneracy cosmic shear exhibits in that parameter space. A joint
analysis of the convergence PDF and shear 2-point function also reduces the
impact of shape measurement systematics, to which the PDF is less susceptible,
and improves the total figure of merit by a factor of , depending on the
level of systematics. Finally, we present a correction factor necessary for
calculating the unbiased Fisher information from finite differences using a
limited number of cosmological simulations.Comment: 10 pages, 5 figure
Detecting Galaxy-Filament Alignments in the Sloan Digital Sky Survey III
Previous studies have shown the filamentary structures in the cosmic web
influence the alignments of nearby galaxies. We study this effect in the LOWZ
sample of the Sloan Digital Sky Survey using the "Cosmic Web Reconstruction"
filament catalogue. We find that LOWZ galaxies exhibit a small but
statistically significant alignment in the direction parallel to the
orientation of nearby filaments. This effect is detectable even in the absence
of nearby galaxy clusters, which suggests it is an effect from the matter
distribution in the filament. A nonparametric regression model suggests that
the alignment effect with filaments extends over separations of 30-40 Mpc. We
find that galaxies that are bright and early-forming align more strongly with
the directions of nearby filaments than those that are faint and late-forming;
however, trends with stellar mass are less statistically significant, within
the narrow range of stellar mass of this sample.Comment: 14 pages, 13 figures. Accepted to the MNRA
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