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Cosmic Variance (cosmic + variance)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

What is the largest Einstein radius in the universe?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
Masamune Oguri
ABSTRACT The Einstein radius plays a central role in lens studies as it characterizes the strength of gravitational lensing. In particular, the distribution of Einstein radii near the upper cut-off should probe the probability distribution of the largest mass concentrations in the universe. Adopting a triaxial halo model, we compute expected distributions of large Einstein radii. To assess the cosmic variance, we generate a number of Monte Carlo realizations of all-sky catalogues of massive clusters. We find that the expected largest Einstein radius in the universe is sensitive to parameters characterizing the cosmological model, especially ,8: for a source redshift of unity, they are 42+9,7, 35+8,6 and 54+12,7 arcsec (errors denote 1, cosmic variance), assuming best-fitting cosmological parameters of the Wilkinson Microwave Anisotropy Probe five-year (WMAP5), three-year (WMAP3) and one-year (WMAP1) data, respectively. These values are broadly consistent with current observations given their incompleteness. The mass of the largest lens cluster can be as small as , 1015 M,. For the same source redshift, we expect in all sky ,35 (WMAP5), ,15 (WMAP3) and ,150 (WMAP1) clusters that have Einstein radii larger than 20 arcsec. For a larger source redshift of 7, the largest Einstein radii grow approximately twice as large. Whilst the values of the largest Einstein radii are almost unaffected by the level of the primordial non-Gaussianity currently of interest, the measurement of the abundance of moderately large lens clusters should probe non-Gaussianity competitively with cosmic microwave background experiments, but only if other cosmological parameters are well measured. These semi-analytic predictions are based on a rather simple representation of clusters, and hence calibrating them with N -body simulations will help to improve the accuracy. We also find that these ,superlens' clusters constitute a highly biased population. For instance, a substantial fraction of these superlens clusters have major axes preferentially aligned with the line-of-sight. As a consequence, the projected mass distributions of the clusters are rounder by an ellipticity of ,0.2 and have , 40,60 per cent larger concentrations compared with typical clusters with similar redshifts and masses. We argue that the large concentration measured in A1689 is consistent with our model prediction at the 1.2, level. A combined analysis of several clusters will be needed to see whether or not the observed concentrations conflict with predictions of the flat ,-dominated cold dark matter model. [source]

Observational biases in Lagrangian reconstructions of cosmic velocity fields

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008
G. Lavaux
ABSTRACT Lagrangian reconstruction of large-scale peculiar velocity fields can be strongly affected by observational biases. We develop a thorough analysis of these systematic effects by relying on specially selected mock catalogues. For the purpose of this paper, we use the Monge,Ampère,Kantorovitch (MAK) reconstruction method, although any other Lagrangian reconstruction method should be sensitive to the same problems. We extensively study the uncertainty in the mass-to-light assignment due to incompleteness (missing luminous mass tracers), and the poorly determined relation between mass and luminosity. The impact of redshift distortion corrections is analysed in the context of MAK and we check the importance of edge and finite-volume effects on the reconstructed velocities. Using three mock catalogues with different average densities, we also study the effect of cosmic variance. In particular, one of them presents the same global features as found in observational catalogues that extend to 80 h,1 Mpc scales. We give recipes, checked using the aforementioned mock catalogues, to handle these particular observational effects, after having introduced them into the mock catalogues so as to quantitatively mimic the most densely sampled currently available galaxy catalogue of the nearby Universe. Once biases have been taken care of, the typical resulting error in reconstructed velocities is typically about a quarter of the overall velocity dispersion, and without significant bias. We finally model our reconstruction errors to propose an improved Bayesian approach to measure ,m in an unbiased way by comparing the reconstructed velocities to the measured ones in distance space, even though they may be plagued by large errors. We show that, in the context of observational data, it is possible to build a nearly unbiased estimator of ,m using MAK reconstruction. [source]

A deep Chandra survey of the Groth Strip , I. The X-ray data

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
K. Nandra
ABSTRACT We present the results of a 200-ks Chandra observation of part of the Groth Strip region, using the ACIS-I instrument. We present a relatively simple method for the detection of point sources and calculation of limiting sensitivities, which we argue is at least as sensitive and more self-consistent than previous methods presented in the literature. A total of 158 distinct X-ray sources are included in our point-source catalogue in the ACIS-I area. The number counts show a relative dearth of X-ray sources in this region. For example, at a flux limit of 10,15 erg cm,2 s,1, around 20 per cent more soft-band sources are detected in the HDF-N and almost 50 per cent more in the ELAIS-N1 field, which we have analysed by the same method for comparison. We find, however, that these differences are consistent with Poisson variations at <2, significance, and therefore there is no evidence for cosmic variance based on these number counts alone. We determine the average spectra of the objects and find a marked difference between the soft-band-selected sources, which have ,= 1.9 typical of unobscured active galactic nuclei (AGN), and the hard-band-selected sources, which have ,= 1.0. Reassuringly, the sample as a whole has a mean spectrum of ,= 1.4 ± 0.1, the same as the X-ray background. None the less, our results imply that the fraction of sources with significant obscuration is only ,25 per cent, much less than predicted by standard AGN population synthesis models. This is confirmed by direct spectral fitting, with only a handful of objects showing evidence for absorption. After accounting for absorption, all objects are consistent with a mean intrinsic spectrum of ,= 1.76 ± 0.08, very similar to local Seyfert galaxies. The survey area is distinguished by having outstanding multiwaveband coverage. Comparison with these observations and detailed discussion of the X-ray source properties will be presented in future papers. [source]

