31 research outputs found
Cross correlation surveys with the Square Kilometre Array
By the time that the first phase of the Square Kilometre Array is deployed it
will be able to perform state of the art Large Scale Structure (LSS) as well as
Weak Gravitational Lensing (WGL) measurements of the distribution of matter in
the Universe. In this chapter we concentrate on the synergies that result from
cross-correlating these different SKA data products as well as external
correlation with the weak lensing measurements available from CMB missions. We
show that the Dark Energy figures of merit obtained individually from WGL/LSS
measurements and their independent combination is significantly increased when
their full cross-correlations are taken into account. This is due to the
increased knowledge of galaxy bias as a function of redshift as well as the
extra information from the different cosmological dependences of the
cross-correlations. We show that the cross-correlation between a spectroscopic
LSS sample and a weak lensing sample with photometric redshifts can calibrate
these same photometric redshifts, and their scatter, to high accuracy by
modelling them as nuisance parameters and fitting them simultaneously
cosmology. Finally we show that Modified Gravity parameters are greatly
constrained by this cross-correlations because weak lensing and redshift space
distortions (from the LSS survey) break strong degeneracies in common
parameterisations of modified gravity.Comment: 12 pages, 3 figures. This article is part of the 'Cosmology Chapter,
Advancing Astrophysics with the SKA (AASKA14) Conference, Giardini Naxos
(Italy), June 9th-13th 2014
The cosmological impact of intrinsic alignment model choice for cosmic shear
We consider the effect of galaxy intrinsic alignments (IAs) on dark energy constraints from weak gravitational lensing. We summarize the latest version of the linear alignment model of IAs, following a brief note of Hirata & Seljakand further interpretation by Laszlo et al. We show the cosmological bias on the dark energy equation of state parameters w0 and wa$ that would occur if IAs were ignored. We find that w0 and wa are both catastrophically biased, by an absolute value of just greater than unity under the Fisher matrix approximation. This contrasts with a bias several times larger for the earlier IA implementation. Therefore, there is no doubt that IAs must be taken into account for future stage III experiments and beyond. We use a flexible grid of IA and galaxy bias parameters as used in previous work and investigate what would happen if the Universe is described by used the latest IA model, but we assumed the earlier version. We find that despite the large difference between the two IA models, the grid flexibility is sufficient to remove cosmological bias and recover the correct dark energy equation of state. In an appendix, we compare observed shear power spectra to those from a popular previous implementation and explain the difference
Weak Lensing with Sizes, Magnitudes and Shapes
Weak lensing can be observed through a number of effects on the images of
distant galaxies; their shapes are sheared, their sizes and fluxes (magnitudes)
are magnified and their positions on the sky are modified by the lensing field.
Galaxy shapes probe the shear field whilst size, magnitude and number density
probe the convergence field. Both contain cosmological information. In this
paper we are concerned with the magnification of the size and magnitude of
individual galaxies as a probe of cosmic convergence. We develop a Bayesian
approach for inferring the convergence field from a measured size, magnitude
and redshift and demonstrate that the inference on convergence requires
detailed knowledge of the joint distribution of intrinsic sizes and magnitudes.
We build a simple parameterised model for the size-magnitude distribution and
estimate this distribution for CFHTLenS galaxies. In light of the measured
distribution, we show that the typical dispersion on convergence estimation is
~0.8, compared to ~0.38 for shear. We discuss the possibility of physical
systematics for magnification (similar to intrinsic alignments for shear) and
compute the expected gains in the Dark Energy Figure-of-Merit (FoM) from
combining magnification with shear for different scenarios regarding
systematics: when accounting for intrinsic alignments but no systematics on the
magnification signal, including magnification could improve the FoM by upto a
factor of ~2.5, whilst when accounting for physical systematics in both shear
and magnification we anticipate a gain between ~25% and ~65%. In addition to
the statistical gains, the fact that cosmic shear and magnification are subject
to different systematics makes magnification an attractive complement to any
cosmic shear analysis.Comment: 15 pages, 5 figures, accepted by MNRA
Galaxy alignments: An overview
The alignments between galaxies, their underlying matter structures, and the
cosmic web constitute vital ingredients for a comprehensive understanding of
gravity, the nature of matter, and structure formation in the Universe. We
provide an overview on the state of the art in the study of these alignment
processes and their observational signatures, aimed at a non-specialist
audience. The development of the field over the past one hundred years is
briefly reviewed. We also discuss the impact of galaxy alignments on
measurements of weak gravitational lensing, and discuss avenues for making
theoretical and observational progress over the coming decade.Comment: 43 pages excl. references, 16 figures; minor changes to match version
