Cross-correlation Amplitude Research Articles

THE CLUSTERING OF Mg II ABSORPTION SYSTEMS ATz∼ 0.5 AND DETECTION OF COLD GAS IN MASSIVE HALOS

We measure the large-scale clustering of Mg II {lambda}{lambda} 2796,2803 absorbers with respect to a population of luminous red galaxies (LRGs) at z {approx} 0.5. From the cross-correlation measurements between Mg II absorbers and LRGs, we calculate the mean bias of the dark matter halos in which the absorbers reside. We investigate possible systematic uncertainties in the clustering measurements due to the sample selection of LRGs and due to uncertainties in photometric redshifts. First, we compare the cross-correlation amplitudes determined using a flux-limited LRG sample and a volume-limited one. The comparison shows that the relative halo bias of Mg II absorbers using a flux-limited LRG sample can be overestimated by as much as {approx}20%. Next, we assess the systematic uncertainty due to photometric redshift errors using a mock galaxy catalog with added redshift uncertainties comparable to the data. We show that the relative clustering amplitude measured without accounting for photometric redshift uncertainties is overestimated by {approx}10%. After accounting for these two main uncertainties, we find a 1{sigma} anticorrelation between mean halo bias and absorber strength W{sub r} (2796) that translates into a 1{sigma} anticorrelation between mean galaxy mass and W{sub r} (2796). The results indicate that a significant fraction ofmore » the Mg II absorber population of W{sub r} (2796) = 1-1.5 A are found in group-size dark matter halos of log M{sub h} 1.5 A are primarily seen in halos of log M{sub h} < 12.7. A larger data set would improve the precision of both the clustering measurements and the relationship between equivalent width and halo mass. Finally, the strong clustering of Mg II absorbers down to scales of {approx}0.3 h {sup -1} Mpc indicates the presence of cool gas inside the virial radii of the dark matter halos hosting the LRGs.« less

Scaling and universality in coupled driven diffusive models

Inspired by the physics of magnetohydrodynamics (MHD) a simplified coupled Burgers-likemodel in one dimension (1d), a generalization of the Burgers model to coupleddegrees of freedom, is proposed to describe 1dMHD. In addition to MHD, thismodel serves as a 1d reduced model for driven binary fluid mixtures. Here we haveperformed a comprehensive study of the universal properties of the generalizedd-dimensional version of the reduced model. We employ both analytical and numericalapproaches. In particular, we determine the scaling exponents and the amplitude ratios ofthe relevant two-point time-dependent correlation functions in the model. We demonstratethat these quantities vary continuously with the amplitude of the noise cross-correlation.Further our numerical studies corroborate the continuous dependence of long wavelengthand long timescale physics of the model on the amplitude of the noise cross-correlations, asfound in our analytical studies. We construct and simulate lattice–gas models of coupleddegrees of freedom in 1d, belonging to the universality class of our coupled Burgers-likemodel, which display similar behavior. We use a variety of numerical (MonteCarlo and Pseudo-spectral methods) and analytical (dynamic renormalizationgroup, self-consistent mode coupling theory and functional renormalization group)approaches for our work. The results from our different approaches complement oneanother. Possible realizations of our results in various non-equilibrium models arediscussed.

Correlation of CMB with large-scale structure. II. Weak lensing

We investigate the correlation of gravitational lensing of the cosmic microwave background (CMB) with several tracers of large-scale structure, including luminous red galaxies (LRGs), quasars, and radio sources. The lensing field is reconstructed based on the CMB maps from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite; the LRGs and quasars are observed by the Sloan Digital Sky Survey (SDSS); and the radio sources are observed in the NRAO VLA Sky Survey (NVSS). Combining all three large-scale structure samples, we find evidence for a positive cross correlation at the $2.5\ensuremath{\sigma}$ level ($1.8\ensuremath{\sigma}$ for the SDSS samples and $2.1\ensuremath{\sigma}$ for NVSS); the cross correlation amplitude is $1.06\ifmmode\pm\else\textpm\fi{}0.42$ times that expected for the WMAP cosmological parameters. Our analysis extends other recent analyses in that we carefully determine bias-weighted redshift distribution of the sources, which is needed for a meaningful cosmological interpretation of the detected signal. We investigate contamination of the signal by galactic emission, extragalactic radio and infrared sources, thermal and kinetic Sunyaev-Zel'dovich effects, and the Rees-Sciama effect, and find all of them to be negligible.

