235 research outputs found
Tests of redshift-space distortions models in configuration space for the analysis of the BOSS final data release
Observations of redshift-space distortions in spectroscopic galaxy surveys
offer an attractive method for observing the build-up of cosmological
structure, which depends both on the expansion rate of the Universe and our
theory of gravity. In preparation for analysis of redshift-space distortions
from the Baryon Oscillation Spectroscopic Survey (BOSS) final data release we
compare a number of analytic and phenomenological `streaming' models, specified
in configuration space, to mock catalogs derived in different ways from several
N-body simulations. The galaxies in each mock catalog have properties similar
to those of the higher redshift galaxies measured by BOSS but differ in the
details of how small-scale velocities and halo occupancy are determined. We
find that all of the analytic models fit the simulations over a limited range
of scales while failing at small scales. We discuss which models are most
robust and on which scales they return reliable estimates of the rate of growth
of structure: we find that models based on some form of resummation can fit our
N-body data for BOSS-like galaxies above Mpc well enough to return
unbiased parameter estimates.Comment: 12 pages, 11 figures, matches version accepted by MNRA
Model-independent X-ray mass determinations
A new method is introduced for making X-ray mass determinations of spherical
clusters of galaxies. Treating the distribution of gravitating matter as
piecewise constant and the cluster atmosphere as piecewise isothermal, X-ray
spectra of a hydrostatic atmosphere are determined up to a single overall
normalizing factor. In contrast to more conventional approaches, this method
relies on the minimum of assumptions, apart from the conditions of hydrostatic
equilibrium and spherical symmetry. The method has been implemented as an XSPEC
mixing model called CLMASS, which was used to determine masses for a sample of
nine relaxed X-ray clusters. Compared to conventional mass determinations,
CLMASS provides weak constraints on values of M_500, reflecting the quality of
current X-ray data for cluster regions beyond r_500. At smaller radii, where
there are high quality X-ray spectra inside and outside the radius of interest
to constrain the mass, CLMASS gives confidence ranges for M_2500 that are only
moderately less restrictive than those from more familiar mass determination
methods. The CLMASS model provides some advantages over other methods and
should prove useful for mass determinations in regions where there are high
quality X-ray data.Comment: 12 pages, 8 figures, accepted for publication in Ap
Tests of redshift-space distortions models in configuration space for the analysis of the BOSS final data release
Citation: White, M., Reid, B., Chuang, C. H., Tinker, J. L., McBride, C. K., Prada, F., & Samushia, L. (2015). Tests of redshift-space distortions models in configuration space for the analysis of the BOSS final data release. Monthly Notices of the Royal Astronomical Society, 447(1), 234-245. doi:10.1093/mnras/stu2460Observations of redshift-space distortions in spectroscopic galaxy surveys offer an attractive method for observing the build-up of cosmological structure, which depends both on the expansion rate of the Universe and our theory of gravity. In preparation for analysis of redshift-space distortions from the Baryon Oscillation Spectroscopic Survey (BOSS) final data release, we compare a number of analytic and phenomenological models, specified in configuration space, to mock catalogues derived in different ways from several N-body simulations. The galaxies in each mock catalogue have properties similar to those of the higher redshift galaxies measured by BOSS but differ in the details of how small-scale velocities and halo occupancy are determined. We find that all of the analytic models fit the simulations over a limited range of scales while failing at small scales. We discuss which models are most robust and on which scales they return reliable estimates of the rate of growth of structure: we find that models based on some form of resummation can fit our N-body data for BOSS-like galaxies above 30 h(-1) Mpc well enough to return unbiased parameter estimates
A map-based method for eliminating systematic modes from galaxy clustering power spectra with application to BOSS
We develop a practical methodology to remove modes from a galaxy survey power
spectrum that are associated with systematic errors. We apply this to the BOSS
CMASS sample, to see if it removes the excess power previously observed beyond
the best-fit CDM model on very large scales. We consider several
possible sources of data contamination, and check whether they affect the
number of targets that can be observed and the power spectrum measurements. We
describe a general framework for how such knowledge can be transformed into
template fields. Mode subtraction can then be used to remove these systematic
contaminants at least as well as applying corrective weighting to the observed
galaxies, but benefits from giving an unbiased power. Even after applying
templates for all known systematics, we find a large-scale power excess, but
this is reduced compared with that observed using the weights provided by the
BOSS team. This excess is at much larger scales than the BAO scale and does not
