1,640 research outputs found
Comparison between the Blue and the Red Galaxy Alignments Detected in the Sloan Digital Sky Survey
We measure the intrinsic alignments of the blue and the red galaxies
separately by analyzing the spectroscopic data of the Sloan Digital Sky Survey
Data Release 6 (SDSS DR6). For both samples of the red and the blue galaxies
with axial ratios of b/a <= 0.8, we detect a 3 sigma signal of the ellipticity
correlation in the redshift range of 0 <= z <= 0.4 for r-band absolute (model)
magnitude cut of M_r <= -19.2 (no K correction). We note a difference in the
strength and the distance scale for the red and the blue galaxy correlation
eta_{2D}(r): For the bright blue galaxies, it behaves as a quadratic scaling of
the linear density correlation of xi(r) as eta_{2D}(r) proportional to
xi^{2}(r) with strong signal detected only at small distance bin of r <= 3
Mpc/h. While for the bright red galaxies it follows a linear scaling as
eta_{2D}(r) proportional to xi(r) with signals detected at larger distance out
to r~6 Mpc/h. We also test whether the detected correlation signal is intrinsic
or spurious by quantifying the systematic error and find that the effect of the
systematic error on the ellipticity correlation is negligible. It is finally
concluded that our results will be useful for the weak lensing measurements as
well as the understanding of the large scale structure formation.Comment: accepted by ApJL, revised version, 12 pages, 2 figures, 1 table,
systematic error analyzed and beaten down, cross-correlations between the
blue and red galaxies shown, clearer discussion on the different generation
mechanism for the blue and red galaxy alignments adde
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.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
Parallel machine architecture and compiler design facilities
The objective is to provide an integrated simulation environment for studying and evaluating various issues in designing parallel systems, including machine architectures, parallelizing compiler techniques, and parallel algorithms. The status of Delta project (which objective is to provide a facility to allow rapid prototyping of parallelized compilers that can target toward different machine architectures) is summarized. Included are the surveys of the program manipulation tools developed, the environmental software supporting Delta, and the compiler research projects in which Delta has played a role
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 density \Omega_M, the mass power spectrum normalization
\sigma_8, the shape parameter \Gamma and the redshift of the sources z_s.
Constraints on \Omega_M and \sigma_8 (resp. \Gamma and z_s) are then given by
marginalizing over \Gamma and z_s (resp. \Omega_M and \sigma_8). For a flat
LCDM cosmologies, using a photometric redshift prior for the sources and \Gamma
\in [0.1,0.4], we find \sigma_8=(0.57\pm0.04) \Omega_M^{(0.24\mp 0.18)
\Omega_M-0.49} at the 68% confidence level (the error budget includes
statistical noise, full cosmic variance and residual systematic). The estimate
of \Gamma, marginalized over \Omega_M \in [0.1,0.4], \sigma_8 \in [0.7,1.3] and
z_s constrained by photometric redshifts, gives \Gamma=0.25\pm 0.13 at 68%
confidence. Adopting h=0.7, a flat universe, \Gamma=0.2 and \Omega_m=0.3 we
find \sigma_8=0.98 \pm0.06 . Combined with CMB, our results suggest a non-zero
cosmological constant and provide tight constraints on \Omega_M and \sigma_8.
We finaly 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 for its improvement.Comment: 11 pages, submitted to A&
Simulating Reionization: Character and Observability
In recent years there has been considerable progress in our understanding of
the nature and properties of the reionization process. In particular, the
numerical simulations of this epoch have made a qualitative leap forward,
reaching sufficiently large scales to derive the characteristic scales of the
reionization process and thus allowing for realistic observational predictions.
Our group has recently performed the first such large-scale radiative transfer
simulations of reionization, run on top of state-of-the-art simulations of
early structure formation. This allowed us to make the first realistic
observational predictions about the Epoch of Reionization based on detailed
radiative transfer and structure formation simulations. We discuss the basic
features of reionization derived from our simulations and some recent results
on the observational implications for the high-redshift Ly-alpha sources.Comment: 3 pages, to appear in the Proceedings of First Stars III, Santa Fe,
July 2007, AIP Conference Serie
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