published in Space Science Reviews; part of a topical volume on galaxy
alignments, with companion papers at arXiv:1504.05546 and arXiv:1504.0546
Testing gravity on cosmological scales with cosmic shear, cosmic microwave background anisotropies, and redshift-space distortions
We use a range of cosmological data to constrain phenomenological
modifications to general relativity on cosmological scales, through
modifications to the Poisson and lensing equations. We include cosmic microwave
background anisotropy measurements from the Planck satellite, cosmic shear from
CFHTLenS and DES-SV, and redshift-space distortions from BOSS data release 12
and the 6dF galaxy survey. We find no evidence of departures from general
relativity, with the modified gravity parameters constrained to and , where and
refer to deviations from general relativity today and are defined to be
zero in general relativity. We also forecast the sensitivity to those
parameters of the full five-year Dark Energy Survey and of an experiment like
the Large Synoptic Survey Telescope, showing a substantial expected improvement
in the constraint on .Comment: 12 pages, 10 figure
Galaxy alignments: Observations and impact on cosmology
Galaxy shapes are not randomly oriented, rather they are statistically
aligned in a way that can depend on formation environment, history and galaxy
type. Studying the alignment of galaxies can therefore deliver important
information about the physics of galaxy formation and evolution as well as the
growth of structure in the Universe. In this review paper we summarise key
measurements of galaxy alignments, divided by galaxy type, scale and
environment. We also cover the statistics and formalism necessary to understand
the observations in the literature. With the emergence of weak gravitational
lensing as a precision probe of cosmology, galaxy alignments have taken on an
added importance because they can mimic cosmic shear, the effect of
gravitational lensing by large-scale structure on observed galaxy shapes. This
makes galaxy alignments, commonly referred to as intrinsic alignments, an
important systematic effect in weak lensing studies. We quantify the impact of
intrinsic alignments on cosmic shear surveys and finish by reviewing practical
mitigation techniques which attempt to remove contamination by intrinsic
alignments.Comment: 52 pages excl. references, 16 figures; minor changes to match version
published in Space Science Reviews; part of a topical volume on galaxy
alignments, with companion papers arXiv:1504.05456 and arXiv:1504.0554
Cosmology from HI galaxy surveys with the SKA
The Square Kilometer Array (SKA) has the potential to produce galaxy redshift surveys which
will be competitive with other state of the art cosmological experiments in the next decade. In this
chapter we summarise what capabilities the first and the second phases of the SKA will be able
to achieve in its current state of design. We summarise the different cosmological experiments
which are outlined in further detail in other chapters of this Science Book. The SKA will be
able to produce competitive Baryonic Oscillation (BAOs) measurements in both its phases. The
first phase of the SKA will provide similar measurements in optical and IR experiments with
completely different systematic effects whereas the second phase being transformational in terms
of its statistical power. The SKA will produce very accurate Redshift Space Distortions (RSD)
measurements, being superior to other experiments at lower redshifts, due to the large number
of galaxies. Cross correlations of the galaxy redshift data from the SKA with radio continuum
surveys and optical surveys will provide extremely good calibration of photometric redshifts as
well as extremely good bounds on modifications of gravity. Basing on a Principle Component
Analysis (PCA) approach, we find that the SKA will be able to provide competitive constraint
on dark energy and modified gravity models. Due to the large area covered the SKA it will be a
transformational experiment in measuring physics from the largest scales such as non-Gaussian
signals. Finally, the SKA might produce the first real time measurement of the redshift
drift. The SKA will be a transformational machine for cosmology as it grows from an early Phase
1 to its full power
The Cosmological Impact of Intrinsic Alignment Model Choice for Cosmic Shear
We consider the effect of galaxy intrinsic alignments (IAs) on dark energy
constraints from weak gravitational lensing. We summarise the latest version of
the linear alignment model of IAs, following the brief note of Hirata & Seljak
(2010) and further interpretation in Laszlo et al. (2011). We show the
cosmological bias on the dark energy equation of state parameters w0 and wa
that would occur if IAs were ignored. We find that w0 and wa are both
catastrophically biased, by an absolute value of just greater than unity under
the Fisher matrix approximation. This contrasts with a bias several times
larger for the earlier IA implementation. Therefore there is no doubt that IAs
must be taken into account for future Stage III experiments and beyond. We use
a flexible grid of IA and galaxy bias parameters as used in previous work, and
investigate what would happen if the universe used the latest IA model, but we
assumed the earlier version. We find that despite the large difference between
the two IA models, the grid flexibility is sufficient to remove cosmological
bias and recover the correct dark energy equation of state. In an appendix, we
compare observed shear power spectra to those from a popular previous
implementation and explain the differences.Comment: 12 pages, 5 figure