PC 1643+4631A, B: THE LYMAN-α FOREST AT THE EDGE OF COHERENCE

This is the first measurement and detection of coherence in the intergalactic medium (IGM) at substantially high redshift (z ~ 3.8) and on large physical scales (~2.5 h –1 70 Mpc). We perform the measurement by presenting new observations of the high-redshift quasar pair PC 1643+4631A, B and their Lyα-absorber coincidences. With data collected from Keck I Low Resolution Imaging Spectrometer (LRIS) in a 10,200 s integration, we have full coverage of the Lyα forest over the redshift range 2.6 < z < 3.8 at a resolution of 3.6 A (~220 km s–1). This experiment extends multiple sight line quasar absorber studies to higher redshift, higher opacity, larger transverse separation, and into a regime where coherence across the IGM becomes weak and difficult to detect. Noteworthy features from these spectra are the strong damped Lyα absorbers (DLAs) just blueward of both Lyα emission peaks, each within 1000 km s–1 of the emission redshift but separated by 2500 km s–1 from each other. The coherence is measured by fitting discrete Lyα absorbers and by using pixel flux statistics. The former technique results in 222 Lyα absorbers in the A sight line and 211 in B. Relative to a Monte Carlo pairing test (using symmetric, nearest-neighbor matching) the data exhibit a 4σ excess of pairs at low velocity splitting (Δv < 150 km s–1), thus detecting coherence on transverse scales of ~2.5 h –1 70 Mpc. We use spectra extracted from a smoothed particle hydrodynamic (SPH) simulation to analyze symmetric pair matching, transmission distributions as a function of redshift and compute zero-lag cross-correlations to compare with the quasar pair data. The simulations agree with the data with the same strength (~4σ) at similarly low velocity splitting above random chance pairings. In cross-correlation tests, the simulations agree when the mean flux (as a function of redshift) is assumed to follow the prescription given by Kirkman et al. (2005). While the detection of flux correlation (measured through coincident absorbers and cross-correlation amplitude) is only marginally significant, the agreement between data and simulations is encouraging for future work in which even better quality data will provide the best insight into the overarching structure of the IGM and its understanding as shown by SPH simulations.

A Precisely Timed Asynchronous Pattern of ON and OFF Retinal Ganglion Cell Activity during Propagation of Retinal Waves

Patterns of coordinated spontaneous activity have been proposed to guide circuit refinement in many parts of the developing nervous system. It is unclear, however, how such patterns, which are thought to indiscriminately synchronize nearby cells, could provide the cues necessary to segregate functionally distinct circuits within overlapping cell populations. Here, we report that glutamatergic retinal waves possess a substructure in the bursting of neighboring retinal ganglion cells with opposite light responses (ON or OFF). Within a wave, cells fire repetitive nonoverlapping bursts in a fixed order: ON before OFF. This pattern is absent from cholinergic waves, which precede glutamate-dependent activity, providing a developmental sequence of distinct activity-encoded cues. Asynchronous bursting of ON and OFF retinal ganglion cells depends on inhibition between these parallel pathways. Similar asynchronous activity patterns could arise throughout the nervous system, as inhibition matures and might help to separate connections of functionally distinct subnetworks.

Cross-talk-free Fluorescence Cross-Correlation Spectroscopy by the Switching Method

Fluorescence cross-correlation spectroscopy (FCCS) is used as a powerful technique to analyze molecular interactions both in vitro and in vivo. This method basically requires two laser excitations for two target molecules labeled with fluorophores of different colors. Their coincidence in a microscopic detection volume is analyzed using two detectors. Any overlap of emission spectra of the two fluorophores, however, gives rise to false-positive data about their interaction. To overcome this problem, we have developed a new FCCS system, in which two excitation lasers are switched alternately by modulation using an acousto-optic tunable filter (AOTF). In this report, we demonstrate the feasibility of switching FCCS for enzymatic cleavage of proteins in living cells. A fusion protein of two fluorophores (EGFP and mRFP) with a cleavage site of caspase-3 inserted was expressed in HeLa cells, and proteolysis assay was performed during apoptotic cell death. Due to the absence of cross-talk signals, the FCCS measurement with the switching function gave a large change in relative cross-correlation amplitude after protein cleavage. Hence, switching FCCS enables more reliable measurement of molecular interactions than conventional FCCS.

