Ue-Li Pen

Extended mosaic observations with the Cosmic Background Imager

Two years of microwave background observations with the Cosmic Background Imager (CBI) have been combined to give a sensitive, high-resolution angular power spectrum over the range 400 2000 power previously seen with the CBI is reduced. Under the assumption that any signal in excess of the primary anisotropy is due to a secondary Sunyaev-Zeldovich anisotropy in distant galaxy clusters, we use CBI, Arcminute Cosmology Bolometer Array Receiver, and Berkeley-Illinois-Maryland Association array data to place a constraint on the present-day rms mass fluctuation on 8 h-1 Mpc scales, σ8. We present the results of a cosmological parameter analysis on the l < 2000 primary anisotropy data that show significant improvements in the parameters as compared to WMAP alone, and we explore the role of the small-scale cosmic microwave background data in breaking parameter degeneracies.

Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

We present the first large-scale radiative transfer simulat ions of cosmic reionization, in a simulation volume of (100 h 1 Mpc) 3 . This is more than a 2 orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and effici ent code for 3D radiative transfer, C 2 -Ray, which we have recently developed. These simulations allow us to do the first numerical studies of the large-scale structure of reionization w hich at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z � 21, and final overlap occurs by z � 11. The resulting electron-scattering optical depth is in goo d agreement with the firstyear WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earli er, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that small-box simulations of reionization have little predictive power for the evolut ion of the mean ionized fraction. The minimum reliable volume size for such predictions is � 30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show t hat there is a significant boost of the density fluctuations in both the neutral and the ionized c omponents relative to the total at arcminute and larger scales. We find two populations of H II re gions according to their size, numerous, mid-sized (� 10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mp c is reached by z � 13, when our volume is only about 50% ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas densi ty at various scales and for the first time show that both distributions are clearly non-Gaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21-cm emission, Ly-α emitter statistics, Gunn-Peterson optical depth and small-scale CMB secondary anisotropies due to patchy reionization.

Spin-induced Galaxy Alignments and Their Implications for Weak-Lensing Measurements

Large-scale correlations in the orientations of galaxies can result from alignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring protogalaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correlations using the Zeldovich approximation for a realistic distribution of galaxy shapes. Weak gravitational lensing can also induce ellipticity correlations, since the images of neighboring galaxies will be distorted coherently. On comparing these two effects that induce shape correlations, we find that for current weak-lensing surveys with a median redshift of zm = 1, the intrinsic signal is of the order of 1%-10% of the measured signal. However, for shallower surveys with zm ≤ 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations.

Simulating cosmic reionization at large scales – II. The 21-cm emission features and statistical signals

We present detailed predictions for the redshifted 21-cm signal from the epoch of reionization. These predictions are obtained from radiative transfer calculations on the results of large-scale (100 h -1 Mpc), high dynamic range, cosmological simulations. We consider several scenarios for the reionization history, of both early and extended reionizations. From the simulations, we construct and analyse a range of observational characteristics, from the global signal, via detailed images and spectra, to statistical representations of rms fluctuations, angular power spectra, and probability distribution functions to characterize the non-Gaussianity of the 21-cm signal. We find that the different reionization scenarios produce quite similar observational signatures, mostly differing in the redshifts of 50 per cent reionization, and of final overlap. All scenarios show a gradual transition in the global signatures of mean signal and rms fluctuations, which would make these more difficult to observe. Individual features, such as deep gaps and bright peaks, are substantially different from the mean, and mapping these with several arcminutes and 100 s of kHz resolution would provide a direct measurement of the underlying density field and the geometry of the cosmological H II regions, although significantly modified by peculiar velocity distortions. The presence of late emission peaks suggests these to be a useful target for observations. The power spectra during reionization are strongly boosted compared to the underlying density fluctuations. The strongest statistical signal is found around the time of 50 per cent reionization and displays a clear maximum at an angular scale of l ∼ 3000-5000. We find the distribution function of emission features to be strongly non-Gaussian, with an order of magnitude higher probability of bright emission features. These results suggest that, observationally, it may be easier to find individual bright features than deriving the power spectra, which, in turn, is easier than observing individual images.

