2,864 research outputs found
Using angular pair upweighting to improve 3D clustering measurements
No full textThree-dimensional galaxy clustering measurements provide a wealth of cosmological information. However, obtaining spectra of galaxies is expensive, and surveys often only measure redshifts for a subsample of a target galaxy population. Provided that the spectroscopic data is representative, we argue that angular pair upweighting should be used in these situations to improve the 3D clustering measurements. We present a toy model showing mathematically how such a weighting can improve measurements, and provide a practical example of its application using mocks created for the Baryon Oscillation Spectroscopic Survey (BOSS). Our analysis of mocks suggests that if an angular clustering measurement is available over twice the area covered spectroscopically, weighting gives an~10-20 per cent reduction of the variance of the monopole correlation function on the baryon acoustic oscillation scale
Effects of cosmological model assumptions on galaxy redshift survey measurements
No full textThe clustering of galaxies observed in future redshift surveys will provide a wealth of cosmological information. Matching the signal at different redshifts constrains the dark energy driving the acceleration of the expansion of the Universe. In tandem with these geometrical constraints, redshift-space distortions depend on the build up of large-scale structure. As pointed out by many authors, measurements of these effects are intrinsically coupled. We investigate this link and argue that it strongly depends on the cosmological assumptions adopted when analysing data. Using representative assumptions for the parameters of the Euclid survey in order to provide a baseline future experiment, we show how the derived constraints change due to different model assumptions. We argue that even the assumption of a Friedman-Robertson-Walker space-time is sufficient to reduce the importance of the coupling to a significant degree. Taking this idea further, we consider how the data would actually be analysed and argue that we should not expect to be able to simultaneously constrain multiple deviations from the standard Lambda cold dark matter (Lambda CDM) model. We therefore consider different possible ways in which the Universe could deviate from the Lambda CDM model, and show how the coupling between geometrical constraints and structure growth affects the measurement of such deviations
Measuring line-of-sight-dependent fourier-space clustering using FFTs
No full textObserved galaxy clustering exhibits local transverse statistical isotropy around the line of sight (LOS). The variation of the LOS across a galaxy survey complicates the measurement of the observed clustering as a function of the angle to the LOS, as fast Fourier transforms (FFTs) based on Cartesian grids, cannot individually allow for this. Recent advances in methodology for calculating LOS-dependent clustering in Fourier space include the realization that power spectrum LOS-dependent moments can be constructed from sums over galaxies, based on approximating the LOS to each pair of galaxies by the LOS to one of them. We show that we can implement this method using multiple FFTs, each measuring the LOS-weighted clustering along different axes. The N log N nature of FFTs means that the computational speed-up is a factor of > 1000 compared with summing over galaxies. This development should be beneficial for future projects such as DESI and Euclid which will provide an order of magnitude more galaxies than current surveys
Cosmological inference from galaxy-clustering power spectrum: Gaussianization and covariance decomposition
No full textLikelihood fitting to two-point clustering statistics made from galaxy surveys usually assumes a multivariate normal distribution for the measurements, with justification based on the central limit theorem given the large number of overdensity modes. However, this assumption cannot hold on the largest scales where the number of modes is low. Whilst more accurate distributions have previously been developed in idealized cases, we derive a procedure suitable for analysing measured monopole power spectra with window effects, stochastic shot noise, and the dependence of the covariance matrix on the model being fitted all taken into account. A data transformation is proposed to give an approximately Gaussian likelihood, with a variance-correlation decomposition of the covariance matrix to account for its cosmological dependence. By comparing with the modified-t likelihood derived under the usual normality assumption, we find in numerical tests that our new procedure gives more accurate constraints on the local non-Gaussianity parameter fNL, which is sensitive to the large-scale power. A simple data analysis pipeline is provided for straightforward application of this new approach in preparation for forthcoming large galaxy surveys such as DESI and Euclid
Hybrid-basis inference for large-scale galaxy clustering: combining spherical and Cartesian Fourier analyses
No full textFuture precision cosmology from large-scale structure experiments including the Dark Energy Spectroscopic Instrument (DESI) and Euclid will probe wider and deeper cosmic volumes than those covered by previous surveys. The Cartesian power spectrum analysis of anisotropic galaxy clustering based on the Fourier plane wave basis makes a number of assumptions, including the local plane-parallel approximation, that will no longer be valid on very large scales and may degrade cosmological constraints. We propose an approach that utilises a hybrid basis: on the largest scales, clustering statistics are decomposed into spherical Fourier modes which respect the natural geometry of both survey observations and physical effects along the line of sight, such as redshift-space distortions, the Alcock-Paczy ́ nsky and light-cone effects; on smaller scales with far more clustering modes, we retain the computational benefit of the power spectrum analysis aided by fast Fourier transforms. This approach is particularly suited to the likelihood analysis of local primordial non-Gaussianity fNL through the scale-dependent halo bias, and we demonstrate its applicability with Nbody simulations. We also release our public code harmonia for galaxy clustering likelihood inference in spherical Fourier or hybrid-basis analyses
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