465 research outputs found
Large Scale Structure Observations
Galaxy Surveys are enjoying a renaissance thanks to the advent of
multi-object spectrographs on ground-based telescopes. The last 15 years have
seen the fruits of this experimental advance, including the 2-degree Field
Galaxy Redshift Survey (2dFGRS; Colless et al. 2003) and the Sloan Digital Sky
Survey (SDSS; York et al. 2000). Most recently, the Baryon Oscillation
Spectroscopic Survey (BOSS; Dawson et al. 2013), part of the SDSS-III project
(Eisenstein et al. 2011), has provided the largest volume of the low-redshift
Universe ever surveyed with a galaxy density useful for high-precision
cosmology. This set of lecture notes looks at some of the physical processes
that underpin these measurements, the evolution of measurements themselves, and
looks ahead to the next 15 years and the advent of surveys such as the enhanced
Baryon Oscillation Spectroscopic Survey (eBOSS), the Dark Energy Spectroscopic
Instrument (DESI) and the ESA Euclid satellite mission.Comment: Lectures given at Post-Planck Cosmology, Ecole de Physique des
Houches, Les Houches, July 8-Aug 2, 2013, eds. B. Wandelt, C. Deffayet, P.
Peter, to be published by Oxford University Press, and New Horizons for
Observational Cosmology, International School of Physics Enrico Fermi,
Varenna, July 1-6, 2013, eds. A. Melchiorri, A. Cooray, E. Komatsu, to be
published by the Italian Society of Physic
Unbiased clustering estimation in the presence of missing observations
In order to be efficient, spectroscopic galaxy redshift surveys do not obtain
redshifts for all galaxies in the population targeted. The missing galaxies are
often clustered, commonly leading to a lower proportion of successful
observations in dense regions. One example is the close-pair issue for SDSS
spectroscopic galaxy surveys, which have a deficit of pairs of observed
galaxies with angular separation closer than the hardware limit on placing
neighbouring fibers. Spatially clustered missing observations will exist in the
next generations of surveys. Various schemes have previously been suggested to
mitigate these effects, but none works for all situations. We argue that the
solution is to link the missing galaxies to those observed with statistically
equivalent clustering properties, and that the best way to do this is to rerun
the targeting algorithm, varying the angular position of the observations.
Provided that every pair has a non-zero probability of being observed in one
realisation of the algorithm, then a pair-upweighting scheme linking targets to
successful observations, can correct these issues. We present such a scheme,
and demonstrate its validity using realisations of an idealised simple survey
strategy.Comment: 14 pages, 8 figures, published in MNRA
Galaxy 2-Point Covariance Matrix Estimation for Next Generation Surveys
We perform a detailed analysis of the covariance matrix of the spherically
averaged galaxy power spectrum and present a new, practical method for
estimating this within an arbitrary survey without the need for running mock
galaxy simulations that cover the full survey volume. The method uses
theoretical arguments to modify the covariance matrix measured from a set of
small-volume cubic galaxy simulations, which are computationally cheap to
produce compared to larger simulations and match the measured small-scale
galaxy clustering more accurately than is possible using theoretical modelling.
We include prescriptions to analytically account for the window function of the
survey, which convolves the measured covariance matrix in a non-trivial way. We
also present a new method to include the effects of supersample covariance and
modes outside the small simulation volume which requires no additional
simulations and still allows us to scale the covariance matrix. As validation,
we compare the covariance matrix estimated using our new method to that from a
brute force calculation using 500 simulations originally created for analysis
of the Sloan Digital Sky Survey Main Galaxy Sample (SDSS-MGS). We find
excellent agreement on all scales of interest for large scale structure
analysis, including those dominated by the effects of the survey window, and on
scales where theoretical models of the clustering normally break-down, but the
new method produces a covariance matrix with significantly better
signal-to-noise. Although only formally correct in real-space, we also discuss
how our method can be extended to incorporate the effects of Redshift Space
Distortions.Comment: 18 pages, 9 figures. Accepted for publication in MNRAS. Added new
references to introduction and slightly updated text accordingl
An accurate linear model for redshift space distortions in the void-galaxy correlation function
Redshift space distortions within voids provide a unique method to test for
environmental dependence of the growth rate of structures in low density
regions, where effects of modified gravity theories might be important. We
derive a linear theory model for the redshift space void-galaxy correlation
that is valid at all pair separations, including deep within the void, and use
this to obtain expressions for the monopole and quadrupole
contributions. Our derivation highlights terms that have previously been
neglected but are important within the void interior. As a result our model
differs from previous works and predicts new physical effects, including a
change in the sign of the quadrupole term within the void radius. We show how
the model can be generalised to include a velocity dispersion. We compare our
model predictions to measurements of the correlation function using mock void
and galaxy catalogues modelled after the BOSS CMASS galaxy sample using the Big
MultiDark -body simulation, and show that the linear model with dispersion
provides an excellent fit to the data at all scales, Mpc. While the RSD model matches simulations, the linear bias
approximation does not hold within voids, and care is needed in fitting for the
growth rate. We show that fits to the redshift space correlation recover the
growth rate to a precision of using the simulation volume
of .Comment: 16 pages, 12 figures. v3: updated to match version published in
MNRAS. Several minor changes to text for better explanations, with reference
to subsequent results (arXiv:1805.09349). No changes to theory, results or
conclusion
Using correlations between CMB lensing and large-scale structure to measure primordial non-Gaussianity
We apply a new method to measure primordial non-Gaussianity, using the
cross-correlation between galaxy surveys and the CMB lensing signal to measure
galaxy bias on very large scales, where local-type primordial non-Gaussianity
predicts a divergence. We use the CMB lensing map recently published by
the Planck collaboration, and measure its external correlations with a suite of
six galaxy catalogues spanning a broad redshift range. We then consistently
combine correlation functions to extend the recent analysis by Giannantonio et
al. (2013), where the density-density and the density-CMB temperature
correlations were used. Due to the intrinsic noise of the Planck lensing map,
which affects the largest scales most severely, we find that the constraints on
the galaxy bias are similar to the constraints from density-CMB temperature
correlations. Including lensing constraints only improves the previous
statistical measurement errors marginally, and we obtain (1) from the combined data set. However, the lensing
measurements serve as an excellent test of systematic errors: we now have three
methods to measure the large-scale, scale-dependent bias from a galaxy survey:
auto-correlation, and cross-correlation with both CMB temperature and lensing.
