31 research outputs found
High Performance P3M N-body code: CUBEP3M
This paper presents CUBEP3M, a publicly-available high performance
cosmological N-body code and describes many utilities and extensions that have
been added to the standard package. These include a memory-light runtime SO
halo finder, a non-Gaussian initial conditions generator, and a system of
unique particle identification. CUBEP3M is fast, its accuracy is tuneable to
optimize speed or memory, and has been run on more than 27,000 cores, achieving
within a factor of two of ideal weak scaling even at this problem size. The
code can be run in an extra-lean mode where the peak memory imprint for large
runs is as low as 37 bytes per particles, which is almost two times leaner than
other widely used N-body codes. However, load imbalances can increase this
requirement by a factor of two, such that fast configurations with all the
utilities enabled and load imbalances factored in require between 70 and 120
bytes per particles. CUBEP3M is well designed to study large scales
cosmological systems, where imbalances are not too large and adaptive
time-stepping not essential. It has already been used for a broad number of
science applications that require either large samples of non-linear
realizations or very large dark matter N-body simulations, including
cosmological reionization, halo formation, baryonic acoustic oscillations, weak
lensing or non-Gaussian statistics. We discuss the structure, the accuracy,
known systematic effects and the scaling performance of the code and its
utilities, when applicable.Comment: 20 pages, 17 figures, added halo profiles, updated to match MNRAS
accepted versio
The BAHAMAS project: the CMB--large-scale structure tension and the roles of massive neutrinos and galaxy formation
Recent studies have presented evidence for tension between the constraints on Omega_m and sigma_8 from the cosmic microwave background (CMB) and measurements of large-scale structure (LSS). This tension can potentially be resolved by appealing to extensions of the standard model of cosmology and/or untreated systematic errors in the modelling of LSS, of which baryonic physics has been frequently suggested. We revisit this tension using, for the first time, carefully-calibrated cosmological hydrodynamical simulations, which thus capture the back reaction of the baryons on the total matter distribution. We have extended the BAHAMAS simulations to include a treatment of massive neutrinos, which currently represents the best motivated extension to the standard model. We make synthetic thermal Sunyaev-Zel'dovich effect, weak galaxy lensing, and CMB lensing maps and compare to observed auto- and cross-power spectra from a wide range of recent observational surveys. We conclude that: i) in general there is tension between the primary CMB and LSS when adopting the standard model with minimal neutrino mass; ii) after calibrating feedback processes to match the gas fractions of clusters, the remaining uncertainties in the baryonic physics modelling are insufficient to reconcile this tension; and iii) invoking a non-minimal neutrino mass, typically of 0.2-0.4 eV (depending on the priors on the other relevant cosmological parameters and the datasets being modelled), can resolve the tension. This solution is fully consistent with separate constraints on the summed neutrino mass from the primary CMB and baryon acoustic oscillations, given the internal tensions in the Planck primary CMB dataset
KiDS-450: Enhancing cosmic shear with clipping transformations
We present the first “clipped” cosmic shear measurement using data from the Kilo-Degree
Survey (KiDS-450). “Clipping” transformations suppress the signal from the highest density,
non-linear regions of cosmological fields. We demonstrate that these transformations improve
constraints on S8 = σ8(Ωm/0.3)0.5 when used in combination with conventional two-point
statistics. For the KiDS-450 data, we find that the combined measurements improve the constraints
on S8 by 17%, compared to shear correlation functions alone. We determine the expectation
value of the clipped shear correlation function using a suite of numerical simulations,
and develop methodology to mitigate the impact of masking and shot noise. Future improvements
in numerical simulations and mass reconstruction methodology will permit the precise
calibration of clipped cosmic shear statistics such that clipping can become a standard tool in
weak lensing analyses
The BAHAMAS project: the CMB-large-scale structure tension and the roles of massive neutrinos and galaxy formation
Large scale structure and cosmolog
Thinking Outside the Box: Effects of Modes Larger than the Survey on Matter Power Spectrum Covariance
Considering the matter power spectrum covariance matrix, it has recently been
found that there is a potentially dominant effect on mildly non-linear scales
due to power in modes of size equal to and larger than the survey volume. This
{\it beat coupling} effect has been derived analytically in perturbation theory
and while it has been tested with simulations, some questions remain
unanswered. Moreover, there is an additional effect of these large modes, which
has so far not been included in analytic studies, namely the effect on the
estimated {\it average} density which enters the power spectrum estimate. In
this article, we work out analytic, perturbation theory based expressions
including both the beat coupling and this {\it local average effect} and we
show that while, when isolated, beat coupling indeed causes large excess
covariance in agreement with the literature, in a realistic scenario this is
compensated almost entirely by the local average effect, leaving only of the excess. We test our analytic expressions by comparison to a suite of
large N-body simulations. For the variances, we find excellent agreement with
the analytic expressions for Mpc at , while the
correlation coefficients agree to beyond Mpc. As expected, the
range of agreement increases towards higher redshift and decreases slightly
towards . We finish by including the large-mode effects in a full
covariance matrix description for arbitrary survey geometry and confirming its
validity using simulations. This may be useful as a stepping stone towards
building an actual galaxy (or other tracer's) power spectrum covariance matrix.
[abridged]Comment: 24 pages, 10 figures. Version accepted for publication in JCAP. Added
Figure 5 and Appendix
RCSLenS: The Red Cluster Sequence Lensing Survey
We present the Red Cluster Sequence Lensing Survey (RCSLenS), an application of the methods developed for the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) to the ∼785 deg2, multi-band imaging data of the Red-sequence Cluster Survey 2. This project represents the largest public, sub-arcsecond seeing, multi-band survey to date that is suited for weak gravitational lensing measurements. With a careful assessment of systematic errors in shape measurements and photometric redshifts, we extend the use of this data set to allow cross-correlation analyses between weak lensing observables and other data sets. We describe the imaging data, the data reduction, masking, multi-colour photometry, photometric redshifts, shape measurements, tests for systematic errors, and a blinding scheme to allow for more objective measurements. In total, we analyse 761 pointings with r-band coverage, which constitutes our lensing sample. Residual large-scale B-mode systematics prevent the use of this shear catalogue for cosmic shear science. The effective number density of lensing sources over an unmasked area of 571.7 deg2 and down to a magnitude limit of r ∼ 24.5 is 8.1 galaxies per arcmin2 (weighted: 5.5 arcmin−2) distributed over 14 patches on the sky. Photometric redshifts based on four-band griz data are available for 513 pointings covering an unmasked area of 383.5 deg2. We present weak lensing mass reconstructions of some example clusters as well as the full survey representing the largest areas that have been mapped in this way. All our data products are publicly available through Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/rcslens/query.html in a format very similar to the CFHTLenS data release