Codebooks from almost difference sets

In direct spread CDMA systems, codebooks meeting the Welch bounds are used to distinguish among the signals of different users. Recently, constructions of codebooks with difference sets meeting Welch's bound on the maximum cross-correlation amplitude were developed. In this paper, a generic construction of codebooks using almost difference sets is considered and several classes of codebooks nearly meeting the Welch bound are obtained. The parameters of the codebooks constructed in this paper are new.

Equilibrium during static and dynamic tasks in blind subjects: no evidence of cross-modal plasticity

Can visual information be replaced by other sensory information in the control of static and dynamic equilibrium? We investigated the balancing behaviour of acquired and congenitally blind subjects (25 severe visually impaired subjects--15 males and 10 females, mean age 36 +/- 13.5 SD) and age and gender-matched normal subjects under static and dynamic conditions. During quiet stance, the centre of foot pressure displacement was recorded and body sway analysed. Under dynamic conditions, subjects rode a platform continuously moving in the antero-posterior direction, with eyes open (EO) and closed (EC). Balance was inferred by the movement of markers fixed on malleolus, hip and head. Amplitude of oscillation and cross-correlation between body segment movements were computed. During stance, in normal subjects body sway was larger EC than EO. In blind subjects, sway was similar under both visual conditions, in turn similar to normal subjects EC. Under dynamic conditions, in normal subjects head and hip were partially stabilized in space EO but translated as much as the platform EC. In blind subjects head and hip displacements were similar in the EO and the EC condition; with respect to normal subjects EC, body displacement was significantly larger with a stronger coupling between segments. Under both static and dynamic conditions, acquired and congenitally blind subjects had a similar behaviour. We conclude that long-term absence of visual information cannot be substituted by other sensory inputs. These results are at variance with the notion of compensatory cross-modal plasticity in blind subjects and strengthen the hypothesis that vision plays an obligatory role in the processing and integration of other sensory inputs for the selection of the balancing strategy in the control of equilibrium.

Quantitative detection of the ligand-dependent interaction between the androgen receptor and the co-activator, Tif2, in live cells using two color, two photon fluorescence cross-correlation spectroscopy

Two-photon, two-color fluorescence cross-correlation spectroscopy (TPTCFCCS) was used to directly detect ligand-dependent interaction between an eCFP-fusion of the androgen receptor (eCFP-AR) and an eYFP fusion of the nuclear receptor co-activator, Tif2 (eYFP-Tif2) in live cells. As expected, these two proteins were co-localized in the nucleus in the presence of ligand. Analysis of the cross-correlation amplitude revealed that AR was on average 81% bound to Tif2 in the presence of agonist, whereas the fractional complex formation decreased to 56% in the presence of antagonist. Residual AR-Tif2 interaction in presence of antagonist is likely mediated by its ligand-independent activation function. These studies demonstrate that using TPTCFCCS it is possible to quantify ligand-dependent interaction of nuclear receptors with co-regulator partners in live cells, making possible a vast array of structure-function studies for these important transcriptional regulators.

Complex Codebooks From Combinatorial Designs

Codebooks (also called signal sets) meeting the Welch bounds are desirable in code-division multiple-access (CDMA) systems. In 2003, binary codebooks meeting Welch's bounds were constructed using difference sets by Ding, Golin, and Kloslashve. Recently, a generic construction of codebooks with cyclic difference sets meeting Welch's bound on the maximum cross-correlation amplitude was developed by Xia In this correspondence, the idea of Xia is extended, and related constructions of optimal codebooks with both cyclic and noncyclic difference sets are presented. These codebooks are optimal in the sense that they also meet this Welch bound. In addition, complex codebooks that almost meet Welch's bound on the maximum cross-correlation amplitude are also constructed with almost difference sets