As the publicly available Planck lensing maps have had their largest-scale
modes at multipoles removed, which are the most sensitive to the
scale-dependent bias, we consider mock CMB lensing data covering all
multipoles. We find that, while the effect of indeed
increases significantly on the largest scales, so do the contributions of both
cosmic variance and the intrinsic lensing noise, so that the improvement is
small.Comment: 5 pages, 3 figures. Additional references added. Submitted to MNRA
Galaxy peculiar velocities and evolution-bias
Galaxy bias can be split into two components: a formation-bias based on the
locations of galaxy creation, and an evolution-bias that details their
subsequent evolution. In this letter we consider evolution-bias in the peaks
model. In this model, galaxy formation takes place at local maxima in the
density field, and we analyse the subsequent peculiar motion of these galaxies
in a linear model of structure formation. The peak restriction yields
differences in the velocity distribution and correlation between the galaxy and
the dark matter fields, which causes the evolution-bias component of the total
bias to evolve in a scale-dependent way. This mechanism naturally gives rise to
a change in shape between galaxy and matter correlation functions that depends
on the mean age of the galaxy population. This model predicts that older
galaxies would be more strongly biased on large scales compared to younger
galaxies. Our arguments are supported by a Monte-Carlo simulation of galaxy
pairs propagated using the Zel'dovich-approximation for describing linear
peculiar galaxy motion.Comment: 5 pages, 4 figures, MNRAS accepte
L-PICOLA: A parallel code for fast dark matter simulation
Robust measurements based on current large-scale structure surveys require
precise knowledge of statistical and systematic errors. This can be obtained
from large numbers of realistic mock galaxy catalogues that mimic the observed
distribution of galaxies within the survey volume. To this end we present a
fast, distributed-memory, planar-parallel code, L-PICOLA, which can be used to
generate and evolve a set of initial conditions into a dark matter field much
faster than a full non-linear N-Body simulation. Additionally, L-PICOLA has the
ability to include primordial non-Gaussianity in the simulation and simulate
the past lightcone at run-time, with optional replication of the simulation
volume. Through comparisons to fully non-linear N-Body simulations we find that
our code can reproduce the power spectrum and reduced bispectrum of dark
matter to within 2% and 5% respectively on all scales of interest to
measurements of Baryon Acoustic Oscillations and Redshift Space Distortions,
but 3 orders of magnitude faster. The accuracy, speed and scalability of this
code, alongside the additional features we have implemented, make it extremely
useful for both current and next generation large-scale structure surveys.
L-PICOLA is publicly available at https://cullanhowlett.github.io/l-picolaComment: 22 Pages, 20 Figures. Accepted for publication in Astronomy and
Computin
Improving the modelling of redshift-space distortions - II. A pairwise velocity model covering large and small scales
We develop a model for the redshift-space correlation function, valid for
both dark matter particles and halos on scales Mpc. In its simplest
formulation, the model requires the knowledge of the first three moments of the
line-of-sight pairwise velocity distribution plus two well-defined
dimensionless parameters. The model is obtained by extending the
Gaussian-Gaussianity prescription for the velocity distribution, developed in a
previous paper, to a more general concept allowing for local skewness, which is
required to match simulations. We compare the model with the well known
Gaussian streaming model and the more recent Edgeworth streaming model. Using
N-body simulations as a reference, we show that our model gives a precise
description of the redshift-space clustering over a wider range of scales. We
do not discuss the theoretical prescription for the evaluation of the velocity
moments, leaving this topic to further investigation.Comment: 18 pages, 10 figures, published in MNRA
The effect of redshift-space distortions on projected 2-pt clustering measurements
Although redshift-space distortions only affect inferred distances and not
angles, they still distort the projected angular clustering of galaxy samples
selected using redshift dependent quantities. From an Eulerian view-point, this
effect is caused by the apparent movement of galaxies into or out of the
sample. From a Lagrangian view-point, we find that projecting the
redshift-space overdensity field over a finite radial distance does not remove
all the anisotropic distortions. We investigate this effect, showing that it
strongly boosts the amplitude of clustering for narrow samples and can also
reduce the significance of baryonic features in the correlation function. We
argue that the effect can be mitigated by binning in apparent galaxy
pair-centre rather than galaxy position, and applying an upper limit to the
radial galaxy separation. We demonstrate this approach, contrasting against
standard top-hat binning in galaxy distance, using sub-samples taken from the
Hubble Volume simulations. Using a simple model for the radial distribution
expected for galaxies from a survey such as the Dark Energy Survey (DES), we
show that this binning scheme will simplify analyses that will measure baryon
acoustic oscillations within such galaxy samples. Comparing results from
different binning schemes has the potential to provide measurements of the
amplitude of the redshift-space distortions. Our analysis is relevant for other
photometric redshift surveys, including those made by the Panoramic Survey
Telescope & Rapid Response System (Pan-Starrs) and the Large Synoptic Survey
Telescope (LSST).Comment: 13 pages, 15 figures, accepted by MNRAS, corrected typos, revised
argument in section 3, figure added in section 3, results unchange
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