affect the main results of BOSS. However, it will be important for the
measurement of a scale-dependent bias due to primordial non-Gaussianity. The
excess is beyond that allowed by any simple model of non-Gaussianity matching
Planck data, and is not matched in other surveys. We show that this power
excess can further be reduced but is still present using "phenomenological"
templates, designed to consider further potentially unknown sources of
systematic contamination. As all discrepant angular modes can be removed using
"phenomenological" templates, the potentially remaining contaminant acts
radially.Comment: 19 pages, accepted by MNRA
Parametrization for the Scale Dependent Growth in Modified Gravity
We propose a scale dependent analytic approximation to the exact linear
growth of density perturbations in Scalar-Tensor (ST) cosmologies. In
particular, we show that on large subhorizon scales, in the Newtonian gauge,
the usual scale independent subhorizon growth equation does not describe the
growth of perturbations accurately, as a result of scale-dependent relativistic
corrections to the Poisson equation. A comparison with exact linear numerical
analysis indicates that our approximation is a significant improvement over the
standard subhorizon scale independent result on large subhorizon scales. A
comparison with the corresponding results in the Synchronous gauge demonstrates
the validity and consistency of our analysis.Comment: 10 pages, 5 figures. Minor modifications and references added to
match published versio
Interpreting large-scale redshift-space distortion measurements
The simplest theory describing large-scale redshift-space distortions (RSD),
based on linear theory and distant galaxies, depends on the growth of
cosmological structure, suggesting that strong tests of General Relativity can
be constructed from galaxy surveys. As data sets become larger and the expected
constraints more precise, the extent to which the RSD follow the simple theory
needs to be assessed in order that we do not introduce systematic errors into
the tests by introducing inaccurate simplifying assumptions. We study the
impact of the sample geometry, non-linear processes, and biases induced by our
lack of understanding of the radial galaxy distribution on RSD measurements.
Using LasDamas simulations of the Sloan Digital Sky Survey II (SDSS-II)
Luminous Red Galaxy (LRG) data, these effects are shown to be important at the
level of 20 per cent. Including them, we can accurately model the recovered
clustering in these mock catalogues on scales 30 -- 200 Mpc/h. Applying this
analysis to robustly measure parameters describing the growth history of the
Universe from the SDSS-II data, gives
and
when no prior is imposed on the
growth-rate, and the background geometry is assumed to follow a CDM
model with the WMAP + SNIa priors. The standard WMAP constrained CDM
model with General Relativity predicts
and
, which is fully consistent with
these measurements.Comment: 20 pages, 17 figures, 1 tabl
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: measuring structure growth using passive galaxies
We explore the benefits of using a passively evolving population of galaxies
to measure the evolution of the rate of structure growth between z=0.25 and
z=0.65 by combining data from the SDSS-I/II and SDSS-III surveys. The
large-scale linear bias of a population of dynamically passive galaxies, which
we select from both surveys, is easily modeled. Knowing the bias evolution
breaks degeneracies inherent to other methodologies, and decreases the
uncertainty in measurements of the rate of structure growth and the
normalization of the galaxy power-spectrum by up to a factor of two. If we
translate our measurements into a constraint on sigma_8(z=0) assuming a
concordance cosmological model and General Relativity (GR), we find that using
a bias model improves our uncertainty by a factor of nearly 1.5. Our results
are consistent with a flat Lambda Cold Dark Matter model and with GR.Comment: Accepted for publication in MNRAS (clarifications added, results and
conclusions unchanged
Luminous Red Galaxies in Simulations: Cosmic Chronometers?
There have been a number of attempts to measure the expansion rate of the
universe at high redshift using Luminous Red Galaxies (LRGs) as "chronometers".
The method generally assumes that stars in LRGs are all formed at the same
time. In this paper, we quantify the uncertainties on the measurement of H(z)
which arise when one considers more realistic, extended star formation
histories. In selecting galaxies from the Millennium Simulation for this study,
we show that using rest-frame criteria significantly improves the homogeneity
of the sample and that H(z) can be recovered to within 3% at z~0.42 even when
extended star formation histories are considered. We demonstrate explicitly
that using Single Stellar Populations to age-date galaxies from the
semi-analytical simulations provides insufficient accuracy for this experiment
but accurate ages are obtainable if the complex star formation histories
extracted from the simulation are used. We note, however, that problems with
SSP-fitting might be overestimated since the semi-analytical models tend to
over predict the late-time star-formation in LRGs. Finally, we optimize an
observational program to carry out this experiment.Comment: 11 pages, 10 figures. Accepted to MNRAS
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
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