MST-based stepwise connection strategies for multipass Radar data, with application to coregistration and equalization

This paper proposes a unified framework for predicting optimized pairing strategies for interferometric processing of multipass synthetic aperture radar data. The approach consists in a minimum spanning tree (MST) structure based on a distance function encoding an a priori model for the interferometric quality of each image pair. Using a distance function modeled after the interferometric coherence allows reproducing many small baseline strategies presented in the recent literature. A novel application of the method to the processing steps of image coregistration and equalization is illustrated, using a test European Remote Sensing Satellite dataset. Widespread methods used for these two operations rely on the computation of the amplitude cross correlation over a large number of corresponding tie patches distributed over the scene. Geometric shift and radiometric equalization parameters are estimated over the patches and used, respectively, within a polynomial warp model and a radiometric correction scheme. The number of reliable patches available behaves similarly to the interferometric synthetic aperture radar (InSAR) coherence with respect to the baselines, and can be assimilated to a quality figure for the derivation of the MST. Results show an improvement in the quality of the stepwise (SW)-processed image stack with respect to the classical single-master procedure, confirming that the SW approach is able to provide better conditions for the estimation of correlation-related InSAR parameters

Simultaneous Multicolor Fluorescence Cross-Correlation Spectroscopy to Detect Higher Order Molecular Interactions Using Single Wavelength Laser Excitation

Fluorescence cross-correlation spectroscopy is a powerful method for the study of molecular interactions and dynamics in solution and even in living cells. Usually, in the optical setup, either two laser beams have to be superimposed in their respective confocal volumes or two-photon excitation is used for a dual-color detection system. It has been shown recently that fluorescence cross correlation can be achieved with spectrally similar fluorophores using single wavelength excitation fluorescence cross-correlation spectroscopy (SW-FCCS). In this study, we show that SW-FCCS allows the simultaneous excitation of up to three fluorophores in which the cross correlation of their fluctuation signals is detected separately in three detection channels. The experimental and theoretical model to describe triple pairwise cross correlations incorporating cross talk and possible changes in emission characteristics such as quenching upon binding are outlined. The effectiveness of SW-FCCS to detect binding of three interacting partners is experimentally verified with a standard ligand-receptor model, biotin-streptavidin, where differently labeled biotin ligands and their binding to a third-color labeled streptavidin are studied. The cross-correlation amplitudes and their changes with stoichiometric binding are analyzed and the upper limits of dissociation constants are determined. Performed with appropriate negative controls, SW-FCCS can determine interaction patterns between ligands and receptors.

Elastographic image quality vs. tissue motion in vivo

Elastography is a noninvasive method of imaging tissue elasticity using standard ultrasound equipment. In conventional elastography, axial strain elastograms are generated by cross-correlating pre- and postcompression digitized radio frequency (RF) echo frames acquired from the tissue before and after a small uniaxial compression, respectively. The time elapsed between the pre- and the postcompression frames is referred to as the interframe interval. For in vivo elastography, the interframe interval is critical because uncontrolled physiologic motion such as heartbeat, muscle motion, respiration and blood flow introduce interframe decorrelation that reduces the quality of elastograms. To obtain a measure of this decorrelation, in vivo experimental data (from human livers and thyroids) at various interframe intervals were obtained from 20 healthy subjects. To further examine the effect of the different interframe intervals on the elastographic image quality, the experimental data were also used in combination with elastographic simulation data. The deterioration of elastographic image quality was objectively evaluated by computing the area under the strain filter (SF) at a given resolution. The experimental results of this study demonstrate a statistical exponential behavior of the temporal decay of the echo signal cross-correlation amplitudes from the in vivo tissues due to uncontrollable motion. The results also indicate that the dynamic range and height of the SF are reduced at increased interframe intervals, suggesting that good objective image quality may be achieved provided only that a high frame rate is maintained in elastographic applications.

