3,813 research outputs found
An Inversion Method for Measuring Beta in Large Redshift Surveys
A precision method for determining the value of Beta= Omega_m^{0.6}/b, where
b is the galaxy bias parameter, is presented. In contrast to other existing
techniques that focus on estimating this quantity by measuring distortions in
the redshift space galaxy-galaxy correlation function or power spectrum, this
method removes the distortions by reconstructing the real space density field
and determining the value of Beta that results in a symmetric signal. To remove
the distortions, the method modifies the amplitudes of a Fourier plane-wave
expansion of the survey data parameterized by Beta. This technique is not
dependent on the small-angle/plane-parallel approximation and can make full use
of large redshift survey data. It has been tested using simulations with four
different cosmologies and returns the value of Beta to +/- 0.031, over a factor
of two improvement over existing techniques.Comment: 16 pages including 6 figures Submitted to The Astrophysical Journa
Complete Treatment of Galaxy Two-Point Statistics: Gravitational Lensing Effects and Redshift-Space Distortions
We present a coherent theoretical framework for computing gravitational
lensing effects and redshift-space distortions in an inhomogeneous universe and
investigate their impacts on galaxy two-point statistics. Adopting the
linearized FRW metric, we derive the gravitational lensing and the generalized
Sachs-Wolfe effects that include the weak lensing distortion, magnification,
and time delay effects, and the redshift-space distortion, Sachs-Wolfe, and
integrated Sachs-Wolfe effects, respectively. Based on this framework, we first
compute their effects on observed source fluctuations, separating them as two
physically distinct origins: the volume effect that involves the change of
volume and is always present in galaxy two-point statistics, and the source
effect that depends on the intrinsic properties of source populations. Then we
identify several terms that are ignored in the standard method, and we compute
the observed galaxy two-point statistics, an ensemble average of all the
combinations of the intrinsic source fluctuations and the additional
contributions from the gravitational lensing and the generalized Sachs-Wolfe
effects. This unified treatment of galaxy two-point statistics clarifies the
relation of the gravitational lensing and the generalized Sachs-Wolfe effects
to the metric perturbations and the underlying matter fluctuations. For near
future dark energy surveys, we compute additional contributions to the observed
galaxy two-point statistics and analyze their impact on the anisotropic
structure. Thorough theoretical modeling of galaxy two-point statistics would
be not only necessary to analyze precision measurements from upcoming dark
energy surveys, but also provide further discriminatory power in understanding
the underlying physical mechanisms.Comment: 20 pages, 5 figures, Fig.4 corrected, appendix added, accepted for
publication in Physical Review
Measuring the galaxy power spectrum with future redshift surveys
Precision measurements of the galaxy power spectrum P(k) require a data
analysis pipeline that is both fast enough to be computationally feasible and
accurate enough to take full advantage of high-quality data. We present a
rigorous discussion of different methods of power spectrum estimation, with
emphasis on the traditional Fourier method, the linear (Karhunen-Loeve; KL),
and quadratic data compression schemes, showing in what approximations they
give the same result. To improve speed, we show how many of the advantages of
KL data compression and power spectrum estimation may be achieved with a
computationally faster quadratic method. To improve accuracy, we derive
analytic expressions for handling the integral constraint, since it is crucial
that finite volume effects are accurately corrected for on scales comparable to
the depth of the survey. We also show that for the KL and quadratic techniques,
multiple constraints can be included via simple matrix operations, thereby
rendering the results less sensitive to galactic extinction and mis-estimates
of the radial selection function. We present a data analysis pipeline that we
argue does justice to the increases in both quality and quantity of data that
upcoming redshift surveys will provide. It uses three analysis techniques in
conjunction: a traditional Fourier approach on small scales, a pixelized
quadratic matrix method on large scales and a pixelized KL eigenmode analysis
to probe anisotropic effects such as redshift-space distortions.Comment: Major revisions for clarity. Matches accepted ApJ version. 23 pages,
with 2 figs included. Color figure and links at
http://www.sns.ias.edu/~max/galpower.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/galpower.html (faster from Europe) or
from [email protected]
Lake-size dependency of wind shear and convection as controls on gas exchange
High-frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u*) and convection (w*) to turbulence in the surface mixed layer. Seasonal patterns of u* and w* were dissimilar; u* was often highest in the spring, while w * increased throughout the summer to a maximum in early fall. Convection was a larger mixed-layer turbulence source than wind shear (u */w*-1 for lakes* and w* differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake-air gas exchange, especially for small lakes. © 2012 by the American Geophysical Union.Jordan S. Read, David P. Hamilton, Ankur R. Desai, Kevin C. Rose, Sally MacIntyre, John D. Lenters, Robyn L. Smyth, Paul C. Hanson, Jonathan J. Cole, Peter A. Staehr, James A. Rusak, Donald C. Pierson, Justin D. Brookes, Alo Laas, and Chin H. W
The Role of 3D Fusion Computed Tomography in the Enhancement of the Safety Profile of FEVAR.