Beating lensing cosmic variance with galaxy tomography

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2004
Ue-Li Pen
ABSTRACT We discuss the use of cross-correlations between galaxies with distance information and projected weak lensing dark matter maps to obtain a fully three-dimensional dark matter map and power spectrum. On large scales (l, 100) one expects the galaxies to be biased, but not stochastic. I show that this allows a simultaneous solution of the full three-dimensional evolving galaxy bias and the dark matter power spectrum simultaneously. Within the photometric redshift information of the Canada,France,Hawaii (CFH) lensing legacy survey, this allows a threefold reduction of statistical error, while a cross-correlation with the Canadian Large Adaptive Reflector (CLAR) or other deep spectroscopic surveys allows a tenfold improvement in dark matter power accuracy on linear scales. This makes lensing surveys more sensitive to the cosmic equation of state and the neutrino masses. [source]

Cosmic momentum field and mass fluctuation power spectrum

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
Changbom Park
We introduce the cosmic momentum field as a new measure of the large-scale peculiar velocity and matter fluctuation fields. The momentum field is defined as the peculiar velocity field traced and weighted by galaxies, and is equal to the velocity field in the linear regime. We show that the radial component of the momentum field can be considered as a scalar field with the power spectrum which is practically one-third of that of the total momentum field. We present a formula for the power spectrum directly calculable from the observed radial peculiar velocity data. The momentum power spectrum is measured for the MAT sample in the Mark III catalogue of peculiar velocities of galaxies. Using the momentum power spectrum we find the amplitude of the matter power spectrum is and at the wavenumbers 0.049 and 0.074 h Mpc,1, respectively, where , is the density parameter. The 68 per cent confidence limits include the cosmic variance. The measured momentum and density power spectra together indicate that the parameter or where bO is the bias factor for optical galaxies. [source]

Voids in the Las Campanas Redshift Survey versus cold dark matter models

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000
V. Müller
We analysed the distribution of void sizes in the two-dimensional slices of the Las Campanas Redshift Survey (LCRS). Fourteen volume-limited subsamples were extracted from the six slices to cover a large part of the survey and to test the robustness of the results against cosmic variance. Thirteen samples were randomly culled to produce homogeneously selected samples. We then studied the relationship between the cumulative area covered by voids and the void size as a property of the void hierarchy. We found that the distribution of void sizes scaled with the mean galaxy separation, ,. In particular, we found that the size of voids covering half of the area is given by Dmed,,+(12±3) h,1 Mpc. Next, by employing an environmental density threshold criterion to identify mock galaxies, we were able to extend this analysis to mock samples from dynamical N -body simulations of cold dark matter (CDM) models. To reproduce the observed void statistics, overdensity thresholds of ,th,0,,,1 are necessary. We compared standard (SCDM), open (OCDM), vacuum energy dominated (,CDM) and broken scale invariant CDM models (BCDM): we found that both the void size distribution and the two-point correlation function provided important and complementary information on the large-scale matter distribution. The dependence of the void statistics on the threshold criterion for the mock galaxy identification showed that the galaxy biasing was more crucial for the void size distribution than were differences between the cosmological models. [source]

Toward an unbiased sample of X-ray selected normal galaxies outside the local Universe

ASTRONOMISCHE NACHRICHTEN, Issue 2 2008
A. GeorgakakisArticle first published online: 14 FEB 200
Abstract This paper shows that our understanding of the statistical properties of X-ray selected normal galaxies (e.g. X-ray luminosity function) can be significantly improved by combining a wide-area XMM-Newton survey with the moderare resolution and high S/N optical spectroscopy of the SDSS. Such a combined dataset has the potential to minimise uncertainties that affect existing normal galaxy samples at X-rays, such as small number statistics, cosmic variance, AGN contamination and incompleteness at bright X-ray luminosities. It is demonstrated that a 100 deg2 XMM-Newton survey in the SDSS area to the limit fX(0.5,2 keV) , 5 × 10,15 erg cm,2 s,1 will detect over 400 X-ray selected normal galaxies with excellent control over systematic biases, thereby providing tight contraints on the X-ray luminosity function at z , 0.1. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]