Cosmology

Abstract:

This paper describes the definition of a typical next-generation space-based weak gravitational lensing experiment. We first adopt a set of top-level science requirements from the literature, based on the scale and depth of the galaxy sample, and the avoidance of systematic effects in the measurements which would bias the derived shear values. We then identify and categorize the contributing factors to the systematic effects, combining them with the correct weighting, in such a way as to fit within the top-level requirements. We present techniques which permit the performance to be evaluated and explore the limits at which the contributing factors can be managed. Besides the modelling biases resulting from the use of weighted moments, the main contributing factors are the reconstruction of the instrument point spread function, which is derived from the stellar images on the image, and the correction of the charge transfer inefficiency in the CCD detectors caused by radiation damage. © 2013 The Authors.

Abstract:

LOFAR, the Low Frequency Radio Array, is a new pan-European radio telescope that is almost fully operational. One of its main drivers is to make deep images of the low frequency radio sky. To be able to do this a number of challenges need to be addressed. These include the high data rates, removal of radio frequency interference, calibration of the beams and correcting for the corrupting influence of the ionosphere. One of the key science goals is to study merger shocks, particle acceleration mechanisms and the structure of magnetic fields in nearby and distant merging clusters. Recent studies with the GMRT and WSRT radio telescopes of the "Sausage" and the "Toothbrush" clusters have given a very good demonstration of the power of radio observations to study merging clusters. Recently we discovered that both clusters contain relic and halo sources, large diffuse regions of radio emission not associated with individual galaxies. The 2 Mpc northern relic in the Sausage cluster displays highly aligned magnetic fields and and exhibits a strong spectral index gradient that is a consequence of cooling of the synchrotron emitting particles in the post-shock region. We have argued that these observations provide strong evidence that shocks in merging clusters are capable of accelerating particles. For the Toothbrush cluster we observe a puzzling linear relic that extends over 2 Mpc. The proposed scenario is that a triplemerger can lead to such a structure. With LOFAR's sensitivity it will not only be possible to trace much weaker shocks, but also to study those shocks due to merging clusters up to redshifts of at least one. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We investigate the extent to which cosmic size magnification may be used to complement cosmic shear in weak gravitational lensing surveys, with a view to obtaining high-precision estimates of cosmological parameters. Using simulated galaxy images, we find that unbiased estimation of the convergence field is possible using galaxies with angular sizes larger than the point spread function (PSF) and signal-to-noise ratio in excess of 10. The statistical power is similar to, but not quite as good as, cosmic shear, and it is subject to different systematic effects. Application to ground-based data will be challenging, with relatively large empirical corrections required to account for the fact that many galaxiesare smaller than the PSF, but for space-based data with 0.1-0.2 arcsec resolution, the size distribution of galaxies brighter than i≃24 is almost ideal for accurate estimation of cosmic size magnification. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present cosmological constraints from 2D weak gravitational lensing by the large-scale structure in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) which spans 154 deg2 in five optical bands. Using accurate photometric redshifts and measured shapes for 4.2 million galaxies between redshifts of 0.2 and 1.3, we compute the 2D cosmic shear correlation function over angular scales ranging between 0.8 and 350 arcmin. Using nonlinear models of the dark-matter power spectrum, we constrain cosmological parameters by exploring the parameter space with Population Monte Carlo sampling. The best constraints from lensing alone are obtained for the small-scale density-fluctuations amplitude σ8 scaled with the total matter density Ωm. For a flat Λcold dark matter (ΛCDM) model we obtain Σ8(Ωm/0.27)0.6 = 0.79 ± 0.03. We combine the CFHTLenS data with 7-year Wilkinson Microwave Anisotropy Probe (WMAP7), baryonic acoustic oscillations (BAO): SDSS-III (BOSS) and a Hubble Space Telescope distance-ladder prior on the Hubble constant to get joint constraints. For a flat ΛCDM model, we find Ωm = 0.283 ± 0.010 and Σ8 = 0.813 ± 0.014. In the case of a curved wCDM universe, we obtain Ωm = 0.27 ± 0.03, Σ8 = 0.83 ± 0.04, w0 = -1.10 ± 0.15 and Ωk = 0.006+0.006-0.004. We calculate the Bayesian evidence to compare flat and curved ΛCDM and dark-energy CDM models. From the combination of all four probes, we find models with curvature to be at moderately disfavoured with respect to the flat case. A simple dark-energy model is indistinguishable from ΛCDM. Our results therefore do not necessitate any deviations from the standard cosmological model. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present the first direct measurement of the galaxy-matter bispectrum as a function of galaxy luminosity, stellar mass and type of spectral energy distribution (SED). Our analysis uses a galaxy-galaxy-galaxy lensing technique (G3L), on angular scales between 9 arcsec and 50 arcmin, to quantify (i) the excess surface mass density around galaxy pairs (excess mass hereafter) and (ii) the excess shear-shear correlations around single galaxies, both of which yield a measure of two types of galaxy-matter bispectra. We apply our method to the state-of-the-art Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), spanning 154 square degrees. This survey allows us to detect a significant change of the bispectra with lens properties. Measurements for lens populations with distinct redshift distributions become comparable by a newly devised normalization technique. That will also aid future comparisons to other surveys or simulations. A significant dependence of the normalized G3L statistics on luminosity within-23=Mr=-18 and stellarmass within 5×109M⊙ =M* <2×1011M⊙ is found (h = 0.73). Both bispectra exhibit a stronger signal for more luminous lenses or those with higher stellar mass (up to a factor of 2-3). This is accompanied by a steeper equilateral bispectrum for more luminous or higher stellar mass lenses for the excess mass. Importantly, we find the excess mass to be very sensitive to galaxy type as recently predicted with semianalytic galaxy models: luminous (Mr < -21) late-type galaxies show no detectable signal, while all excess mass detected for luminous galaxies seems to be associated with early-type galaxies. We also present the first observational constraints on third-order stochastic galaxy biasing parameters. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.