On measuring the covariance matrix of the nonlinear power spectrum from simulations
We show how to estimate the covariance of the power spectrum of a
statistically homogeneous and isotropic density field from a single periodic
simulation, by applying a set of weightings to the density field, and by
measuring the scatter in power spectra between different weightings. We
recommend a specific set of 52 weightings containing only combinations of
fundamental modes, constructed to yield a minimum variance estimate of the
covariance of power. Numerical tests reveal that at nonlinear scales the
variance of power estimated by the weightings method substantially exceeds that
estimated from a simple ensemble method. We argue that the discrepancy is
caused by beat-coupling, in which products of closely spaced Fourier modes
couple by nonlinear gravitational growth to the beat mode between them.
Beat-coupling appears whenever nonlinear power is measured from Fourier modes
with a finite spread of wavevector, and is therefore present in the weightings
method but not the ensemble method. Beat-coupling inevitably affects real
galaxy surveys, whose Fourier modes have finite width. Surprisingly, the
beat-coupling contribution dominates the covariance of power at nonlinear
scales, so that, counter-intuitively, it is expected that the covariance of
nonlinear power in galaxy surveys is dominated not by small scale structure,
but rather by beat-coupling to the largest scales of the survey.Comment: 19 pages, 4 figures. To appear in Monthly Notices of the Royal
Astronomical Society. Revised to match accepted versio
A New Statistic for Analyzing Baryon Acoustic Oscillations
We introduce a new statistic omega_l for measuring and analyzing large-scale
structure and particularly the baryon acoustic oscillations. omega_l is a
band-filtered, configuration space statistic that is easily implemented and has
advantages over the traditional power spectrum and correlation function
estimators. Unlike these estimators, omega_l can localize most of the acoustic
information into a single dip at the acoustic scale while also avoiding
sensitivity to the poorly constrained large scale power (i.e., the integral
constraint) through the use of a localized and compensated filter. It is also
sensitive to anisotropic clustering through pair counting and does not require
any binning. We measure the shift in the acoustic peak due to nonlinear effects
using the monopole omega_0 derived from subsampled dark matter catalogues as
well as from mock galaxy catalogues created via halo occupation distribution
(HOD) modeling. All of these are drawn from 44 realizations of 1024^3 particle
dark matter simulations in a 1h^{-1}Gpc box at z=1. We compare these shifts
with those obtained from the power spectrum and conclude that the results
agree. This indicates that any distance measurements obtained from omega_0 and
P(k) will be consistent with each other. We also show that it is possible to
extract the same amount of acoustic information using either omega_0 or P(k)
from equal volume surveys.Comment: 12 pages, 7 figures. ApJ accepted. Edit: Now updated with final
accepted versio
Silica “SHB”chiral Pc-L* Cu(I) complexes for continuous flow cyclopropanation reactions with carbon dioxide as a carrier
We have recently reported that copper(I) complexes of the new C1-symmetric pyridine-based 12-membered tetraaza macrocycles, Pyridine Containing Ligands (Pc-L*), are competent catalysts in the asymmetric cyclopropanation. In order to improve our catalytic system Cu(I) complexes based on Pc-L* ligands were heterogeneised on mesoporous ordered and non-ordered silicas (Davisil B, MCM-41, etc.) by the Supported by HydrogenBond (SHB) method.
Supported catalysts C were tested in enantioselective cyclopropanation in batch conditions showing good catalytic activities employing ethyl diazoacetate (EDA) as carbene precursor in n-hexane. The silica support has a strong influence on the diastereoselective outcome of the reaction, favoring the formation of the more challenging cis-isomer. Then, C were tested as catalyst for the cyclopropanation reaction under flow conditions focusing our attention on the use of supercritical CO2. Under optimised conditions, the catalyst was stable over at least 10 h of continuous flow, without drop in activity or selectivity
Geographically touring the eastern bloc: British geography, travel cultures and the Cold War
This paper considers the role of travel in the generation of geographical knowledge of the eastern bloc by British geographers. Based on oral history and surveys of published work, the paper examines the roles of three kinds of travel experience: individual private travels, tours via state tourist agencies, and tours by academic delegations. Examples are drawn from across the eastern bloc, including the USSR, Poland, Romania, East Germany and Albania. The relationship between travel and publication is addressed, notably within textbooks, and in the Geographical Magazine. The study argues for the extension of accounts of cultures of geographical travel, and seeks to supplement the existing historiography of Cold War geography
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