Discriminating weak lensing from intrinsic spin correlations using the curl-gradient decomposition

The distortion field defined by the ellipticities of galaxy shapes as projected on the sky can be uniquely decomposed into a gradient and a curl component. If the observed ellipticities are induced by weak gravitational lensing, then the distortion field is curl-free. Here we show that, in contrast, the distortion field resulting from intrinsic spin alignments is not curl-free. This provides a powerful discriminant between lensing and intrinsic contributions to observed ellipticity correlations. We also show how these contributions can be disentangled statistically from the ellipticity correlations or computed locally from circular integrals of the ellipticity field. This allows for an unambiguous detection of intrinsic galaxy alignments in the data. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors. These techniques can be applied equally well to the polarization of the microwave background, where it can be used to separate curl-free scalar perturbations from those produced by gravity waves or defects.

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

Power spectra in global defect theories of cosmic structure formation

An efficient technique for computing perturbation power spectra in field ordering theories of cosmic structure formation is introduced, enabling computations to be carried out with unprecedented precision. Large scale simulations are used to measure unequal time correlators of the source stress energy, taking advantage of scaling during matter and radiation domination, and causality, to make optimal use of the available dynamic range. The correlators are then re-expressed in terms of a sum of eigenvector products, a representation which we argue is optimal, enabling the computation of the final power spectra to be performed at high accuracy. Microwave anisotropy and matter perturbation power spectra for global strings, monopoles, textures and non-topological textures are presented and compared with recent observations.

A simulation-calibrated limit on the H i power spectrum from the GMRT Epoch of Reionization experiment

The Giant Metrewave Radio Telescope Epoch of Reionization experiment is an ongoing effort to measure the power spectrum from neutral hydrogen at high redshift. We have previously reported an upper limit of (70 mK)^2 at wavenumbers of k ≈ 0.65 h Mpc^(−1) using a basic piecewise-linear foreground subtraction. In this paper, we explore the use of a singular value decomposition to remove foregrounds with fewer assumptions about the foreground structure. Using this method, we also quantify, for the first time, the signal loss due to the foreground filter and present new power spectra adjusted for this loss, providing a revised measurement of a 2σ upper limit at (248 mK)^2 for k = 0.50 h Mpc^(−1). While this revised limit is larger than previously reported, we believe it to be more robust and still represents the best current constraint on reionization at z ≈ 8.6.

Likelihood analysis of cosmic shear on simulated and VIRMOS-DESCART data ?

We present a maximum likelihood analysis of cosmological parameters from measurements of the aperture mass up to 35 arcmin using simulated and real cosmic shear data. A four-dimensional parameter space is explored which examines the mean densityM, the mass power spectrum normalisation8, the shape parameter and the redshift of the sources zs. Constraints onM and8 (resp. and zs) are provided by marginalising over and zs (resp.M and8). For a flatCDM cosmologies, using a photometric redshift prior for the sources and 2 (0:1; 0:4), we find8= (0:57 0:04) (0:240:18) M 0:49 M at the 68% confidence level (the error budget includes statistical noise, full cosmic variance and residual systematics). The estimate of , marginalised overM 2 (0:1; 0:4),8 2 (0:7; 1:3) and zs constrained by photometric redshifts, gives= 0:25 0:13 at 68% confidence. Adopting h= 0:7, a flat universe,= 0: 2a ndm = 0: 3w e fi nd8 = 0:98 0:06. Combined with CMB measurements, our results suggest a non-zero cosmological constant and provide tight constraints onM and8. Finally, we compare our results to the cluster abundance ones, and discuss the possible discrepancy with the latest determinations of the cluster method. In particular we point out the actual limitations of the mass power spectrum prediction in the non-linear regime, and the importance in improving this.

Cosmic Shear from Galaxy Spins.

We discuss the origin of galactic angular momentum and the statistics of the present-day spin distribution. It is expected that the galaxy spin axes are correlated with the intermediate principal axis of the gravitational shear tensor. This allows one to reconstruct the shear field and thereby the full gravitational potential from the observed galaxy spin fields. We use the direction of the angular momentum vector without any information of its magnitude, which requires a measurement of the position angle and inclination on the sky of each disk galaxy. We present the maximum likelihood shear inversion procedure, which involves a constrained linear minimization. The theory is tested against numerical simulations. We find the correlation strength of nonlinear structures with the initial shear field and show that accurate large-scale density reconstructions are possible at the expected noise level.