Spatial clustering of Ultra Steep Spectrum sources and galaxies

We present measurements of the clustering properties of galaxies in the field of redshift range 0.5 ≤ z ≤ 1.5 Ultra Steep Spectrum radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories Very Large Array Sky Survey. Galaxies in these USS fields were identified in deep near-infrared observations, complete down to K s = 20, using the IRIS2 instrument at the Anglo-Australian Telescope. We used the redshift distribution of K s < 20 galaxies taken from Cimatti et al. (2002) to constrain the correlation length r 0 . We find a strong correlation signal of galaxies with K s < 20 around our USS sample. A comoving correlation length r 0 = 14.0 ± 2.8 h -1 Mpc and y = 1.98 ± 0.15 are derived in a flat cosmological model universe. We compare our findings with those obtained in a cosmological N-body simulation populated with GALFORM semi-analytic galaxies. We find that clusters of galaxies with masses in the range M = 10 13.4-14.2 2 h -1 M ⊙ have a cluster-galaxy cross-correlation amplitude comparable to those found between the USS hosts and galaxies. These results suggest that distant radio galaxies are excellent tracers of galaxy overdensities and pinpoint the progenitors of present day rich clusters of galaxies.

New light on dark cosmos

Recent studies by a number of independent collaborations have correlated the cosmic microwave background temperatures measured by the Wilkinson Microwave Anisotropy Probe satellite with different galaxy surveys that trace the matter distribution with light from the whole range of the electromagnetic spectrum: radio, far-infrared, optical and X-ray surveys. The new data systematically find positive correlations, indicating a rapid slow down in the growth of structure in the Universe. Individual cross-correlation measurements are of low significance, but we show that combining data at different redshifts introduces important new constraints. Contrary to what happens at low redshifts, for a fixed Ω m , the higher the dark energy content, Ω A the lower the integrated Sachs-Wolfe effect cross-correlation amplitude. At 68 per cent confidence level, the data find new independent evidence of dark energy: Ω A = 0.42-1.22. The data also confirm, to higher significance, the presence of a large dark matter component, Ω m = 0.18-0.34, exceeding the density of baryonic matter, but far from the critical value. Combining these new constraints with the prior of a flat universe, or the prior of an accelerating universe, provides strong new evidence for a dark cosmos. Combination with supernova data yields Ω A = 0.71 ± 0.13, Ω m = 0.29 ± 0.04. If we also assume a flat universe, we find Ω A = 0.70 ± 0.05 and w = -1.02 ± 0.17 for a constant dark energy equation of state.

Spatial and dynamical properties of voids in a Λ cold dark matter universe

We study the statistical properties of voids in the distribution of mass, dark-matter haloes and galaxies (BJ 1011.5 h−1 M⊙, similar to the median halo mass, Mmed= 1011.3 h−1 M⊙. We also find that the number density of galaxy- and halo-defined voids can be up to two orders of magnitude higher than mass-defined voids for large void radii, however, we observe that this difference is reduced to about half an order of magnitude when the positions are considered in redshift space. As expected, there are outflow velocities that show their maximum at larger void-centric distances for larger voids. We find a linear relation for the maximum outflow velocity, vmax=v0rvoid. The void-centric distance where this maximum occurs follows a suitable power-law fit of the form log(d)=(rvoid/A)B. At sufficiently large distances, we find mild infall motions on to the subdense regions. The galaxy velocity field around galaxy-defined voids is consistent with the results of haloes with masses above the median, showing milder outflows than the mass around mass-defined voids. We find that a similar analysis in redshift space would make both outflows and infalls appear with a lower amplitude. We also find that the velocity dispersion of galaxies and haloes is larger in the direction parallel to the void walls by ≃10–20 per cent. Given that voids are by definition subdense regions, the cross-correlation function between galaxy-defined voids and galaxies is close to ξ=−1 out to separations comparable to the void size, and at larger separations the correlation function level increases, approaching the values of the auto-correlation function of galaxies. The cross-correlation amplitude of mass-defined voids versus mass has a more gentle behaviour remaining negative at larger distances. The cross- to auto-correlation function ratio as a function of the distance normalized to the void radius shows a small scatter around a relation that depends only on the object used to define the voids (galaxies or haloes for instance). The distortion pattern observed in ξ(σ, π) is that of an elongation along the line of sight that extends out to large separations. Positive ξ contours evidence finger-of-god motions at the void walls. Elongations along the line of sight are roughly comparable between galaxy-, halo- and mass-defined voids.

Probing the Intergalactic Medium-Galaxy Connection toward PKS 0405-123. II. A Cross-Correlation Study of Lyα Absorbers and Galaxies at z &lt; 0.5

We present a pilot study of the clustering properties of Lyα absorbers with respect to known galaxies based on 112 Lyα absorbers and 482 galaxies identified at z < 0.5 along the sight line toward PKS 0405-123. The principal goal is to determine the origin of Lyα absorbers based on their cross-correlation amplitude with known galaxies and to investigate a possible N(H I) dependence of the cross-correlation function. The main results of our study are as follows: (1) The cross-correlation function ξga measured using only emission-line-dominated galaxies and Lyα absorbers with log N(H I) ≥ 14 shows a comparable strength to the galaxy autocorrelation function ξgg on comoving, projected distance scales of less than 1 h-1 Mpc, while there remains a lack of cross-correlation signal when using only absorption-line-dominated galaxies. This signifies a morphology-dependent ξga and indicates that strong absorbers with log N(H I) ≥ 14 and emission-line galaxies reside in the same halo population. (2) A maximum likelihood analysis shows that Lyα absorbers with log N(H I) < 13.6 are consistent with being more randomly distributed with respect to known galaxies. (3) Finally, we find based on this single sight line that the amplitude of ξga does not depend sensitively on N(H I) for strong absorbers, with log N(H I) ≥ 13.6.

Indirect limit on the amplitude of primordial gravitational wave background from CMB-galaxy cross correlation

While large scale cosmic microwave background (CMB) anisotropies involve a combination of the scalar and tensor fluctuations, the scalar amplitude can be independently determined through the CMB-galaxy cross correlation. Using recently measured cross correlation amplitudes, arising from the cross correlation between galaxies and the integrated Sachs-Wolfe (ISW) effect in CMB anisotropies, we obtain a constraint r<0.5 at 68% confidence level on the tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude gravity waves at a level of a few percent, relative to the density field, in a low-Lambda dominated universe ({omega}{sub {lambda}}{approx}0.5). In future, joining cross correlation ISW measurements, which captures cosmological parameter information, with independent determinations of the matter density and CMB anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from temperature anisotropy maps when all other cosmological parameters except for the tensor amplitude are known and the combination with CMB-galaxy correlation allows this limit to be reached easily be accounting for degeneracies in certain cosmological parameters.

Bradykinesia Induced by Dopamine D2 Receptor Blockade Is Associated with Reduced Motor Cortex Activity in the Rat

Disruption of motor cortex activity is hypothesized to play a major role in the slowed movement (bradykinesia) associated with reduced dopaminergic function. We recorded single neurons in the motor cortex of free-moving rats performing a forelimb-reaching task. The same neurons were examined before and after induction of bradykinesia with the D2 dopamine receptor antagonist haloperidol. Within-cell changes in the firing rate and firing pattern of individual cells and the correlation between simultaneously recorded cells after injection of haloperidol were statistically compared with vehicle-only control experiments. During haloperidol-induced bradykinesia (mean movement time increase, +231%), there was an average 11% decrease in baseline firing rate. Movement-related peaks in firing rate were more dramatically affected, with an overall reduction in peak amplitudes of 40%. Bradykinesia was also associated with decreased intensity of bursting and amplitude of cross-correlation peaks at rest. The results show for the first time that significant reductions can be detected in motor cortex activity at rest in animals with impaired ability to generate movements induced by reduced dopamine action and confirm that impaired movements are associated with reduced cortical activation. Together, these changes in neural activity may reduce recruitment and rate modulation of motor units in the spinal cord.

Achieving the Welch Bound With Difference Sets

Consider a codebook containing N unit-norm complex vectors in a K-dimensional space. In a number of applications, the codebook that minimizes the maximal cross-correlation amplitude (I/sub max/) is often desirable. Relying on tools from combinatorial number theory, we construct analytically optimal codebooks meeting, in certain cases, the Welch lower bound. When analytical constructions are not available, we develop an efficient numerical search method based on a generalized Lloyd algorithm, which leads to considerable improvement on the achieved I/sub max/ over existing alternatives. We also derive a composite lower bound on the minimum achievable I/sub max/ that is effective for any codebook size N.